Magnesium is a chemical element with the symbol Mg

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust, where it constitutes about 2% by mass, and ninth in the known universe as a whole.Sony VAIO VGN-SR140N/S Battery

This abundance of magnesium is related to the fact that it is easily built up in supernova stars from a sequential addition of three helium nuclei to carbon (which in turn is made from three helium nuclei). Due to magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater.Sony VAIO VGN-SR165E/B Battery

Magnesium is the 11th most abundant element by mass in the human body; its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Hundreds of enzymesthus require magnesium ions to function. Sony VAIO VGN-SR165E/P Battery

Magnesium is also the metallic ion at the center of chlorophyll, and is thus a common additive to fertilizers. Magnesium compounds are used medicinally as common laxatives, antacids (e.g., milk of magnesia), and in a number of situations where stabilization of abnormal nerve excitation and blood vessel spasm is required (e.g., to treat eclampsia). Sony VAIO VGN-SR165E/S Battery

Magnesium ions are sour to the taste, and in low concentrations help to impart a natural tartness to fresh mineral waters.

The free element (metal) is not found naturally on Earth, as it is highly reactive (though once produced, it is coated in a thin layer of oxide [see passivation], which partly masks this reactivity). Sony VAIO VGN-SR190EBJ Battery

The free metal burns with a characteristic brilliant white light, making it a useful ingredient in flares. The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. Commercially, the chief use for the metal is as an alloying agent to make aluminium-magnesium alloys, sometimes called "magnalium" or "magnelium". Sony VAIO VGN-SR190EBQ Battery

Since magnesium is less dense than aluminium, these alloys are prized for their relative lightness and strength.

Physical and chemical properties

Elemental magnesium is a fairly strong, silvery-white, light-weight metal (two thirds the density of aluminium). Sony VAIO VGN-SR190EEJ/C Battery

It tarnishesslightly when exposed to air, although unlike the alkali metals, storage in an oxygen-free environment is unnecessary because magnesium is protected by a thin layer of oxide that is fairly impermeable and difficult to remove. Like its lower periodic table group neighbor calcium, magnesium reacts with water at room temperature, though it reacts much more slowly than calcium.Sony VAIO VGN-SR190NAB Battery

When submerged in water, hydrogen bubbles will almost unnoticeably begin to form on the surface of the metal, though if powdered it will react much more rapidly. The reaction will occur faster with higher temperatures (see precautions). Magnesium's ability to react with water can be harnessed to produce energy and run a magnesium-based engine.Sony VAIO VGN-SR190NBB Battery

Magnesium also reacts exothermically with most acids, such as hydrochloric acid (HCl). As with aluminium, zinc and many other metals, the reaction with hydrochloric acid produces the chloride of the metal and releases hydrogen gas.Sony VAIO VGN-SR190NDB Battery

Magnesium is a highly flammable metal, but while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk. Once ignited, it is difficult to extinguish, being able to burn in nitrogen (formingmagnesium nitride), carbon dioxide (forming magnesium oxide and carbon) and water (forming magnesium oxide and hydrogen). Sony VAIO VGN-SR190NEB Battery

This property was used in incendiary weapons used in the firebombing of cities in World War II, the only practical civil defense being to smother a burning flare under dry sand to exclude the atmosphere. On burning in air, magnesium produces a brilliant white light which includes strong ultraviolet. Sony VAIO VGN-SR190NGB Battery

Thus magnesium powder (flash powder) was used as a source of illumination in the early days of photography. Later, magnesium ribbon was used in electrically ignited flash bulbs. Magnesium powder is used in the manufacture of fireworks and marine flares where a brilliant white light is required. Sony VAIO VGN-SR190PAB Battery

Flame temperatures of magnesium and magnesium alloys can reach 3,100 °C (3,370 K; 5,610 °F),[7] although flame height above the burning metal is usually less than 300 mm (12 in).[8] Magnesium may be used as an ignition source for thermite, an otherwise difficult to ignite mixture of aluminium and iron oxide powder.Sony VAIO VGN-SR190PCB Battery

Magnesium compounds are typically white crystals. Most are soluble in water, providing the sour-tasting magnesium ion Mg2+. Small amounts of dissolved magnesium ion contribute to the tartness and taste of natural waters. Magnesium ion in large amounts is an ionic laxative, and magnesium sulfate (common name: Epsom salt) is sometimes used for this purpose.Sony VAIO VGN-SR190PFB Battery

So-called "milk of magnesia" is a water suspension of one of the few insoluble magnesium compounds, magnesium hydroxide. The undissolved particles give rise to its appearance and name. Milk of magnesia is a mild base commonly used as an antacid, which has some laxative side effect.Sony VAIO VGN-SR19VN Battery

Isotopes

Magnesium has three stable isotopes: 24Mg, 25Mg and 26Mg. All are present in significant amounts (see table of isotopes above). About 79% of Mg is 24Mg. The isotope 28Mg is radioactive and in the 1950s to 1970s was made commercially by several nuclear power plants for use in scientific experiments. Sony VAIO VGN-SR19VN Battery

This isotope has a relatively short half-life (21 hours) and so its use was limited by shipping times.

26Mg has found application in isotopic geology, similar to that of aluminium. 26Mg is a radiogenic daughter product of 26Al, which has a half-life of 717,000 years. Large enrichments of stable 26Mg have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. Sony VAIO VGN-SR19VRN Battery

The anomalous abundance of 26Mg is attributed to the decay of its parent 26Al in the inclusions. Therefore, the meteorite must have formed in the solar nebula before the 26Al had decayed. Hence, these fragments are among the oldest objects in the solar system and have preserved information about its early history.Sony VAIO VGN-SR19XN Battery

It is conventional to plot 26Mg/24Mg against an Al/Mg ratio. In an isochron dating plot, the Al/Mg ratio plotted is27Al/24Mg. The slope of the isochron has no age significance, but indicates the initial 26Al/27Al ratio in the sample at the time when the systems were separated from a common reservoir.Sony VAIO VGN-SR19XN Battery

Creation

Stable forms of magnesium are produced in stars. It is made from fusing helium and neon, in the so-called alpha process. Creation requires temperatures above 600 megakelvins and masses greater than 3 solar masses.Sony VAIO VGN-SR210J/S Battery

Occurrence

Although magnesium is found in over 60 minerals, only dolomite, magnesite, brucite,carnallite, talc, and olivine are of commercial importance.

The Mg2+ cation is the second most abundant cation in seawater (occurring at about 12% of the mass of sodium there), which makes seawater and sea-salt an attractive commercial source of Mg.Sony VAIO VGN-SR21M/S Battery

To extract the magnesium, calcium hydroxide is added to seawater to formmagnesium hydroxide precipitate.

MgCl2 + Ca(OH)2 ? Mg(OH)2 + CaCl2

Magnesium hydroxide (brucite) is insoluble in water so it can be filtered out, and reacted with hydrochloric acid to obtain concentrated magnesium chloride.Sony VAIO VGN-SR21RM/H Battery

Mg(OH)2 + 2 HCl ? MgCl2 + 2 H2O

From magnesium chloride, electrolysis produces magnesium.

In the United States, magnesium is principally obtained by electrolysis of fused magnesium chloride from brines, wells, andsea water. At the cathode, the Mg2+ ion is reduced by two electrons to magnesium metal:Sony VAIO VGN-SR21RM/S Battery

Mg2+ + 2 e?? Mg

At the anode, each pair of Cl? ions is oxidized to chlorine gas, releasing two electrons to complete the circuit:

2 Cl? ? Cl2 (g) + 2 e?
The United States has traditionally been the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Sony VAIO VGN-SR220J/B Battery

Today, the US market share is at 7%, with a single domestic producer left, US Magnesium, a Renco Group company in Utah born from now-defunct Magcorp.[10]

As of 2005, China has taken over as the dominant supplier, pegged at 60% world market share, which increased from 4% in 1995.Sony VAIO VGN-SR220J/H Battery

Unlike the above-described electrolyticprocess, China is almost completely reliant on a different method of obtaining the metal from its ores, the silicothermic Pidgeon process (the reduction of the oxide at high temperatures with silicon).

History

The name magnesium originates from the Greek word for a district in Thessaly called Magnesia. Sony VAIO VGN-SR240J/B Battery

It is related to magnetiteand manganese, which also originated from this area, and required differentiation as separate substances. See manganese for this history.[citation needed]

Magnesium is the seventh most abundant element in the Earth's crust by mass or molarity.[Sony VAIO VGN-SR240N/B Battery

It is found in large deposits of magnesite, dolomite, and other minerals, and in mineral waters, where magnesium ion is soluble. In 1618, a farmer at Epsom in England attempted to give his cows water from a well there. The cows refused to drink because of the water's bitter taste, but the farmer noticed that the water seemed to heal scratches and rashes. Sony VAIO VGN-SR290JTH Battery

The substance became known as Epsom salts and its fame spread; it was eventually recognized to be hydrated magnesium sulfate, MgSO4.

The metal itself was first produced by Sir Humphry Davy in England in 1808 using electrolysis of a mixture of magnesia and mercury oxide.Sony VAIO VGN-SR290JTJ Battery

Antoine Bussy prepared it in coherent form in 1831. Davy's first suggestion for a name was magnium, but the name magnesium is now used.

As Metal

Magnesium is the third most commonly used structural metal, following iron and aluminium. It has been called the lightest useful metal by The Periodic Table of Videos.Sony VAIO VGN-SR290JTQ Battery

The main applications of magnesium are, in order: component of aluminium alloys, in die-casting (alloyed with zinc), to removesulfur in the production of iron and steel, the production of titanium in the Kroll process.

Magnesium, in its purest form, can be compared with aluminium, and is strong and light, so it is used in several high volume part manufacturing applications, including automotive and truck components.Sony VAIO VGN-SR290JVB/C Battery

Specialty, high-grade car wheels of magnesium alloy are called "mag wheels", although the term is often more broadly misapplied to include aluminum wheels. In 1957, a Corvette SS, designed for racing, was constructed with magnesium body panels. Sony VAIO VGN-SR290JVH/C Battery

An earlier Mercedes-Benz race car model, the Mercedes-Benz 300 SLR, had a body made from Elektron, a magnesium alloy; these cars ran (with successes) at Le Mans, the Mille Miglia, and other world-class race events in 1955 (though one was involved in the single worst accident in auto racing history, in terms of human casualties, at the Le Mans race.) Sony VAIO VGN-SR290NTB Battery

Porsche's all-out quest to decrease the weight of their race cars led to the use of magnesium frames in the famous 917/053 that won Le Mans in 1971, and still holds the absolute distance record. The 917/30 Can-Am car also featured a magnesium frame, helping it to make the most of its prodigious 1100–1500 hp. Volkswagen Group has used magnesium in its engine components for many years.Sony VAIO VGN-SR29VN/S Battery

For a long time, Porsche used magnesium alloy for its engine blocks due to the weight advantage. There is renewed interest in magnesium engine blocks, as featured in the 2006 BMW 325i and 330i models. The BMW engine uses an aluminium alloy insert for the cylinder walls and cooling jackets surrounded by a high-temperature magnesium alloy AJ62A. Sony VAIO VGN-SR29XN/S Battery

The application of magnesium AE44 alloy in the 2006 Corvette Z06 engine cradle has advanced the technology of designing robust automotive parts in magnesium. Both these alloys are recent developments in high-temperature low creep magnesium alloys. Mitsubishi Motors also uses magnesium (branded magnesium alloy) for its paddle shifters. Sony VAIO VGN-SR2RVN/S Battery

The general strategy for such alloys is to form intermetallic precipitates at the grain boundaries, for example by adding mischmetal or calcium. New alloy development and lower costs, which are becoming competitive to aluminium, will further the number of automotive applications.Sony VAIO VGN-SR390NAB Battery

The second application field of magnesium is electronic devices. Because of low weight, good mechanical and electrical properties, magnesium is widely used for manufacturing of mobile phones, laptop computers, cameras, and other electronic components.Sony VAIO VGN-SR390NAH Battery

Historically, magnesium was one of the main aerospace construction metals and was used for German military aircraft as early as World War I and extensively for German aircraft in World War II. The Germans coined the name 'Elektron' for magnesium alloy. The term is still used today. Sony VAIO VGN-SR41M/P Battery

Because of perceived hazards with magnesium parts in the event of fire, the application of magnesium in the commercial aerospace industry was generally restricted to engine related components. Currently the use of magnesium alloys in aerospace is increasing, mostly driven by the increasing importance of fuel economy and the need to reduce weight.Sony VAIO VGN-SR41M/S Battery

The development and testing of new magnesium alloys continues, notably Elektron 21, which has successfully undergone extensive aerospace testing for suitability in engine, internal and airframe components. The European Community runs three R&D magnesium projects in the Aerospace priority of Six Framework Program.Sony VAIO VGN-SR49VN/H Battery

Niche Uses of the Metal

Magnesium, being available and relatively nontoxic, has variety of uses:

• Magnesium is flammable, burning at a temperature of approximately 3,100 °C (3,370 K; 5,610 °F), and the autoignition temperature of magnesium ribbon is approximately 630 °C (903 K; 1,166 °F) in air.Sony VAIO VGN-SR51B/P Battery
• It produces intense, bright, white light when it burns. Magnesium's high burning temperature makes it a useful tool for starting emergency fires during outdoor recreation. Other uses include flash photography, flares, pyrotechnics and fireworks sparklers.Sony VAIO VGN-SR51B/S Battery

• In the form of turnings or ribbons, to prepare Grignard reagents, which are useful in organic synthesis.
• As an additive agent in conventional propellants and the production of nodular graphite in cast iron.
• As a reducing agent for the production of uranium and other metals from their salts.Sony VAIO VGN-SR51MF Battery
• As a sacrificial (galvanic) anode to protect underground tanks, pipelines, buried structures, and water heaters.
• Alloyed with zinc to produce the zinc sheet used in photoengraving plates in the printing industry, dry-cell battery walls, androofing.Sony VAIO VGN-SR51MF/P Battery
• As a metal, this element's principal use is as an alloying additive to aluminium with these aluminium-magnesium alloys being used mainly for beverage cans.

