Magnesium in biological systems
From Wikipedia, the free encyclopedia
Magnesium is an essential element in biological systems. Magnesium occurs typically as the Mg2+ ion. It is an essential mineral nutrient for life<ref name="Leroy 1926">Leroy, J. (1926). "Necessite du magnesium pour la croissance de la souris". Comptes Rendus de Seances de la Societe de Biologie 94: 431-433.</ref><ref name="Lusk 1968">Lusk, J.E., Williams, R.J.P., and Kennedy, E.P. (1968). "Magnesium and the growth of Escherichia coli". Journal of Biological Chemistry 243: 2618-2624.</ref><ref name="Marschner 1995">Marschner, H. (1995). Mineral Nutrition in Higher Plants. San Diego: Academic Press.</ref> and is present in every cell type in every organism.
Contents |
[edit] Function
A balance of magnesium is vital to the well being of all organisms. But what is it about magnesium that has led to its status as an indispensable component of cells? Magnesium is a relatively abundant ion in the lithosphere and is highly bioavailable in the hydrosphere. This ready availability, in combination with a useful and very unusual chemistry, may have led to its evolution as an ion for signalling, 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. Biological membranes are impermeable to Mg2+ (and other ions) so transport proteins must facilitate the flow of Mg2+, both into and out of cells and intracellular compartments.
[edit] Biological range, distribution, and regulation
In animals it has been shown that individual cell types maintain differing levels of magnesium.<ref name="Valberg 1965">Valberg, L.S., Holt, J.M., Paulson, E., and Szivek, J. (1965). "Spectrochemical analysis of sodium, potassium, calcium, magnesium, copper, and zinc in normal human erythrocytes". Journal of Clinical Investigation 44: 379–389.</ref><ref name="Seiler 1966">Seiler, R.H., Ramirez, O., Brest, A.N., and Moyer, J.H. (1966). "Serum and erythrocytic magnesium levels in congestive heart failure: effect of hydrochlorothiazide". American Journal of Cardiology 17: 786-791.</ref><ref name="Walser 1967">Walser, M. (1967). "Magnesium metabolism". Ergebnisse der Physiologie Biologischen Chemie und Experimentellen Pharmakologie 59: 185-296.</ref><ref name="Iyengar 1978">Iyengar, G.V., Kollmer, W.E., and Bowen, H.J.M. (1978). The Elemental Composition of Human Tissues and Body Fluids. Weinheim, New York: Verlag Chemie.</ref> and it seems likely that the same is true for plants.<ref name="Stelzer 1990">Stelzer, R., Lehmann, H., Krammer, D., and Luttge, U. (1990). "X-Ray microprobe analysis of vacuoles of spruce needle mesophyll, endodermis and transfusion parenchyma cells at different seasons of the year". Botanica Acta 103: 415-423.</ref><ref name="Shaul 1999">Shaul, O., Hilgemann, D.W., de-Almeida-Engler, J., Van, M.M., Inze, D., and Galili, G. (1999). "Cloning and characterization of a novel Mg2+/H+ exchanger". EMBO Journal 18: 3973-3980.</ref> This suggests that the balance of uptake and efflux of magnesium may be regulated in different ways in different cell types. A delicate balance of internal free magnesium must also be maintained for the correct function of the cell 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<ref name="Thomas 1991">Thomas, R.C., Coles, J.A., and Deitmer, J.W. (1991). "Homeostatic muffling". Nature 350: 564.</ref>). Additionally, in plants, and more recently in animals, it has been recognised that magnesium is an important regulatory signal, both activating and mediating many biochemical reactions. The best example of this is perhaps the regulation of key enzymes involved in the fixation of carbon in chloroplasts.<ref name="Berkowitz 1993">Berkowitz, G.A., Wu, W. (1993). "Magnesium, potassium flux and photosynthesis". Magnesium Research 6: 257-265.</ref><ref name="Shaul 2002">Shaul, O. (2002). "Magnesium transport and function in plants: the tip of the iceberg". BioMetals 15: 309-323.</ref>
As magnesium is important to each individual cell, deficiency will cause disease phenotypes in the affected organism. In single-celled organisms the phenotype is very easy to see: bacteria and yeast both show greatly reduced growth rates when magnesium is limiting. Additionally, when the magnesium uptake systems are deleted genetically from these organisms they must be supplemented with very high external concentrations of magnesium to achieve normal growth rates.<ref name="Hmiel 1989">Hmiel, S.P., Snavely, M.D., Florer, J.B., Maguire, M.E., and Miller, C.G. (1989). "Magnesium transport in Salmonella typhimurium: genetic characterization and cloning of three magnesium transport loci". Journal of Bacteriology 171: 4742–4751.</ref><ref name="MacDiarmid 1998">MacDiarmid, C.W., Gardner, R.C. (1998). "Overexpression of the Saccharomyces cerevisiae magnesium transport system confers resistance to aluminum ion". J. Biol. Chem. 273: 1727-1732.</ref> In yeast, magnesium deficiency in a subcellular compartment, the mitochondria, also leads to a disease state.<ref name="Wiesenberger 1992">Wiesenberger, G., Waldherr, M., and Schweyen, R.J. (1992). "The nuclear gene MRS2 is essential for the excision of group II introns from yeast mitochondrial transcripts in vivo". J. Biol. Chem. 267: 6963-6969.</ref>
Plant stress responses can be observed in plants that are lacking in magnesium. The first observable signs of magnesium stress in plants for both starvation and toxicity is a depression of the rate of photosynthesis, presumably because of the strong relationships between magnesium and chloroplasts/chlorophyll. The later effects of magnesium deficiency on plants are a significant reduction in growth and reproductive viability.<ref name="Marschner 1995"/>
In animals, magnesium deficiency (hypomagnesemia or ‘grass tetany’) is seen in ruminant animals when the environmental availability of magnesium is low, and is identified by a loss of balance due to muscle weakness.<ref name="Grunes 1970">Grunes, D.L., Stout, P.R., and Brownwell, J.R. (1970). "Grass tetany of ruminants". Advances in Agronomy 22: 332-374.</ref> A number of genetically attributable hypomagnesmia disorders have also been identified in humans.<ref name="Paunier 1968">Paunier, L., Radde, I.C., Kooh, S.W., Conen, P.E., and Fraser, D. (1968). "Primary hypomagnesemia with secondary hypocalcemia in an infant". Pediatrics 41: 385-402.</ref><ref name="Weber 2000">Weber, S., Hoffmann, K., Jeck, N., Saar, K., Boeswald, M., Kuwertz-Broeking, E., Meij, I.I.C., Knoers, N.V.A.M., Cochat, P., Sulakova, T., Bonzel, K.E., Soergel, M., Manz, F., Schaerer, K., Seyberth, H.