As Magnesium Compounds

The magnesium ion is necessary for all life (see magnesium in biology), so magnesium salts are frequently included in variousfoods, fertilizers (magnesium is a component of chlorophyll), and culture media.Sony VAIO VGN-SR51MF/S Battery

Magnesium compounds, primarily magnesium oxide (MgO), are used as refractory a material in furnace linings for producing iron,steel, nonferrous metals, glass and cement. Magnesium oxide and other magnesium compounds are also used in the agricultural, chemical, and construction industries.Sony VAIO VGN-SR51MF/W Battery

Magnesium reacted with an alkyl halide gives a Gringard reagent, which is a very useful tool for preparing alcohols.

Niche and Illustrative Uses of Magnesium Compounds

• Magnesium sulfate, as the heptahydrate called Epsom salts, is used as bath salts, as a laxative, and as a highly soluble fertilizer.
• Magnesium hydroxide, suspended in water, is used in milk of magnesia antacids and laxatives.Sony VAIO VGN-SR51MR Battery
• Magnesium chloride, oxide, gluconate, malate, orotate and citrate are all used as oral magnesium supplements. Oral magnesium supplements have been claimed to be therapeutic for some individuals who suffer from Restless Leg Syndrome (RLS).Sony VAIO VGN-SR59VG Battery
• Magnesium borate, magnesium salicylate, and magnesium sulfate are all used as antiseptics.
• Magnesium bromide is used as a mild sedative (this action is due to the bromide, not the magnesium).
• Dead-burned magnesite (magnesium carbonate) is used for refractory purposes such as brick and liners in furnaces and converters.Sony VAIO VGN-SR59VG/H Battery
• Magnesium carbonate powder is used by such athletes as gymnasts and weightlifters to improve their grip on a gymnastic apparatus, lifting bar, or other object.
• Magnesium stearate is a slightly flammable white powder with lubricating properties. Sony VAIO VGN-SR70B/S Battery
• In pharmaceutical technology, it is used in the manufacturing of numerous kinds of tablets to prevent the tablets from sticking to the equipment during the tablet compression process (i.e., when the tablet's substance is pressed into tablet form).
• Magnesium sulfite is used in the manufacture of paper (sulfite process).Sony VAIO VGN-SR72B/P Battery
• Magnesium phosphate is used to fireproof wood used in construction.
• Magnesium hexafluorosilicate is used in mothproofing of textiles. Sony VAIO VGN-SR72B/S Battery

Biological role

Because of the important interaction between phosphate and magnesium ions, magnesium ions are essential to the basic nucleic acid chemistry of life, and thus are essential to all cells of all known living organisms.Sony VAIO VGN-SR73JB/S Battery

Over 300 enzymes require the presence of magnesium ions for their catalytic action, including all enzymes utilizing or synthesizing ATP, or those that use other nucleotides to synthesize DNA and RNA. ATP exists in cells normally as a chelate of ATP and a magnesium ion.Sony VAIO VGN-SR74FB/S Battery

Plants have an additional use for magnesium in that chlorophylls are magnesium-centered porphyrins. Magnesium deficiency in plants causes late-season yellowing between leaf veins, especially in older leaves, and can be corrected by applying Epsom salts (which is rapidly leached), or else crushed dolomitic limestone to the soil.Sony VAIO VGN-SR90FS Battery

Magnesium is a vital component of a healthy human diet. Human magnesium deficiency (including conditions that show few overt symptoms) is relatively rare although only 32% of the United States meet the RDA-DRI; low levels of magnesium in the body has been associated with the development of a number of human illnesses such as asthma, diabetes, and osteoporosis.Sony VAIO VGN-SR90NS Battery

Adult human bodies contain about 24 grams of magnesium, with 60% in the skeleton, 39% intracellular (20% in skeletal muscle), and 1% extracellular. Serum levels are typically 0.7–1.0 mmol/L or 1.8–2.4 mEq/L. Serum magnesium levels may appear normal even in cases of underlying intracellular deficiency, although no known mechanism maintains a homeostatic level in the blood other than renal excretion of high blood levels.Sony VAIO VGN-SR90S Battery

Intracellular magnesium is correlated with intracellular potassium. Magnesium is absorbed in the gastrointestinal tract, with more absorbed when status is lower. In humans, magnesium appears to facilitate calcium absorption . Low and high protein intake inhibit magnesium absorption, and other factors such as phosphate, phytate, and fat affect absorption.Sony VAIO VGN-SR90US Battery

Absorbed dietary magnesium is largely excreted through the urine, although most magnesium "administered orally" is excreted through the feces. Magnesium status may be assessed roughly through serum and erythrocyte Mg concentrations and urinary and fecal excretion, but intravenous magnesium loading tests are likely the most accurate and practical in most people.Sony VAIO VGN-SR91NS Battery

In these tests, magnesium is injected intravenously; a retention of 20% or more indicates deficiency.Other nutrient deficiencies are identified through biomarkers, but none are established for magnesium.

Spices, nuts, cereals, coffee, cocoa, tea, and vegetables are rich sources of magnesium. Green leafy vegetables such as spinach are also rich in magnesium as they contain chlorophyll. Sony VAIO VGN-SR91PS Battery

Observations of reduced dietary magnesium intake in modern Western countries compared to earlier generations may be related to food refining and modern fertilizers that contain no magnesium.

Numerous magnesium dietary supplements are available. Magnesium oxide, one of the most common because it has high magnesium content per weight, has been reported to be the least bioavailable.Sony VAIO VGN-SR91S Battery

Magnesium citrate has been reported as more bioavailable than oxide or amino-acid chelate (glycinate) forms.

Excess magnesium in the blood is freely filtered at the kidneys, and for this reason it is difficult to overdose on magnesium from dietary sources alone.Sony VAIO VGN-SR91US Battery

With supplements, overdose is possible, however, particularly in people with poor renal function; occasionally, with use of high cathartic doses of magnesium salts, severe hypermagnesemia has been reported to occur even without renal dysfunction. Sony VAIO VGN-SR92NS Battery

Alcoholism can produce a magnesium deficiency, which is easily reversed by oral or parenteral administration, depending on the degree of deficiency.

Detection in biological fluids

Magnesium concentrations in plasma or serum may be measured to monitor for efficacy and safety in those receiving the drug therapeutically,Sony VAIO VGN-SR92PS Battery

to confirm the diagnosis in potential poisoning victims or to assist in the forensic investigation in a case of fatal overdosage. The newborn children of mothers who received parenteral magnesium sulfate during labor may exhibit toxicity at serum magnesium levels that were considered appropriate for the mothers.Sony VAIO VGN-SR92S Battery

Magnesium in Treatment-resistant Depression

There has been some speculation that magnesium deficiency can lead to depression. Cerebral spinal fluid (CSF) magnesium has been found low in treatment-resistant suicidal depression and in patients that have attempted suicide. Sony VAIO VGN-SR92US Battery

Brain magnesium has been found low in TRD using phosphorus nuclear magnetic resonance spectroscopy, an accurate means for measuring brain magnesium. Blood and CSF magnesium do not appear well-correlated with major depression.Magnesium chloride in relatively small doses was found to be as effective in the treatment of depressed elderly type 2 diabetics with hypomagnesemia as imipramine 50 mg daily. Sony VAIO VGN-SR93DS Battery

Safety precautions

Magnesium metal and its alloys are explosive hazards; they are highly flammable in their pure form when molten or in powder or ribbon form. Burning or molten magnesium metal reacts violently with water. Sony VAIO VGN-SR93JS Batery

When working with powdered magnesium, safety glasses with welding eye protection are employed, because the bright white light produced by burning magnesium contains ultraviolet light that can permanently damage the retinas of the eyes.

Magnesium is capable of reducing water to the highly flammable hydrogen gas:Sony VAIO VGN-SR93PS Batttery

Mg (s) + 2 H2O (l) ? Mg(OH)2 (s) + H2 (g)

As a result, water cannot be used to extinguish magnesium fires; the hydrogen gas produced will only intensify the fire. Dry sand is an effective smothering agent but is usable only on relatively level and flat surfaces.

Magnesium also reacts with carbon dioxide to form magnesium oxide and carbon:Sony VAIO VGN-SR93YS Battery

2 Mg (s) + CO2 ? 2 MgO (s) + C (s)

Hence, carbon dioxide fire extinguishers cannot be used for extinguishing magnesium fires either.

Burning magnesium is usually quenched by using a Class D dry chemical fire extinguisher, or by covering the fire with sand or magnesium foundry flux to remove its air source.Sony VAIO VGN-SR94FS Battery

Magnesium is an essential element in biological systems. Magnesium occurs typically as the Mg2+ ion. It is an essential mineral nutrient for life and is present in every cell type in every organism. For example, ATP (adenosine triphosphate), the main source of energy in cells, must be bound to a magnesium ion in order to be biologically active. Sony VAIO VGN-SR94GS Battery

What is called ATP is often actually Mg-ATP. Similarly, magnesium plays a role in the stability of all polyphosphate compounds in the cells, including those associated with DNA and RNA synthesis.

Over 300 enzymes require the presence of magnesium ions for their catalytic action, including all enzymes utilizing or synthesizing ATP,Sony VAIO VGN-SR94HS Battery

or those that use other nucleotides to synthesize DNA and RNA. In plants, magnesium is necessary for synthesis of chlorophyll and photosynthesis.

Function

A balance of magnesium is vital to the well being of all organisms. Magnesium is a relatively abundant ion in the lithosphere and is highly bioavailable in thehydrosphere. Sony VAIO VGN-SR94VS Battery

This availability, in combination with a useful and very unusual chemistry, may have led to its usefulness in evolution as an ion for signaling, enzyme activation and catalysis. However, the unusual nature of ionic magnesium has also led to a major challenge in the use of the ion in biological systems. Sony VAIO VGN-SZ35B/B Battery

Biological membranes are impermeable to magnesium (and other ions) so transport proteins must facilitate the flow of magnesium , both into and out of cells and intracellular compartments.

Biological range, distribution, and regulation

In animals it has been shown that different cell types maintain different concentrations of magnesium.Sony VAIO VGN-SZ54B/B Battery

It seems likely that the same is true forplants. This suggests that different cell types may regulate influx and efflux of magnesium in different ways based on their unique metabolic needs. Interstitial and systemic concentrations of free magnesium must be delicately maintained by the combined processes of buffering (binding of ions to proteins and other molecules) and muffling (the transport of ions to storage or extracellular spaces ).Sony VAIO VGN-SZ55B/B Battery

In plants, and more recently in animals, magnesium has been recognized as an important signaling ion, both activating and mediating many biochemical reactions. The best example of this is perhaps the regulation of carbon fixation in chloroplasts in the Calvin cycle.Sony VAIO VGN-SZ55GN/B Battery

The importance of magnesium to proper cellular function cannot be overstated. Deficiency of the nutrient results in disease in the affected organism. In single-celled organisms such as bacteria and yeast, low levels of magnesium manifests in greatly reduced growth rates.Sony VAIO VGN-SZ56 Battery

In magnesium transport knockout strains of bacteria, healthy rates are maintained only with exposure to very high external concentrations of the ion. In yeast, mitochondrial magnesium deficiency also leads to disease.

Plants deficient in magnesium show stress responses. The first observable signs of both magnesium starvation and overexposure in plants is a decrease in the rate ofphotosynthesis. Sony VAIO VGN-SZ561N Battery

This is due to the central position of the Mg++ ion in the chlorophyll molecule. The later effects of magnesium deficiency on plants are a significant reduction in growth and reproductive viability.[3] Magnesium can also be toxic to plants, although this is typically seen only in droughtconditions.Sony VAIO VGN-SZ562N Battery

In animals, magnesium deficiency (hypomagnesemia) is seen when the environmental availability of magnesium is low. In ruminant animals, particularly vulnerable to magnesium availability in pasture grasses, the condition is known as ‘grass tetany’. Hypomagnesemia is identified by a loss of balance due to muscle weakness.Sony VAIO VGN-SZ57N Battery

A number of genetically attributable hypomagnesemia disorders have also been identified in humans.

Overexposure to magnesium may be toxic to individual cells, though these effects have been difficult to show experimentally. Sony VAIO VGN-SZ58GN Battery

In humans the condition is termed hypermagnesemia, and is well documented, though it is usually caused by loss of kidney function. In healthy individuals, excess magnesium is rapidly excreted in the urine (Harrison’s Principles of Internal Medicine, Online Edition).Sony VAIO VGN-SZ58GN/C Battery

Human health

Magnesium deficiency in humans was first described in the medical literature in 1934. The adult human daily nutritional requirement, which is affected by various factors including gender, weight and size, is 300-400 mg/day. Sony VAIO VGN-SZ58N Battery

Inadequate magnesium intake frequently causes muscle spasms, and has been associated with cardiovascular disease, diabetes, high blood pressure, anxiety disorders, migraines, osteoporosis and cerebral infarction. Acute deficiency (see hypomagnesemia) is rare, and is more common as a drug side effect (such as chronic alcohol or diuretic use)Sony VAIO VGN-SZ61MN/B Battery

than from low food intake per se, but it can also occur within people fed intravenously for extended periods of time. The incidence of chronic deficiency resulting in less than optimal health is debated.

The DRI upper tolerated limit for supplemental magnesium is 350 mg/day (calculated as milligrams (mg) of elemental magnesium in the salt).Sony VAIO VGN-SZ61VN/X Battery

(Supplements based on amino acid chelates, such as glycinate, lysinate etc., are much better tolerated by the digestive system and do not have the side effects of the older compounds used, while sustained release supplements prevent the occurrence of diarrhea.)Sony VAIO VGN-SZ61WN/C Battery

The most common symptom of excess oral magnesium intake is diarrhea. Since the kidneys of adult humans excrete excess magnesium efficiently, oral magnesium poisoning in adults with normal renal function is very rare. Infants, which have less ability to excrete excess magnesium even when healthy, should not be given magnesium supplements, except under a physician's care.Sony VAIO VGN-SZ640N/B Battery

Magnesium salts (usually in the form of magnesium sulfate or chloride when given parenterally) are used therapeutically for a number of medical conditions, see Epsom salts for a list of conditions which have been treated with supplemental magnesium ion.Sony VAIO VGN-SZ645P3 Battery

Magnesium is absorbed with reasonable efficiency (30% to 40%) by the body from any soluble magnesium salt, such as the chloride or citrate. Magnesium is similarly absorbed from Epsom salts, although the sulfate in these salts adds to their laxative effect at higher doses. Sony VAIO VGN-SZ650N/C Battery

Magnesium absorption from the insoluble oxide and hydroxide salts (milk of magnesia) is erratic and of poorer efficiency, since it depends on the neutralization and solution of the salt by the acid of the stomach, which may not be (and usually is not) complete.

Magnesium orotate may be used as adjuvant therapy in patients on optimal treatment for severe congestive heart failure, increasing survival rate and improving clinical symptoms and patient's quality of life.Sony VAIO VGN-SZ660N/C Battery

Nerve Conduction

Magnesium can affect muscle relaxation through direct action on the cell membrane. Mg++ ions close certain types of calcium channels, which conduct a positively charged calcium ion into the neuron. With an excess of magnesium, more channels will be blocked and the nerve will have less activity.Sony VAIO VGN-SZ670N/C Battery

Hypertension

Magnesium-containing Epsom salts are especially used in treating the hypertension of eclampsia. Even if the case is not eclampsia, there may be antihypertensiveeffects of having a substantial portion of the intake of sodium chloride (NaCl) exchanged for e.g. magnesium chloride; Sony VAIO VGN-SZ680ND Battery

NaCl is an osmolite and increases arginine vasopressin (AVP) release, which increases extracellular volume and thus results in increased blood pressure. However, not all osmolites have this effect on AVP release, so with magnesium chloride, the increase in osmolarity may not cause such a hypertensive response.Sony VAIO VGN-SZ691N/X Battery

Food sources

Green vegetables such as spinach provide magnesium because of the abundance of chlorophyll molecules which contain the ion. Nuts(especially cashews and almonds), seeds, dark chocolate, roasted soybeans, bran, and some whole grains are also good sources of magnesium.Sony VAIO VGN-SZ71E/B Battery

Although many foods contain magnesium, it is usually found in low levels. As with most nutrients, daily needs for magnesium are unlikely to be met by one serving of any single food. Eating a wide variety of fruits, vegetables, and grains will help ensure adequate intake of magnesium.Sony VAIO VGN-SZ71VN/X Battery

Because magnesium readily dissolves in water, refined foods, which are often processed or cooked in water and dried, are generally poor sources of the nutrient. For example, whole wheat bread has twice as much magnesium as white bread because the magnesium-rich germ and bran are removed when white flour is processed.Sony VAIO VGN-SZ71WN/C Battery

The table of food sources of magnesium suggests many dietary sources of magnesium.

"Hard" water can also provide magnesium, but "soft" water does not contain the ion. Dietary surveys do not assess magnesium intake from water, which may lead to underestimating total magnesium intake and its variability.Sony VAIO VGN-SZ74B/B Battery

Too much magnesium may make it difficult for the body to absorb calcium . Not enough magnesium can lead to hypomagnesemiaas described above, with irregular heartbeats, high blood pressure (a sign in humans but not some experimental animals such as rodents), insomnia and muscle spasms (fasciculation). Sony VAIO VGN-SZ750N/C Battery

However, as noted, symptoms of low magnesium from pure dietary deficiency are thought to be rarely encountered.

Biological chemistry

Mg2+ is the fourth most abundant metal ion in cells (in moles) and the most abundant free divalent cation — as a result it is deeply and intrinsically woven into cellular metabolism.Sony VAIO VGN-SZ75B/B Battery

Indeed, Mg2+-dependent enzymes appear in virtually every metabolic pathway: specific binding of Mg2+ to biological membranes is frequently observed, Mg2+ is also used as a signalling molecule, and much of nucleic acid biochemistry requires Mg2+, including all reactions which require release of energy from ATP.Sony VAIO VGN-SZ76 Battery

In nucleotides, the triple phosphate moiety of the compound is invariably stabilized by association with Mg2+ in all enzymic processes.

Chlorophyll

In photosynthetic organisms Mg2+ has the additional vital role of being the coordinating ion in the chlorophyll molecule.Sony VAIO VGN-SZ770N/C Battery

This role was discovered by R. M. Willstätter, who received the Nobel Prize in Chemistry 1915 for the purification and structure of chlorophyll.

Enzymes

The chemistry of the Mg2+ ion, as applied to enzymes, uses the full range of this ion’s unusual reaction chemistry to fulfill a range of functions.Sony VAIO VGN-SZ77N Battery

Mg2+ interacts with substrates, enzymes and occasionally both (Mg2+ may form part of the active site). Mg2+ generally interacts with substrates through inner sphere coordination, stabilising anions or reactive intermediates, also including binding to ATP and activating the molecule to nucleophilic attack. Sony VAIO VGN-SZ780 Battery

When interacting with enzymes and other proteins Mg2+ may bind using inner or outer sphere coordination, to either alter the conformation of the enzyme or take part in the chemistry of the catalytic reaction. In either case, because Mg2+ is only rarely fully dehydrated during ligand binding, Sony VAIO VGN-SZ78N Battery

it may be a water molecule associated with the Mg2+ that is important rather than the ion itself. The Lewis acidity of Mg2+ (pKa 11.4) is used to allow both hydrolysis and condensation reactions (most commonly phosphate ester hydrolysis and phosphoryl transfer) that would otherwise require pH values greatly removed from physiological values.Sony VAIO VGN-SZ791N/X Battery

Essential role in the biological activity of ATP

ATP (adenosine triphosphate), the main source of energy in cells, must be bound to a magnesium ion in order to be biologically active. What is called ATP is often actually Mg-ATP.

Nucleic acids

Nucleic acids have an important range of interactions with Mg2+. Sony VAIO VGN-SZ84NS Battery

The binding of Mg2+ to DNA and RNA stabilises structure; this can be observed in the increased melting temperature (Tm) of double-stranded DNA in the presence of Mg2+. Additionally, ribosomes contain large amounts of Mg2+ and the stabilisation provided is essential to the complexation of this ribo-protein.Sony VAIO VGN-SZ84PS Battery

A large number of enzymes involved in the biochemistry of nucleic acids bind Mg2+ for activity, using the ion for both activation and catalysis. Finally, the autocatalysis of many ribozymes (enzymes containing only RNA) is Mg2+ dependent (e.g. the yeast mitochondrial group II self splicing introns ).Sony VAIO VGN-SZ84S Battery

Magnesium ions can be critical in maintaining the positional integrity of closely clustered phosphate groups. These clusters appear in numerous and distinct parts of the cell nucleus and cytoplasm. For instance hexahydrated Mg2+ ions bind in the deep major groove and at the outer mouth of A-form nucleic acid duplexes.Sony VAIO VGN-SZ84US Battery

Cell membranes and walls

Biological cell membranes and cell walls are polyanionic surfaces. This has important implications for the transport of ions, particularly because it has been shown that different membranes preferentially bind different ions.Sony VAIO VGN-SZ85NS Battery

Both Mg2+ and Ca2+ regularly stabilise membranes by the cross-linking of carboxylated and phosphorylated head groups of lipids. However, the envelope membrane of E. coli has also been shown to bind Na+, K+, Mn2+ and Fe3+. The transport of ions is dependent on both the concentration gradient of the ion and the electric potential (??) across the membrane, Sony VAIO VGN-SZ85S Battery

which will be affected by the charge on the membrane surface. For example, the specific binding of Mg2+ to the chloroplast envelope has been implicated in a loss of photosynthetic efficiency by the blockage of K+ uptake and the subsequent acidification of the chloroplast stroma.Sony VAIO VGN-SZ85US Battery

Proteins

The Mg2+ ion tends to bind only weakly to proteins (Ka ? 105 ) and this can be exploited by the cell to switch enzymatic activity on and off by changes in the local concentration of Mg2+. Although the concentration of free cytoplasmic Mg2+ is on the order of 1 mmol/L,Sony VAIO VGN-SZ94NS Battery

the total Mg2+ content of animal cells is 30 mmol/L and in plants the content of leaf endodermal cells has been measured at values as high as 100 mmol/L (Stelzer et al., 1990), much of which is buffered in storage compartments. The cytoplasmic concentration of free Mg2+ is buffered by binding to chelators (e.g. ATP),Sony VAIO VGN-SZ94PS Battery

but also more importantly by storage of Mg2+ in intracellular compartments. The transport of Mg2+ between intracellular compartments may be a major part of regulating enzyme activity. The interaction of Mg2+ with proteins must also be considered for the transport of the ion across biological membranes.Sony VAIO VGN-SZ94S Battery

Manganese

In biological systems, only manganese (Mn2+) is readily capable of replacing Mg2+, but only in a limited set of circumstances. Mn2+is very similar to Mg2+ in terms of its chemical properties, including inner and outer shell complexation. Mn2+ effectively binds ATP and allows hydrolysis of the energy molecule by most ATPases. Sony VAIO VGN-SZ94US Battery

Mn2+ can also replace Mg2+ as the activating ion for a number of Mg2+-dependent enzymes, although some enzyme activity is usually lost. Sometimes such enzyme metal preferences vary among closely related species: for example, the reverse transcriptase enzyme of lentiviruses like HIV, SIV and FIV is typically dependent on Mg2+, whereas the analogous enzyme for other retroviruses prefers Mn2+.Sony VAIO VGN-SZ95NS Battery

Importance in drug binding

An article investigating the structural basis of interactions between clinically relevant antibiotics and the 50S ribosome appeared in Nature in October 2001. High resolution x-ray crystallography established that these antibiotics only associate with the 23S rRNA of a ribosomal subunit, and no interactions are formed with a subunit's protein portion.Sony VAIO VGN-SZ95S Battery

The article stresses that the results show "the importance of putative Mg2+ ions for the binding of some drugs".

By radioactive isotopes

The use of radioactive tracer elements in ion uptake assays allows the calculation of km, Ki and Vmax and determines the initial change in the ion content of the cells. Sony VAIO VGN-SZ95US Battery

28Mg decays by the emission of a high energy beta or gamma particle, which can be measured using a scintillation counter. However, the radioactive half-life of 28Mg, the most stable of the radioactive magnesium isotopes, is only 21 hours. This severely restricts the experiments involving the nuclide. Sony VAIO VGN-TZ121 Battery

Additionally, since 1990 no facility has routinely produced 28Mg and the price per mCi is now predicted to be approximately US$30,000. The chemical nature of Mg2+ is such that it is closely approximated by few other cations. However, Co2+, Mn2+ and Ni2+ have been used successfully to mimic the properties of Mg2+ in some enzyme reactions, Sony VAIO VGN-TZ13 Battery

and radioactive forms of these elements have been employed successfully in cation transport studies. The difficulty of using metal ion replacement in the study of enzyme function is that the relationship between the enzyme activities with the replacement ion compared to the original is very difficult to ascertain.Sony VAIO VGN-TZ130N/B Battery

By fluorescent indicators

A number of chelators of divalent cations have different fluorescence spectra in the bound and unbound states. Chelators for Ca2+are well established, have high affinity for the cation, and low interference from other ions. Mg2+ chelators lag behind and the major fluorescence dye for Mg2+ (mag-fura 2 ) actually has a higher affinity for Ca2+.Sony VAIO VGN-TZ131 Battery

This limits the application of this dye to cell types where the resting level of Ca2+ is < 1 ?M and does not vary with the experimental conditions under which Mg2+ is to be measured. Recently, Otten et al. (2001) have described work into a new class of compounds that may prove more useful, having significantly better binding affinities for Mg2+.Sony VAIO VGN-TZ132/N Battery

The use of the fluorescent dyes is limited to measuring the free Mg2+. If the ion concentration is buffered by the cell by chelation or removal to subcellular compartments, the measured rate of uptake will only give minimum values of km and Vmax.

By electrophysiology

First, ion-specific microelectrodes can be used to measure the internal free ion concentration of cells and organelles. Sony VAIO VGN-TZ132N Battery

The major advantages are that readings can be made from cells over relatively long periods of time, and that unlike dyes very little extra ion buffering capacity is added to the cells.

Second, the technique of two-electrode voltage-clamp allows the direct measurement of the ion flux across the membrane of a cell.Sony VAIO VGN-TZ150N/B Battery

The membrane is held at an electric potential and the responding current is measured. All ions passing across the membrane contribute to the measured current.

Third, the technique of patch-clamp which uses isolated sections of natural or artificial membrane in much the same manner as voltage-clamp but without the secondary effects of a cellular system. Sony VAIO VGN-TZ150N/N Battery

Under ideal conditions the conductance of individual channels can be quantified. This methodology gives the most direct measurement of the action of ion channels.

By absorption spectroscopy

Flame atomic absorption spectroscopy (AAS) determines the total magnesium content of a biological sample.Sony VAIO VGN-TZ160CB Battery

This method is destructive; biological samples must be broken down in concentrated acids to avoid clogging the fine nebulising apparatus. Beyond this the only limitation is that samples need to be in a volume of approximately 2 mL and at a concentration range of 0.1 – 0.4 µmol/L for optimum accuracy.Sony VAIO VGN-TZ160N/B Battery

As this technique cannot distinguish between Mg2+ already present in the cell and that taken up during the experiment only content not uptake can be quantified.

Inductively coupled plasma (ICP) using either the mass spectrometry (MS) or atomic emission spectroscopy (AES) modifications also allows the determination of the total ion content of biological samples.Sony VAIO VGN-TZ16GN/B Battery

These techniques are more sensitive than flame AAS and are capable of measuring the quantities of multiple ions simultaneously. However, they are also significantly more expensive.

Magnesium transport

The chemical and biochemical properties of Mg2+ present the cellular system with a significant challenge when transporting the ion across biological membranes. Sony VAIO VGN-TZ16N Battery

The dogma of ion transport states that the transporter recognises the ion then progressively removes the water of hydration, removing most or all of the water at a selective pore before releasing the ion on the far side of the membrane.Sony VAIO VGN-TZ16N/B Battery

Due to the properties of Mg2+, large volume change from hydrated to bare ion, high energy of hydration and very low rate of ligand exchange in the inner coordination sphere, these steps are probably more difficult than for most other ions. To date, only the ZntA protein of Paramecium has been shown to be a Mg2+ channel.Sony VAIO VGN-TZ17/N Battery

The mechanisms of Mg2+ transport by the remaining proteins are beginning to be uncovered with the first three dimensional structure of a Mg2+ transport complex being solved in 2004.

The hydration shell of the Mg2+ ion has a very tightly bound inner shell of six water molecules and a relatively tightly bound second shell containing 12 – 14 water molecules (Markham et al., 2002).Sony VAIO VGN-TZ170N/B Battery

Thus recognition of the Mg2+ ion probably requires some mechanism to interact initially with the hydration shell of Mg2+, followed by a direct recognition/binding of the ion to the protein. Due to the strength of the inner sphere complexation between Mg2+ and any ligand, Sony VAIO VGN-TZ170N/N Battery

multiple simultaneous interactions with the transport protein at this level might significantly retard the ion in the transport pore. Hence, it is possible that much of the hydration water is retained during transport, allowing the weaker (but still specific) outer sphere coordination.Sony VAIO VGN-TZ17GN/B Battery

In spite of the mechanistic difficulty, Mg2+ must be transported across membranes, and a large number of Mg2+ fluxes across membranes from a variety of systems have been described. However, only a small selection of Mg2+ transporters have been characterised at the molecular level.Sony VAIO VGN-TZ17N Battery

Ligand ion channel blockade

Magnesium ions (Mg2+) in cellular biology are usually in almost all senses opposite to Ca2+ ions, because they are bivalent too, but have greater electronegativity and thus hold on to water molecules stronger, preventing passage through the channel (even though magnesium is smaller). Thus Mg2+ ions block Ca2+ channels (NMDA channels) for example, etc.Sony VAIO VGN-TZ18/N Battery

Plant physiology of magnesium

The previous sections have dealt in detail with the chemical and biochemical aspects of Mg2+ and its transport across cellular membranes. This section will apply this knowledge to aspects of whole plant physiology, in an attempt to show how these processes interact with the larger and more complex environment of the multicellular organism.Sony VAIO VGN-TZ180N/RC Battery

Nutritional requirements and interactions

Mg2+ is essential for plant growth and is present in higher plants in amounts on the order of 80 ?mol g?1 dry weight. The amounts of Mg2+ vary in different parts of the plant and are dependent upon nutritional status. In times of plenty, excess Mg2+ may be stored in vascular cells (Stelzer et al., 1990;Sony VAIO VGN-TZ185N/WC Battery

and in times of starvation Mg2+ is redistributed, in many plants, from older to newer leaves.

Mg2+ is taken up into plants via the roots. Interactions with other cations in the rhizosphere can have a significant effect on the uptake of the ion.(Kurvits and Kirkby, 1980;Sony VAIO VGN-TZ18GN/X Battery

The structure of root cell walls is highly permeable to water and ions, and hence ion uptake into root cells, can occur anywhere from the root hairs to cells located almost in the centre of the root (limited only by the Casparian strip). Plant cell walls and membranes carry a great number of negative charges and the interactions of cations with these charges is key to the uptake of cations by root cells allowing a local concentrating effect. Sony VAIO VGN-TZ18N Battery

Mg2+ binds relatively weakly to these charges, and can be displaced by other cations, impeding uptake and causing deficiency in the plant.

Within individual plant cells the Mg2+ requirements are largely the same as for all cellular life; Sony VAIO VGN-TZ190N/B Battery

Mg2+ is used to stabilise membranes, is vital to the utilisation of ATP, is extensively involved in the nucleic acid biochemistry, and is a cofactor for many enzymes (including the ribosome). Also, Mg2+ is the coordinating ion in the chlorophyll molecule. It is the intracellular compartmentalisation of Mg2+ in plant cells that leads to additional complexity.Sony VAIO VGN-TZ190N/BC Battery

Four compartments within the plant cell have reported interactions with Mg2+. Initially Mg2+ will enter the cell into the cytoplasm (by an as yet unidentified system), but free Mg2+ concentrations in this compartment are tightly regulated at relatively low levels (?2 mmol/L) and so any excess Mg2+ is either quickly exported or stored in the second intracellular compartment, the vacuole.Sony VAIO VGN-TZ191N/XC Battery

The requirement for Mg2+ in mitochondria has been demonstrated in yeast and it seems highly likely that the same will apply in plants. The chloroplasts also require significant amounts of internal Mg2+, and low concentrations of cytoplasmic Mg2+. Sony VAIO VGN-TZ195N/XC Battery

In addition, it seems likely that the other subcellular organelles (e.g. Golgi, endoplasmic reticulum, etc.) also require Mg2+.

Distributing magnesium ions within the plant

Once in the cytoplasmic space of root cells Mg2+, along with the other cations, is probably transported radially into the stele and the vascular tissue.Sony VAIO VGN-TZ198N/RC Battery

From the cells surrounding the xylem the ions are released or pumped into the xylem and carried up through the plant. In the case of Mg2+, which is highly mobile in both the xylem and phloem, the ions will be transported to the top of the plant and back down again in a continuous cycle of replenishment. Sony VAIO VGN-TZ250N/B Battery

Hence, uptake and release from vascular cells is probably a key part of whole plant Mg2+ homeostasis. Figure 1 shows how few processes have been connected to their molecular mechanisms (only vacuolar uptake has been associated with a transport protein, AtMHX).Sony VAIO VGN-TZ250N/N Battery

The diagram shows a schematic of a plant and the putative processes of Mg2+ transport at the root and leaf where Mg2+ is loaded and unloaded from the vascular tissues. Mg2+ is taken up into the root cell wall space (1) and interacts with the negative charges associated with the cell walls and membranes.Sony VAIO VGN-TZ250N/P Battery

Mg2+ may be taken up into cells immediately (symplastic pathway) or may travel as far as the Casparian band (4) before being absorbed into cells (apoplastic pathway; 2). The concentration of Mg2+ in the root cells is probably buffered by storage in root cell vacuoles (3). Note that cells in the root tip do not contain vacuoles. Sony VAIO VGN-TZ27/N Battery

Once in the root cell cytoplasm Mg2+ travels towards the centre of the root by plasmodesmata, where it is loaded into the xylem (5) for transport to the upper parts of the plant. When the Mg2+ reaches the leaves it is unloaded from the xylem into cells (6) and again is buffered in vacuoles (7). Sony VAIO VGN-TZ270N/B Battery

Whether cycling of Mg2+ into the phloem occurs via general cells in the leaf (8) or directly from xylem to phloem via transfer cells (9) is unknown. Mg2+ may return to the roots in the phloem sap.

When a Mg2+ ion has been absorbed by a cell requiring it for metabolic processes, it is generally assumed that the ion stays in that cell for as long as the cell is active.Sony VAIO VGN-TZ27N Battery

In vascular cells this is not always the case; in times of plenty Mg2+ is stored in the vacuole, takes no part in the day-to-day metabolic processes of the cell (Stelzer et al., 1990) , and is released at need. But for most cells it is death by senescence or injury that releases Mg2+ and many of the other ionic constituents, recycling them into healthy parts of the plant.Sony VAIO VGN-TZ28/N Battery

Additionally, when Mg2+ in the environment is limiting some species are able to mobilise Mg2+ from older tissues. These processes involve the release of Mg2+ from its bound and stored states and its transport back into the vascular tissue, where it can be distributed to the rest of the plant.Sony VAIO VGN-TZ285N/RC Battery

In times of growth and development Mg2+ is also remobilised within the plant as source and sink relationships change.

The homeostasis of Mg2+ within single plant cells is maintained by processes occurring at the plasma membrane and at the vacuole membrane (see Figure 2). The major driving force for the translocation of ions in plant cells is ?pH.Sony VAIO VGN-TZ28N Battery

H+-ATPases pump H+ ions against their concentration gradient to maintain the pH differential that can be used for the transport of other ions and molecules. H+ ions are pumped out of the cytoplasm into the extracellular space or into the vacuole. The entry of Mg2+ into cells may occur through one of two pathways, Sony VAIO VGN-TZ290EAB Battery

via channels using the ?? (negative inside) across this membrane or by symport with H+ ions. To transport the Mg2+ ion into the vacuole requires a Mg2+/H+ antiport transporter (such as AtMHX). It is interesting to note that the H+-ATPases are dependent on Mg2+ (bound to ATP) for activity, so that Mg2+ is required to maintain its own homeostasis.Sony VAIO VGN-TZ290EAN Battery

A schematic of a plant cell is shown including the four major compartments currently recognised as interacting with Mg2+. H+-ATPases maintain a constant ?pH across the plasma membrane and the vacuole membrane. Mg2+ is transported into the vacuole using the energy of ?pH (in A. thaliana by AtMHX).Sony VAIO VGN-TZ290EAP Battery

Transport of Mg2+ into cells may use either the negative ?? or the ?pH. The transport of Mg2+ into mitochondria probably uses ?? as in the mitochondria of yeast, and it is likely that chloroplasts take Mg2+ by a similar system. The mechanism and the molecular basis for the release of Mg2+ from vacuoles and from the cell is not known.Sony VAIO VGN-TZ295N/XC Battery

Likewise the light-regulated Mg2+ concentration changes in chloroplasts are not fully understood, but do require the transport of H+ions across the thylakoid membrane.

Magnesium, chloroplasts and photosynthesis

Mg2+ is the coordinating metal ion in the chlorophyll molecule, and in plants where the ion is in high supply about 6% of the total Mg2+ is bound to chlorophyll.Sony VAIO VGN-TZ298N/XC Battery

Thylakoid stacking is stabilised by Mg2+ and is important for the efficiency of photosynthesis, allowing phase transitions to occur.

Mg2+ is probably taken up into chloroplasts to the greatest extent during the light induced development from proplastid to chloroplast or etioplast to chloroplast.Sony VAIO VGN-TZ33/B Battery

At these times the synthesis of chlorophyll and the biogenesis of the thylakoid membrane stacks absolutely require the divalent cation.

Whether Mg2+ is able to move into and out of chloroplasts after this initial developmental phase has been the subject of several conflicting reports.Sony VAIO VGN-TZ33/N Battery

Deshaies et al.(1984) found that Mg2+ did move in and out of isolated chloroplasts from young pea plants, but Gupta and Berkowitz (1989) were unable to reproduce the result using older spinach chloroplasts. Deshaies et al. had stated in their paper that older pea chloroplasts showed less significant changes in Mg2+ content than those used to form their conclusions. Sony VAIO VGN-TZ33/W Battery

Perhaps the relative proportion of immature chloroplasts present in the preparations might explain these observations.

The metabolic state of the chloroplast changes considerably between night and day. During the day the chloroplast is actively harvesting the energy of light and converting it into chemical energy.Sony VAIO VGN-TZ350N/B Battery

The activation of the metabolic pathways involved comes from the changes in the chemical nature of the stroma on the addition of light. H+ is pumped out of the stroma (into both the cytoplasm and the lumen) leading to an alkaline pH. Mg2+ (along with K+) is released from the lumen into the stroma, in an electroneutralisation process to balance the flow of H+.Sony VAIO VGN-TZ350N/N Battery

Finally, thiol groups on enzymes are reduced by a change in the redox state of the stroma. Examples of enzymes activated in response to these changes are fructose 1,6-bisphosphatase, sedoheptulose bisphosphatase and ribulose-1,5-bisphosphate carboxylase.Sony VAIO VGN-TZ350N/P Battery

During the dark period, if these enzymes were active a wasteful cycling of products and substrates would occur.

Two major classes of the enzymes that interact with Mg2+ in the stroma during the light phase can be identified. Firstly, enzymes in the glycolytic pathway most often interact with two atoms of Mg2+. Sony VAIO VGN-TZ370N/B Battery

The first atom is as an allosteric modulator of the enzymes’ activity, while the second forms part of the active site and is directly involved in the catalytic reaction. The second class of enzymes include those where the Mg2+ is complexed to nucleotide di- and tri-phosphates (ADP and ATP) and the chemical change involves phosphoryl transfer. Sony VAIO VGN-TZ37N/G Battery

Mg2+ may also serve in a structural maintenance role in these enzymes (e.g. enolase).

Magnesium stress

Plant stress responses can be observed in plants that are under or over supplied with Mg2+. Sony VAIO VGN-TZ37N/P Battery

The first observable signs of Mg2+ stress in plants for both starvation and toxicity is a depression of the rate of photosynthesis, presumably because of the strong relationships between Mg2+ and chloroplasts/chlorophyll. In pine trees, even before the visible appearance of yellowing and necrotic spots, the photosynthetic efficiency of the needles drops markedly.Sony VAIO VGN-TZ37N/R Battery

In Mg2+ deficiency, reported secondary effects include carbohydrate immobility, loss of RNA transcription and loss of protein synthesis. However, due to the mobility of Mg2+ within the plant, the deficiency phenotype may be present only in the older parts of the plant. Sony VAIO VGN-TZ37N/X Battery

For example, in Pinus radiata starved of Mg2+ one of the earliest identifying signs is the chlorosis in the needles on the lower branches of the tree. This is because Mg2+ has been recovered from these tissues and moved to growing (green) needles higher in the tree.Sony VAIO VGN-TZ38N/X Battery

A Mg2+ deficit can be caused by the lack of the ion in the media (soil), but more commonly comes from inhibition of its uptake. Mg2+binds quite weakly to the negatively charged groups in the root cell walls, so that excesses of other cations such as K+, NH4+, Ca2+and Mn2+ can all impede uptake.(Kurvits and Kirkby, 1980;Sony VAIO VGN-TZ398U/XC Battery

In acid soils Al3+ is a particularly strong inhibitor of Mg2+ uptake. The inhibition by Al3+ and Mn2+ is more severe than can be explained by simple displacement, hence it is possible that these ions bind to the Mg2+ uptake system directly.In bacteria and yeast, such binding by Mn2+ has already been observed. Sony VAIO VGN-TZ50B Battery

Stress responses in the plant develop as cellular processes halt due to a lack of Mg2+ (e.g. maintenance of ?pH across the plasma and vacuole membranes). Interestingly, in Mg2+-starved plants under low light conditions the percentage of Mg2+ bound to chlorophyll has been recorded at 50%. Presumably, this imbalance has detrimental effects on other cellular processes.Sony VAIO VGN-TZ90HS Battery

Mg2+ toxicity stress is more difficult to develop. When Mg2+ is plentiful the plants generally take up the ion and store it (Stelzer et al., 1990). However, if this is followed by drought then ionic concentrations within the cell can increase dramatically.Sony VAIO VGN-TZ90NS Battery

High cytoplasmic Mg2+ concentrations block a K+ channel in the inner envelope membrane of the chloroplast, in turn inhibiting the removal of H+ ions from the chloroplast stroma. This leads to an acidification of the stroma that inactivates key enzymes in carbon fixation, which all leads to the production of oxygen free radicals in the chloroplast that then cause oxidative damage.Sony VAIO VGN-TZ90S Battery

Magnesium transporters are proteins that transport magnesium across the cell membrane. All forms of life require magnesium, yet the molecular mechanisms of Mg2+uptake from the environment and the distribution of this vital element within the organism are only slowly being elucidated. Sony VAIO VPC CW2MFX/PU Battery

In bacteria, Mg2+ is probably mainly supplied by the CorA protein[1] and, where the CorA protein is absent, by the MgtE protein.In yeast the initial uptake is via the Alr1p and Alr2p proteins,[4]but at this stage the only internal Mg2+ distributing protein identified is Mrs2p.[5] Within the protozoa only one Mg2+ transporter (XntAp) has been identified.Sony VAIO VPC S11V9E/B Battery

In metazoa, Mrs2p[7] and MgtE homologues[8] have been identified, along with two novel Mg2+ transport systems TRPM6/TRPM7[9][10] and PCLN-1.[11] Finally, in plants, a family of Mrs2p homologues has been identified[12][13] along with another novel protein, AtMHX.Sony VAIO VPCB119GJ/B Battery

The evolution of Mg2+ transport appears to have been rather complicated. Proteins apparently based on MgtE are present in bacteria and metazoa, but are missing in fungi and plants, whilst proteins apparently related to CorA are present in all of these groups. Sony VAIO VPCB11AGJ Battery

The two active transport transporters present in bacteria, MgtA and MgtB, do not appear to have any homologies in higher organisms. There are also Mg2+ transport systems that are found only in the higher organisms.

There are a large number of proteins yet to be identified that transport Mg2+.Sony VAIO VPCB11AVJ Battery

Even in the best studied eukaryote, yeast, Borrelly[15] has reported a Mg2+/H+exchanger without an associated protein, which is probably localised to the Golgi. At least one other major Mg2+ transporter in yeast is still unaccounted for, the one affecting Mg2+transport in and out of the yeast vacuole. Sony VAIO VPCB11V9E Battery

In higher, multicellular organisms, it seems that many Mg2+ transporting proteins await discovery.

The CorA-domain-containing Mg2+ transporters (CorA, Alr-like and Mrs2-like) have a similar but not identical array of affinities for divalent cations. In fact, this observation can be extended to all of the Mg2+ transporters identified so far.Sony VAIO VPCB11X9E Battery

This similarity suggests that the basic properties of Mg2+ strongly influence the possible mechanisms of recognition and transport. However, this observation also suggests that using other metal ions as tracers for Mg2+ uptake will not necessarily produce results comparable to the transporter’s ability to transport Mg2+. Ideally, Mg2+ should be measured directly.Sony VAIO VPCCW18FJ/P Battery

Since 28Mg2+ is practically unobtainable, much of the old data will need to be reinterpreted with new tools for measuring Mg2+transport, if different transporters are to be compared directly. The pioneering work of Kolisek[17] and Froschauer[18] using mag-fura 2 has shown that free Mg2+ can be reliably measured in vivo in some systems. Sony VAIO VPCCW18FJ/R Battery

By returning to the analysis of CorA with this new tool, we have gained an important baseline for the analysis of new Mg2+ transport systems as they are discovered. However, it is important that the amount of transporter present in the membrane is accurately determined if comparisons of transport capability are to be made. Sony VAIO VPCCW18FJ/W Battery

This bacterial system might also be able to provide some utility for the analysis of eukaryotic Mg2+ transport proteins, but differences in biological systems of prokaryotes and eukaryotes will have to be considered in any experiment.Sony VAIO VPCCW19FJ/W Battery

Comparing the functions of the characterised Mg2+ transport proteins is currently almost impossible, even though the proteins have been investigated in different biological systems using different methodologies and technologies. Finding a system where all the proteins can be compared directly would be a major advance. Sony VAIO VPCCW1AFJ Battery

If the proteins could be shown to be functional in bacteria (S. typhimurium), then a combination of the techniques of mag-fura 2, quantification of protein in the envelope membrane, and structure of the proteins (X-ray crystal or cryo-TEM) might allow the determination of the basic mechanisms involved in the recognition and transport of the Mg2+ ion.Sony VAIO VPCCW1AHJ Battery

However, perhaps the best advance would be the development of methods allowing the measurement of the protein’s function in the patch-clamp system using artificial membranes.

Early research

In 1968, Lusk described the limitation of bacterial (Escherichia coli) growth on Mg2+-poor media, suggesting that bacteria required Mg2+ and were likely to actively take this ion from the environment. Sony VAIO VPCCW1S1E Battery

The following year, the same group and another group, Silver, independently described the uptake and efflux of Mg2+ in metabolically active E. coli cells using 28Mg2+. By the end of 1971, two papers had been published describing the interference of Co2+, Ni2+ and Mn2+ on the transport of Mg2+ in E. coli and in Aerobacter aerogenes and Bacillus megaterium.Sony VAIO VPCCW1S1E/B Battery

In the last major development before the cloning of the genes encoding the transporters, it was discovered that there was a second Mg2+ uptake system that showed similar affinity and transport kinetics to the first system, but had a different range of sensitivities to interfering cations. This system was also repressible by high extracellular concentrations of Mg2+ .Sony VAIO VPCCW1S1E/L Battery

CorA

The CorA gene and its corresponding protein are the most exhaustively studied Mg2+ transport system in any organism. Most of the published literature on the CorA gene comes from the laboratory of M. E. Maguire. Recently the group of R. J. Schweyen made a significant impact on the understanding of Mg2+ transport by CorA. Sony VAIO VPCCW1S1E/P Battery

The gene was originally named after the cobalt-resistant phenotype in E. coli that was caused by the gene’s inactivation.

The gene was genetically identified in E. coli by Park et al., but wasn’t cloned until Hmiel et al. isolated the Salmonella entericaserovar Typhimurium (S. typhimurium) homologue. Sony VAIO VPCCW1S1E/R Battery

Later it would be shown by Smith and Maguire that the CorA gene was present in 17 gram-negative bacteria. With the large number of complete genome sequences now available for prokaryotes, CorA has been shown to be virtually ubiquitous among the Eubacteria, as well as being widely distributed among the Archaea.Sony VAIO VPCCW1S1E/W Battery

The CorA locus in E. coli contains a single open reading frame of 948 nucleotides, producing a protein of 316 amino acids. This protein is well conserved amongst the Eubacteria and Archaea. Between E. coli and S. typhimurium, the proteins are 98% identical, but in more distantly related species, the similarity falls to between 15 and 20%.Sony VAIO VPCCW21FX/B Battery

In the more distantly related genes, the similarity is often restricted to the C-terminal part of the protein, and a short amino acid motif GMN within this region is very highly conserved. The CorA domain, also known as PF01544 in the pFAM conserved protein domain database, is additionally present in a wide range of higher organisms, and these transporters will be reviewed below.Sony VAIO VPCCW21FX/L Battery

The CorA gene is constitutively expressed in S. typhimurium under a wide range of external Mg2+ concentrations.However, recent evidence suggests that the activity of the protein may be regulated by the PhoPQ two-component regulatory system. This sensor responds to low external Mg2+ concentrations during the infection process of S. typhimurium in humans.Sony VAIO VPCCW21FX/R Battery

In low external Mg2+ conditions, the PhoPQ system was reported to suppress the function of CorA and it has been previously shown that the transcription of the alternative Mg2+ transporters MgtA and MgtB is activated in these conditions. hamnongpol and Groisman suggest that this allows the bacteria to escape metal ion toxicity caused by the transport of other ions, Sony VAIO VPCCW21FX/W Battery

particularly Fe(II), by CorA in the absence of Mg2+. Papp and Maguire offer a conflicting report on the source of the toxicity.

The figure (not to scale) shows the originally published transmembrane (TM) domain topology of the S. typhimurium CorA protein, which was said to have three membrane-spanning regions in the C-terminal part of the protein (shown in blue),Sony VAIO VPCCW26EC Battery

as determined by Smith et al. Evidence for CorA acting as a homotetramer was published by Warren et al. in 2004.In December 2005 the crystal structure of the CorA channel was posted to the RSCB protein structure database. The results showed that the protein has two TM domains and exists as a homopentamer, in direct conflict with the earlier reports.Sony VAIO VPCCW26FX/B Battery

Follow this link to see the structure in 3D. The soluble intracellular parts of the protein are highly charged, containing 31 positively charged and 53 negatively charged residues. Conversely, the TM domains contain only one charged amino acid, which has been shown to be unimportant in the activity of the transporter.Sony VAIO VPCCW28EC Battery

From mutagenesis experiments, it appears that the chemistry of the Mg2+ transport relies on the hydroxyl groups lining the inside of the transport pore; there is also an absolute requirement for the GMN motif (shown in red).

Before the activity of CorA could be studied in vivo, any other Mg2+ transport systems in the bacterial host had to be identified and inactivated or deleted (see below). Sony VAIO VPCCW28FJ/P Battery

A strain of S. typhimurium containing a functional CorA gene but lacking MgtA and MgtB was constructed (also see below), and the uptake kinetics of the transporter were analysed. This strain showed nearly normal growth rates on standard media (50 ?M Mg2+), but the removal of all three genes created a bacterial strain requiring 100 mM external Mg2+ for normal growth.Sony VAIO VPCCW28FJ/R Battery

Mg2+ is transported into cells containing only the CorA transport system with similar kinetics and cation sensitivities as the Mg2+uptake described in the earlier papers, and has additionally been quantified (see table). The uptake of Mg2+ was seen to plateau as in earlier studies, and although no actual mechanism for the decrease in transport has been determined, Sony VAIO VPCCW28FJ/W Battery

so it has been assumed that the protein is inactivated. Co2+ and Ni2+ are toxic to S. typhimurium cells containing a functional CorA protein and this toxicity stems from the blocking of Mg2+ uptake (competitive inhibition) and the accumulation of these ions inside the cell.Sony VAIO VPCCW29FJ/W Battery

Co2+and Ni2+ have been shown to be transported by CorA by using radioactive tracer analysis, although with lower affinities (km) and velocities (Vmax) than for Mg2+ (see table). The km values for Co2+ and Ni2+ are significantly above those expected to be encountered by the cells in their normal environment, Sony VAIO VPCCW2AFJ Battery

so it is unlikely that the CorA transport system mediates the uptake of these ions under natural conditions. To date, the evidence for Mn2+ transport by CorA is limited to E. coli.

The table lists the transport kinetics of the CorA Mg2+ transport system. This table has been compiled from the publications of Snavely et al. (1989b), Gibson et al. (1991) and Smith et al. (1998a)Sony VAIO VPCCW2AHJ Battery

and summarises the kinetic data for the CorA transport protein expressed from the wild type promoter in bacteria lacking MgtA and MgtB. km and Vmax were determined at 20 °C as the uptake of Mg2+ at 37 °C was too rapid to measure accurately.

Recently the Mg2+-dependent fluorescence of mag-fura 2 was used to measure the free Mg2+ content of S. typhimurium cells in response to external Mg2+, which showed that CorA is the major uptake system for Mg2+ in bacteria.Sony VAIO VPCCW2S1E Battery

The authors also showed for the first time that the changes in the electric potential (??) across the plasma membrane of the cell affected both the rate of Mg2+ uptake and the free Mg2+ content of the cell; depolarisation suppressed transport, while hyperpolarisation increased transport. Sony VAIO VPCCW2S1E/B Battery

The kinetics of transport were defined only by the rate of change of free Mg2+ inside the cells (250 ?M s?1). Because no quantification of the amount of CorA protein in the membrane was made, this value is cannot be compared with other experiments on Mg2+transporters.Sony VAIO VPCCW2S1E/L Battery

The efflux of Mg2+ from bacterial cells was first observed by Lusk and Kennedy (1969) and is mediated by the CorA Mg2+ transport system in the presence of high extracellular concentrations of Mg2+. The efflux can also be triggered by Co2+, Mn2+ and Ni2+, although not to the same degree as Mg2+.Sony VAIO VPCCW2S1E/P Battery

No Co2+ efflux through the CorA transport system was observed. The process of Mg2+ efflux additionally requires one of the CorB, CorC or CorD genes. Interestingly, the mutation of any single one of these genes leads to a Co2+ resistance a little less than half of that provided by a CorA mutant. Sony VAIO VPCCW2S1E/R Battery

This effect may be due to the inhibition of Mg2+ loss that would otherwise occur in the presence of high levels of Co2+. It is currently unknown whether Mg2+ is more toxic when the CorBCD genes are deleted.

It has been speculated that the Mg2+ ion will initially interact with any transport protein through its hydration shell.Sony VAIO VPCCW2S1E/W Battery

Cobalt (III) hexaammine, Co(III)Hex, is a covalently bound (non-labile) analog for the first shell of hydration for several divalent cations, including Mg2+. The radius of the Co(III)Hex molecule is 244 pm, very similar to the 250 pm radius of the first hydration shell of Mg2+. Sony VAIO VPCCW2S5C CN1 Battery

This analog is a potent inhibitor of the CorA transport system, more so than Mg2+, Co2+ or Ni2+.[40] The additional strength of the Co(III)Hex inhibition might come from the blocking of the transport pore due to the inability of the protein to ‘dehydrate’ the substrate. Sony VAIO VPCF112FX/B Battery

It was also shown that Co(III)Hex was not transported into the cells,suggesting that at least partial dehydration would be required for the transport of the normal substrate (Mg2+). Nickel (II) hexaammine, with a radius of 255 pm, did not inhibit the CorA transport system, suggesting a maximum size limit exists for the binding of the CorA substrate ion.Sony VAIO VPCF115FG/B Battery

These results suggest that the important property involved in the recognition of Mg2+ by CorA is the size of the ion with its first shell of hydration. Hence, the volume change generally quoted for the bare to hydrated Mg2+ ion of greater than 500-fold, including the second sphere of hydration,Sony VAIO VPCF116FGBI Battery

may not be biologically relevant, and may be a reason for the first sphere volume change of 56-fold to be more commonly used.

MgtA and MgtB

The presence of these two genes was first suspected when Nelson and Kennedy (1972) showed that there were Mg2+-repressible and non-repressible Mg2+ uptake systems in E. coli. Sony VAIO VPCF117FJ/W Battery

The non-repressible uptake of Mg2+ is mediated by the CorA protein. In S. typhimurium the repressible Mg2+ uptake was eventually shown to be via the MgtA and MgtB proteins.

Both MgtA and MgtB are regulated by the PhoPQ system and are actively transcribed during the process of infection of human patients by S. typhimurium.Sony VAIO VPCF117HG/BI Battery

Although neither gene is required for pathogenicity, the MgtB protein does enhance the long term survival of the pathogen in the cell. The genes are also upregulated in vitro when the Mg2+ concentration falls below 50 ?M (Snavely et al., 1991a). Although the proteins have km values similar to CorA and transport rates approximately 10 times less, Sony VAIO VPCF118FJ/W Battery

the genes may be part of a Mg2+ scavenging system. Chamnongpol and Groisman (2002) presents evidence that the role of these proteins may be to compensate for the inactivation of the CorA protein by the PhoPQ regulon. The authors suggest that the CorA protein is inactivated to allow the avoidance of metal toxicity via the protein in the low Mg2+ environments S. typhimurium is subjected to by cells after infection.Sony VAIO VPCF119FC Battery

The proteins are both P-type ATPases and neither gene shows any similarity to CorA. The MgtA and MgtB proteins are 75% similar (50% identical), although it seems that MgtB may have been acquired by horizontal gene transfer as part of Salmonella Pathogenicity Island 3.Sony VAIO VPCF119FC/BI Battery

The TM topology of the MgtB protein has been experimentally determined, showing that the protein has ten TM-spanning helices with the termini of the protein in the cytoplasm (see figure ). MgtA is present in widely divergent bacteria, but is not nearly as common as CorA, while MgtB appears to have a quite restricted distribution.Sony VAIO VPCF119FJ/BI Battery

No hypotheses for the unusual distribution have been suggested.

The figure, adapted from Smith et al. (1993b), shows the experimentally determined membrane topology of the MgtB protein in S. typhimurium. The TM domains are shown in light blue and the orientation in the membrane and the positions of the N- and C-termini are indicated. The figure is not drawn to scale.Sony VAIO VPCF11AFJ Battery

While the MgtA and MgtB proteins are very similar, they do show some minor differences in activity. MgtB is very sensitive to temperature, losing all activity (with regard to Mg2+transport) at a temperature of 20°C. Additionally, MgtB and MgtA are inhibited by different ranges of cations (Table A10.1 ).Sony VAIO VPCF11AGJ Battery

The table lists cation transport characteristics of the MgtA and MgtB proteins in S. typhimurium as well as the kinetic data for the MgtA and MgtB transport proteins at 37 °C. The Vmax numbers listed in parentheses are those for uptake at 20 °C. The inhibition of Mg2+ transport by Mn2+ via MgtA showed unusual kinetics (see Figure 1 of Snavely et al., 1989b )Sony VAIO VPCF11AHJ Battery

The MgtA and MgtB proteins are ATPases, using one molecule of ATP per transport cycle, whereas the Mg2+ uptake via CorA is simply electrochemically favourable. Chamnongpol and Groisman (2002) have suggested that the MgtA and MgtB proteins form part of a metal toxicity avoidance system.Sony VAIO VPCF11JFX/B Battery

Alternatively, as most P-type ATPases function as efflux mediating transporters, it has been suggested that the MgtA and MgtB proteins act as efflux proteins for a currently unidentified cation, and Mg2+ transport is either non-specific or exchanged to maintain the electro-neutrality of the transport process.Sony VAIO VPCF11M1E Battery

Further experiments will be required to define the physiological function of these proteins.

MgtE

Two papers describe MgtE, a fourth Mg2+ uptake protein in bacteria unrelated to MgtA/B or CorA. Sony VAIO VPCF11M1E/H Battery

This gene has been sequenced and the protein, 312 amino acids in size, is predicted to contain either four or five TM spanning domains that are closely arranged in the C-terminal part of the protein (see figure). This region of the protein has been identified in thePfam database as a conserved protein domain Sony VAIO VPCF11MFX/B Battery

(PF01769) and species containing proteins that have this protein domain are roughly equally distributed throughout the Eubacteria and Archaea, although it is quite rare in comparison with the distribution of CorA. However, the diversity of the proteins containing the domain is significantly larger than that of the CorA domain. Sony VAIO VPCF11S1E Battery

The Pfam database lists seven distinct groups of MgtE domain containing proteins, of which six contain an archaic or eubacterial member. The expression of MgtE is frequently controlled by a conserved RNA structure, YkoK leader or M-box.

The figure (right), adapted from Smith et al. (1995) and the PFAM database entry, shows the computer-predicted membrane topology of the MgtE protein in Bacillus firmus OF4. Sony VAIO VPCF11S1E/B Battery

The TM domains are shown in light blue. The CBS domains, named for the protein they were identified in, cystathionine-beta synthase, shown in orange, are identified in the Pfam database as regulatory domains, but the mechanism of action has not yet been described. They are found in several voltage-gated chloride channels.Sony VAIO VPCF11Z1E Battery

The orientation in the membrane and the positions of the N- and C-termini are indicated. This figure is not drawn to scale. This transporter has recently had its structure solved by x-ray crystallography. Published in Nature

The MgtE gene was first identified by Smith et al. (1995) during a screen for CorA-like proteins in bacteria and complements the Mg2+-uptake-deficient S. typhimurium strain MM281 (corA mgtA mgtB), Sony VAIO VPCF11Z1E/BI Battery

restoring wild type growth on standard media. The kinetics of Mg2+ transport for the protein were not determined, as 28Mg2+ was unavailable. As a substitute, the uptake of 57Co2+ was measured and was shown to have a km of 82 ?M and a Vmax of 354 pmol min?1 108 cells?1. Sony VAIO VPCF11ZHJ Battery

Mg2+ was a competitive inhibitor with a Ki of 50 ?M — the Ki of Mg2+ inhibition of 60Co2+ uptake via CorA is 10 ?M. A comparison of the available kinetic data for MgtA and CorA is shown in the table. Clearly, MgtE does not transport Co2+ to the same degree as CorA, and the inhibition of transport by Mg2+ is also less efficient, Sony VAIO VPCF11ZHJ Battery

which suggests that the affinity of MgtE for Mg2+ is lower than that of CorA. The strongest inhibitor of Co2+ uptake was Zn2+, with a Ki of 20 ?M.[3] The transport of Zn2+ by this protein may be as important as that of Mg2+.

The table shows a comparison of the transport kinetics of MgtE and CorA, and key kinetic parameter values for them are listed. Sony VAIO VPCF127HGBI Battery

As shown, the data has been generated at differing incubation temperatures. Km and Ki are not significantly altered by the differing incubation temperature. Conversely, Vmax shows a strong positive correlation with temperature, hence the value of Co2+ Vmax for MgtE is not directly comparable with the values for CorA.Sony VAIO VPCF137HG/BI Battery

Early research

The earliest research showing that yeast takes up Mg2+ appears to be done by Schmidt et al. (1949). However, these authors only showed altered yeast Mg2+ content in a table within the paper, and the report’s conclusions dealt entirely with the metabolism of phosphate. Sony VAIO VPCS111FM/S Battery

A series of experiments by Rothstein shifted the focus more towards the uptake of the metal cations, showing that yeast take up cations with the following affinity series; Mg2+, Co2+, Zn2+ > Mn2+ > Ni2+ > Ca2+ > Sr2+. Additionally, it was suggested that the transport of the different cations is mediated by the same transport system — a situation very much like that in bacteria.Sony VAIO VPCS115EC Battery

In 1998, MacDiarmid and Gardner finally identified the proteins responsible for the observed cation transport phenotype in Saccharomyces cerevisiae.[4] The genes involved in this system and a second mitochondrial Mg2+ transport system, functionally identified significantly after the gene was cloned, are described in the sections below.Sony VAIO VPCS115FG Battery

ALR1 and ALR2

Two genes, ALR1 and ALR2, were isolated in a screen for Al3+ tolerance (resistance) in yeast.[4] Over-expression constructs containing yeast genomic DNA were introduced into wild type yeast and the transformants were screened for growth on toxic levels of Al3+. ALR1 and ALR2 containing plasmids allowed the growth of yeast in these conditions.Sony VAIO VPCS117GG Battery

The Alr1p and Alr2p proteins consist of 859 and 858 amino acids respectively and are 70% identical. In a region in the C-terminal, half of these proteins are weakly similar to the full CorA protein. The computer-predicted TM topology of Alr1p is shown in the figure.Sony VAIO VPCS117GGB Battery

The presence of a third TM domain was suggested by MacDiarmid and Gardner (1998), on the strength on sequence homology, and more recently by Lee and Gardner (2006), on the strength of mutagenesis studies, making the TM topology of these proteins more like that of CorA (see figure). Sony VAIO VPCS118EC Battery

Also, Alr1p contains the conserved GMN motif at the outside end of TM 2 (TM 2') and the mutation of the methionine (M) in this motif to a leucine (L) led to the loss of transport capability.

The figure shows the two possible TM topologies of Alr1p. Sony VAIO VPCS119FJ/B Battery

Part A of the figure shows the computer-predicted membrane topology of the Alr1p protein in yeast and part B shows the topology of Alr1p based on the experimental results of Lee and Gardner (2006).[56] The GMN motif location is indicated in red and the TM domains in light blue. Sony VAIO VPCS119GC Battery

The orientation in the membrane and the positions of the N- and C-termini are indicated, the various sizes of the soluble domains are given in amino acids (AA), and TM domains are numbered by their similarity to CorA. Where any TM domain is missing, the remaining domains are numbered with primes. Sony VAIO VPCS11AFJ Battery

The figure is not drawn to scale. A third ALR-like gene is present in S. cerevisiae and there are two homologous genes in both Schizosaccharomyces pombe andNeurospora crassa. These proteins contain a GMN motif like that of CorA, with the exception of the second N. crassa gene. No ALR-like genes have been identified in species outside of the fungi.Sony VAIO VPCS11AGJ Battery

Membrane fractionation and green fluorescent protein (GFP) fusion studies established that Alr1p is localised to the plasma membrane. The localisation of the Alr1p was observed to be internalised and degraded in the vacuole in response to extracellular cations.Sony VAIO VPCS11AHJ Battery

Mg2+, at very low extracellular concentrations (100 ?M; < 10% of the standard media Mg2+ content), and Co2+ and Mn2+ at relatively high concentrations (> 20× standard media), induced the change in Alr1p protein localisation, and the effect was dependent on functional ubiquitination, endocytosis and vacuolar degradation.Sony VAIO VPCS11AVJ Battery

This mechanism was proposed to allow the regulation of Mg2+ uptake by yeast. However a recent report indicates that several of the observations made by Stadler et al. were not reproducible.For example, regulation of ALR1 mRNA accumulation by Mg2+ supply was not observed, and the stability of the Alr1 protein was not reduced by exposure to excess Mg2+.Sony VAIO VPCS11J7E/B Battery

The original observation of Mg-dependent accumulation of the Alr1 protein under steady-state low-Mg conditions was replicated, but this effect was shown to be an artifact caused by the addition of a small peptide (epitope) to the protein to allow its detection. Despite these problems, Sony VAIO VPCS11M1E/W Battery

Alr1 activity was demonstrated to respond to Mg supply,suggesting that the activity of the protein is regulated directly, as was observed for some bacterial CorA proteins.

A functional Alr1p (wild type) or Alr2p (overexpressed) is required for S. cerevisiae growth in standard conditions (4 mM Mg2+[4]), and Alr1p can support normal growth at Mg2+ concentrations as low as 30 ?M.Sony VAIO VPCS11V9E Battery

57Co2+ is taken up into yeast via the Alr1p protein with a km of 77 – 105 ?M (; C. MacDiarmid and R. C. Gardner, unpublished data), but the Ki for Mg2+ inhibition of this transport is currently unknown. The transport of other cations by the Alr1p protein was assayed by the inhibition of yeast growth. Sony VAIO VPCS11V9E/B Battery

The overexpression of Alr1p led to increased sensitivity to Ca2+, Co2+, Cu2+, La3+, Mn2+, Ni2+ and Zn2+, an array of cations similar to those shown to be transported into yeast by a CorA-like transport system.The increased toxicity of the cations in the presence of the transporter is assumed to be due to the increased accumulation of the cation inside the cell.Sony VAIO VPCS11X9E/B Battery

The evidence that Alr1p is primarily a Mg2+ transporter is that the loss of Alr1p leads to a decreased total cell content of Mg2+, but not of other cations. Additionally, two electrophysiological studies where Alr1p was produced in yeast or Xenopus oocytes showed a Mg2+-dependent current in the presence of the protein :Salih et al., in prep.).Sony VAIO VPCS123FGB Battery

The kinetics of Mg2+ uptake by Alr1p have been investigated by electrophysiology techniques on whole yeast cells.[59] The results suggested that Alr1p is very likely to act as an ion-selective channel. In the same paper, the authors reported that Mg2+ transport by Alr1p varied from 200 pA to 1500 pA, with a mean current of 264 pA. Sony VAIO VPCS125EC Battery

No quantification of the amount of protein producing the current was presented, so the results lack comparability with the bacterial Mg2+ transport proteins.

The alternative techniques of 28Mg2+ radiotracer analysis and mag-fura 2 to measure Mg2+ uptake have not yet been used with Alr1p. Sony VAIO VPCS128EC Battery

28Mg2+ is currently not available and the mag-fura 2 system is unlikely to provide simple uptake data in yeast. The yeast cell maintains a heterogeneous distribution of Mg2+[60] suggesting that multiple systems inside the yeast are transporting Mg2+ into storage compartments. Sony VAIO VPCS129GC Battery

This internal transport will very likely mask the uptake process. The expression of ALR1 in S. typhimurium without Mg2+ uptake genes may be an alternative, but, as stated earlier, the effects of a heterologous expression system would need to be taken into account.Sony VAIO VPCS12C7E/B Battery

MNR2

The MNR2 gene encodes a protein closely related to the Alr proteins, but includes conserved features that define a distinct subgroup of CorA proteins in fungal genomes, suggesting a distinct role in Mg2+ homeostasis. Sony VAIO VPCS12L9E/B Battery

Like an alr1 mutant, growth of an mnr2 mutant was sensitive to Mg2+-deficient conditions, but the mnr2 mutant was observed to accumulate more Mg2+ than a wild-type strain under these conditions. These phenotypes suggested that Mnr2 may regulate Mg2+storage within an intracellular compartment.Sony VAIO VPCS12V9E/B Battery

Consistent with this interpretation, the Mnr2 protein was localized to the membrane of the vacuole, an internal compartment implicated in the storage of excess mineral nutrients by yeast. A direct role of Mnr2 in Mg2+ transport was suggested by the observation that increased Mnr2 expression, Sony VAIO VPCY115FGS Battery

which redirected some Mnr2 protein to the cell surface, also suppressed the Mg2+-requirement of an alr1 alr2 double mutant strain. Interestingly, the mnr2 mutation also altered accumulation of other divalent cations, suggesting this mutation may increase Alr gene expression or protein activity. Sony VAIO VPCY115FX/BI Battery

Recent work supported this model, by showing that Alr1 activity was increased in an mnr2 mutant strain, and that the mutation was associated with induction of Alr1 activity at a higher external Mg concentration than was observed for an Mnr2 wild-type strain.Sony VAIO VPCY115FXBI Battery

These effects were observed without any change in Alr1 protein accumulation, again indicating that Alr1 activity may be regulated directly by the Mg concentration within the cell.

MRS2 and Lpe10

Like the ALR genes, the MRS2 gene was cloned and sequenced before it was identified as a Mg2+ transporter. Sony VAIO VPCY118EC Battery

The MRS2 gene was identified in the nuclear genome of yeast in a screen for suppressors of a mitochondrial gene RNA splicing mutation,[62] and was cloned and sequenced by Wiesenberger et al. (1992). Mrs2p was not identified as a putative Mg2+ transporter until Bui et al. (1999).Sony VAIO VPCY118GX/BI Battery

Gregan et al. (2001a) identified LPE10 by homology to MRS2 and showed that both LPE10 and MRS2 mutants altered the Mg2+content of yeast mitochondria and affected RNA splicing activity in the organelle. Mg2+ transport has been shown to be directly mediated by Mrs2p,[17] but not for Lpe10p.Sony VAIO VPCY119FJ/S Battery

The Mrs2p and Lpe10p proteins are 470 and 413 amino acid residues in size, respectively, and a 250–300 amino acid region in the middle of the proteins shows a weak similarity to the full CorA protein. The TM topologies of the Mrs2p and Lpe10p proteins have been assessed using a protease protection assay and are shown in the figure. Sony VAIO VPCY11AFJ Battery

TM 1 and 2 correspond to TM 2 and 3 in the CorA protein. The conserved GMN motif is at the outside end of the first TM domain, and when the glycine (G) in this motif was mutated to a cysteine (C) in Mrs2p, Mg2+ transport was strongly reduced.

The figure shows the experimentally determined topology of Mrs2p and Lpe10p as adapted from Bui et al.(1999) and Gregan et al.Sony VAIO VPCY11AGJ Battery

(2001a).[64] The GMN motif location is indicated in red and the TM domains in light blue. The orientation in the membrane and the positions of the N- and C-termini are indicated. The various sizes of the soluble domains are given in amino acids (AA), TM domains are numbered, and the figure is not drawn to scale.Sony VAIO VPCY11AHJ Battery

Mrs2p has been localised to the mitochondrial inner membrane by subcellular fractionation and immunodetection[5] and Lpe10p to the mitochondria.[64] Mitochondria lacking Mrs2p do not show a fast Mg2+uptake, only a slow ‘leak’, and overaccumulation of Mrs2p leads to an increase in the initial rate of uptake.Sony VAIO VPCY11AVJ Battery

Additionally, CorA, when fused to the mitochondrial leader sequence of Mrs2p, can partially complement the mitochondrial defect conferred by the loss of either Mrs2p or Lpe10p. Hence, Mrs2p and/or Lpe10p may be the major Mg2+ uptake system for mitochondria. Sony VAIO VPCY11M1E/S Battery

A possibility is that the proteins form heterodimers, as neither protein (when overexpressed) can fully complement the loss of the other.

The characteristics of Mg2+ uptake in isolated mitochondria by Mrs2p were quantified using mag-fura 2. The uptake of Mg2+ by Mrs2p shared a number of attributes with CorA.Sony VAIO VPCY11S1E Battery

First, Mg2+ uptake was directly dependent on the electric potential (??) across the boundary membrane. Second, the uptake is saturated far below that which the ?? theoretically permits, so the transport of Mg2+ by Mrs2p is likely to be regulated in a similar manner to CorA, possibly by the inactivation of the protein. Sony VAIO VPCY11V9E/S Battery

Third, Mg2+ efflux was observed via Mrs2p upon the artificial depolarisation of the mitochondrial membrane by valinomycin. Finally, the Mg2+ fluxes through Mrs2p are inhibited by cobalt (III) hexaammine.

The kinetics of Mg2+ uptake by Mrs2p were determined in the Froschauer et al. (2004) paper on CorA in bacteria.Sony VAIO VPCY218EC/BI Battery

The initial change in free Mg2+ concentration was 150 ?M s-1 for wild type and 750 ?M s-1 for mitochondria from yeast overexpressing MRS2. No attempt was made to scale the observed transport to the amount of transporter present.

Protozoan (Paramecium)

The transport of Mg2+ into Paramecium has been characterised largely by R. R. Preston and his coworkers. Sony VAIO VPCY218EC/G Battery

Electrophysiological techniques on whole Paramecium were used to identify and characterise Mg2+ currents in a series of papers before the gene was cloned by Haynes et al. (2002).

The open reading frame for the XNTA gene is 1707 bp in size, contains two introns and produces a predicted protein of 550 amino acids.Sony VAIO VPCY218EC/L Battery

The protein has been predicted to contain 11 TM domains and also contains the ?1 and ?2 motifs (see figure) of the SLC8 (Na+/Ca2+exchanger ) and SLC24 (K+ dependent Na+/Ca2+exchanger ) human solute transport proteins. The XntAp is equally similar to the SLC8 and SLC24 protein families by amino acid sequence, Sony VAIO VPCY218EC/P Battery

but the predicted TM topology is more like that of SLC24, but the similarity is at best weak and the relationship is very distant. The AtMHX protein from plants also shares a distant relationship with the SLC8 proteins.

The figure shows the predicted TM topology of XntAp. Adapted from Haynes et al. (2002), this figure shows the computer predicted membrane topology of XntAp in Paramecium. Sony VAIO VPCY21S1E/L Battery

The orientation in the membrane was determined using HMMTOP. The TM domains are shown in light blue, the ?1 and ?2 domains are shown in green. The orientation in the membrane and the positions of the N- and C-termini are indicated and the figure is not drawn to scale.Sony VAIO VPCY21S1E/P Battery

The Mg2+-dependent currents carried by XntAp are kinetically like that of a channel protein and have an ion selectivity order of Mg2+ > Co2+, Mn2+ > Ca2+ — a series again very similar to that of CorA.[69] Unlike the other transport proteins reported so far, XntAp is dependent on intracellular Ca2+. Sony VAIO VPCY21S1E/SI Battery

The transport is also dependent on ??, but again Mg2+ is not transported to equilibrium, being limited to approximately 0.4 mM free Mg2+ in the cytoplasm. The existence of an intracellular compartment with a much higher free concentration of Mg2+ (8 mM) was supported by the results.Sony VAIO VPCCW2S5C CN1 Battery

Metazoa

The investigation of Mg2+ in animals, including humans, has lagged behind that in bacteria and yeast. This is largely because of the complexity of the systems involved, but also because of the impression within the field that Mg2+ was maintained at high levels in all cells and was unchanged by external influences. Sony VAIO VPCEA20 Battery

Only in the last 25 years has a series of reports begun to challenge this view, with new methodologies finding that free Mg2+ content is maintained at levels where changes might influence cellular metabolism.

MRS2

A bioinformatic search of the sequence databases identified one homologue of the MRS2 gene of yeast in a range of metazoans.Sony VAIO VPCEB10 Battery

The protein has a very similar sequence and predicted TM topology to the yeast protein, and the GMN motif is intact at the end of the first TM domain. The human protein, hsaMrs2p, has been localised to the mitochondrial membrane in mouse cells using a GFP fusion protein.Sony VAIO VPCEB11FM Battery

Very little is known about the Mg2+ transport characteristics of the protein in mammals, but Zsurka et al. (2001) has shown that the human Mrs2p complements the mrs2 mutants in the yeast mitochondrial Mg2+ uptake system.

SLC41 (MgtE)

The identification of this gene family in the metazoa began with a signal sequence trap method for isolating secreted and membrane proteins.Sony VAIO VPCEB11FM/BI Battery

Much of the identification has come from bioinformatic analyses. Three genes were eventually identified in humans, another three in mouse and three in Caenorhabditis elegans, with a single gene in Anopheles gambiae. The pFAM database lists the MgtE domain as pFAM01769 and additionally identifies a MgtE domain-containing protein inDrosophila melanogaster. Sony VAIO VPCEB11FM/BI Battery

The proteins containing the MgtE domain can be divided into seven classes, as defined by pFAM using the type and organisation of the identifiable domains in each protein. Metazoan proteins are present in three of the seven groups. All of the metazoa proteins contain two MgtE domains, but some of these have been predicted only by context recognition (Coin, Bateman and Durbin, unpublished. See the pFAM website for further details).Sony VAIO VPCEB11FM/T Battery

The human SLC41A1 protein contains two MgtE domains with 52% and 46% respective similarity to the PF01769 consensus sequence and is predicted to contain ten TM domains, five in each MgtE domain (see figure), which suggests that the MgtE protein of bacteria may work as a dimer.Sony VAIO VPCEB11FM/WI Battery

Adapted from Wabakken et al. (2003)[8] and the pFAM database, the figure shows the computer predicted membrane topology of MgtE in H. sapiens. The TM domains are shown in light blue, the orientation in the membrane and the positions of the N- and C-termini are indicated, and the figure is not drawn to scale.Sony VAIO VPCEB11FX Battery

Wabakken et al. (2003)[8] found that the transcript of the SLC41A1 gene was expressed in all human tissues tested, but at varying levels, with the heart and testis having the highest expression of the gene. No explanation of the expression pattern has been suggested with regard to Mg2+-related physiology.Sony VAIO VPCEB11FX/BI Battery

It has not been shown whether the SLC41 proteins transport Mg2+ or complement a Mg2+ transport mutation in any experimental system. However, it has been suggested that as MgtE proteins have no other known function, they are likely to be Mg2+ transporters in the metazoa as they are in the bacteria.Sony VAIO VPCEB11FX/T Battery

This will need to be verified using one of the now standard experiment systems for examining Mg2+ transport.

TRPM6/ TRPM7

The investigation of the TRPM genes and proteins in human cells is an area of intense recent study and, at times, debate. Montell et al.(2002)[75] have reviewed the research into the TRP genes, and a second review by Montell (2003)[76] has reviewed the research into the TRPM genes.Sony VAIO VPCEB11FX/WI Battery

The TRPM family of ion channels has members throughout the metazoa. The TRPM6 and TRPM7 proteins are highly unusual, containing both an ion channel domain and a kinase domain (Figure 1.7), the role of which brings about the most heated debate.Sony VAIO VPCEB11GX Battery

The activity of these two proteins has been very difficult to quantify. TRPM7 by itself appears to be a Ca2+ channel[77] but in the presence of TRPM6 the affinity series of transported cations places Mg2+ above Ca2+. The differences in reported conductance were caused by the expression patterns of these genes. Sony VAIO VPCEB11GX/BI Battery

TRPM7 is expressed in all cell types tested so far, while TRPM6 shows a more restricted pattern of expression.[79] An unfortunate choice of experimental system by Voets et al., (2004)[ ] led to the conclusion that TRPM6 is a functional Mg2+ transporter. However, later work by Chubanov et al. (2004) Sony VAIO VPCEB11GX/T Battery

clearly showed that TRPM7 is required for TRPM6 activity and that the results of Voets et al. are explained by the expression of TRPM7 in the experimental cell line used by Voets et al. in their experiments. Whether TRPM6 is functional by itself is yet to be determined.Sony VAIO VPCEB11GX/WI Battery

The predicted TM topology of the TPRM6 and TRPM7 proteins has been adapted from Nadler et al. (2001),[9] Runnels et al.(2001)[81] and Montell et al. (2002),[75] this figure shows the computer predicted membrane topology of the TRPM6 and TRPM7 proteins in Homo sapiens. At this time, the topology shown should be considered a tentative hypothesis. Sony VAIO VPCEB12FX Battery

The TM domains are shown in light blue, the pore loop in purple, the TRP motif in red and the kinase domain in green. The orientation in the membrane and the positions of the N- and C-termini are indicated and the figure is not drawn to scale.

The conclusions of the Voets et al. (2004) paper are probably incorrect in attributing the Mg2+ dependent currents to TRPM7 alone, and their kinetic data are likely to reflect the combined TRPM7/ TRPM6 channel. Sony VAIO VPCEB12FX/BI Battery

The report presents a robust collection of data consistent with a channel-like activity passing Mg2+, based on both electrophysiological techniques and also mag-fura 2 to determine changes in cytoplasmic free Mg2+.

Paracellular transport

Claudins allow for Mg2+ transport via the paracellular pathway; that is, it mediates the transport of the ion through the tight junctions between cells that form an epithelial cell layer.Sony VAIO VPCEB12FX/BIC Battery

\ In particular, Claudin-16 allows the selective reuptake of Mg2+ in the human kidney. Some patients with mutations in the CLDN19 gene also have altered magnesium transport.

The gene Claudin-16 was cloned by Simon et al. (1999), but only after a series of reports described the Mg2+ flux itself with no gene or protein.Sony VAIO VPCEB12FX/T Battery

The expression pattern of the gene was determined by RT-PCR, and was shown to be very tightly confined to a continuous region of the kidney tubule running from the medullary thick descending limb to the distal convoluted tubule. This localisation was consistent with the earlier reports for the location of Mg2+ re-uptake by the kidney. Sony VAIO VPCEB14FX Battery

Following the cloning, mutations in the gene were identified in patients with familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, strengthening the links between the gene and the uptake of Mg2+.

Plants

The current knowledge of the molecular mechanisms for Mg2+ transport in plants is very limited, with only three publications reporting a molecular basis for Mg2+transport in plants.Sony VAIO VPCEB14FX/BI Battery

However, the importance of Mg2+ to plants has been well described, and physiological and ecophysiological studies about the effects of Mg2+ are numerous. This section will summarise the knowledge of a gene family identified in plants that is distantly related to CorA. Another gene, a Mg2+/H+exchanger (AtMHX ), unrelated to this gene family and to CorA has also been identified, is localised to the vacuolar membrane, and will be described last.Sony VAIO VPCEB14FX/T Battery

The AtMRS2 gene family

Schock et al. (2000) identified and named the family AtMRS2 based on the similarity of the genes to the MRS2 gene of yeast. The authors also showed that the AtMRS2-1 gene could complement a ?mrs2 yeast mutant phenotype. Independently, Li et al.Sony VAIO VPCEB14FX/WI Battery

(2001) published a report identifying the family and showing that two additional members could complement Mg2+ transport deficient mutants, one in S. typhimurium and the other in S. cerevisiae.

The three genes that have been shown to transport Mg2+ are AtMRS2-1, AtMRS2-10 and AtMRS2-11, and these genes produce proteins 442, 443 and 459 amino acids in size, respectively.Sony VAIO VPCEB15FM Battery

Each of the proteins shows significant similarity to Mrs2p of yeast and a weak similarity to CorA of bacteria, contains the conserved GMN amino acid motif at the outside end of the first TM domain, and is predicted to have two TM domains.

The AtMRS2-1 gene, when expressed in yeast from the MRS2 promoter and being fused C-terminally to the first 95 amino acids of the Mrs2p protein,Sony VAIO VPCEB15FM/BI Battery

was directed to the mitochondria, where it complemented a ?mrs2 mutant both phenotypically (mitochondrial RNA splicing was restored) and with respect to the Mg2+ content of the organelle. No data on the kinetics of the transport was presented. Sony VAIO VPCEB15FM/T Battery

The AtMRS2-11 gene was analysed in yeast (in the alr1 alr2 strain), where it was shown that expression of the gene significantly increased the rate of Mg2+ uptake into starved cells over the control, as measured using flame atomic absorption spectroscopy of total cellular Mg2+ content. Sony VAIO VPCEB15FM/WI Battery

However, Alr1p was shown to be significantly more effective at transporting Mg2+ at low extracellular concentrations, suggesting that the affinity of AtMRS2-11 for Mg2+ is lower than that of Alr1p. An electrophysiological (voltage clamp) analysis of the AtMRS2-11 protein in Xenopus oocytes also showed a Mg2+-dependent current at membrane potentials (??) of –100 – –150 mV inside.Sony VAIO VPCEB15FX Battery

These values are physiologically significant, as several membranes in plants maintain ?? in this range. However, the author had difficulty reproducing these results due to an apparent "death" of oocytes containing the AtMRS2-11 protein, and therefore these results should be viewed with caution.Sony VAIO VPCEB15FX/BI Battery

The AtMRS2-10 transporter has been analysed using radioactive tracer uptake analysis.[13] 63Ni2+ was used as the substitute ion and Mg2+ was shown to inhibit the uptake of 63Ni2+ with a Ki of 20 ?M. Uptake was also inhibited by Co(III)Hex and by other divalent cations. Only Co2+ and Cu2+ inhibited transport with Ki values less than 1 mM.Sony VAIO VPCEB15FX/T Battery

The AtMRS2-10 protein was fused to GFP, and was shown to be localised to the plasma membrane.[13] A similar experiment was attempted in the Schock et al. (2000) paper, but the observed localisation was not significantly different from that seen with unfused GFP. Sony VAIO VPCEB15FX/WI Battery

The most likely reason for the lack of a definitive localisation of AtMRS2-1 in the Schock et al. paper is that the authors removed the TM domains from the protein, thereby precluding its insertion into a membrane.

The exact physiological significance of the AtMRS2-1 and AtMRS2-10 proteins in plants has yet to be clarified. Sony VAIO VPCEB16FX Battery

The AtMRS2-11 gene has been overexpressed (from the CaMV 35S promoter) in A. thaliana. The transgenic line has been shown to accumulate high levels of the AtMRS2-11 transcript. A strong Mg2+ deficiency phenotype (necrotic spots on the leaves, see Chapter 1.5 below) was recorded during the screening process (in both the T1 and T2 generations)Sony VAIO VPCEB16FX/B Battery

for a homozygote line, but this phenotype was lost in the T3 generation and could not be reproduced when the earlier generations were screened a second time. The author suggested that environmental affects were the most likely cause of the inconsistent phenotype.Sony VAIO VPCEB16FX/G Battery

AtMHX

The first magnesium transporter isolated in any multicellular organism, AtMHX shows no similarity to any previously isolated Mg2+transport protein.[14] The gene was initially identified in the A. thaliana genomic DNA sequence database, by its similarity to the SLC8 family of Na+/Ca2+ exchanger genes in humans.Sony VAIO VPCEB16FX/L Battery

The cDNA sequence of 1990 bp is predicted to produce a 539-amino acid protein. AtMHX is quite closely related to the SLC8 family at the amino acid level and shares a topology with eleven predicted TM domains (Figure A10.5). There is one major difference in the sequence, in that the long non-membranal loop (see Figure A10.5)Sony VAIO VPCEB16FX/P Battery

is 148 amino acids in the AtMHX protein but 500 amino acids in the SLC8 proteins. However, this loop is not well conserved and is not required for transport function in the SLC8 family.[14]

The AtMHX gene is expressed throughout the plant but most strongly in the vascular tissue.Sony VAIO VPCEB16FX/W Battery

The authors suggest that the physiological role of the protein is to store Mg2+ in these tissues for later release when needed. The protein localisation to the vacuolar membrane supports this suggestion (see also Chapter 1.5).

The protein transports Mg2+ into the vacuolar space and H+ out, as demonstrated by electrophysiological techniques.Sony VAIO VPCEB17FX Battery

The transport is driven by the ?pH maintained between the vacuolar space (pH 4.5 – 5.9) and the cytoplasm (pH 7.3 – 7.6) by an H+-ATPase.[90][91] How the transport of Mg2+ by the protein is regulated was not determined. Currents were observed to pass through the protein in both directions,Sony VAIO VPCEB17FX/B Battery

but the Mg2+ out current required a ‘cytoplasmic’ pH of 5.5, a condition not found in plant cells under normal circumstances. In addition to the transport of Mg2+, Shaul et al. (1999)[14] also showed that the protein could transport Zn2+ and Fe2+, but did not report on the capacity of the protein to transport other divalent cations (e.g. Co2+ and Ni2+) or its susceptibility to inhibition by cobalt (III) hexaammine.Sony VAIO VPCEB17FX/G Battery

The detailed kinetics of Mg2+ transport have not been determined for AtMHX. However, physiological effects have been demonstrated. When A. thaliana plants were transformed with overexpression constructs of the AtMHX gene driven by the CaMV 35S promoter,Sony VAIO VPCEB17FX/L Battery

the plants over-accumulated the protein and showed a phenotype of necrotic lesions in the leaves, which the authors suggest is caused by a disruption in the normal function of the vacuole, given their observation that the total Mg2+ (or Zn2+) content of the plants was not altered in the transgenic plants.Sony VAIO VPCEB17FX/P Battery

The image has been adapted from Shaul et al. (1999)[14] and Quednau et al. (2004), and combined with an analysis using HMMTOP, this figure shows the computer predicted membrane topology of the AtMHX protein in Arabidopsis thaliana. At this time the topology shown should be considered a tentative hypothesis. Sony VAIO VPCEB17FX/W Battery

The TM domains are shown in light blue, the orientation in the membrane and the positions of the N- and C-termini are indicated, and the figure is not drawn to scale. The ?1 and ?2 domains, shown in green, are both quite hydrophobic and may both be inserted into the membrane.Sony VAIO VPCEB190X Battery,Sony VAIO VPCEB19FX Battery,Sony VAIO VPCEB19GX Battery,Sony VAIO VPCEB1AFX Battery,Sony VAIO VPCEB1AFX/B Battery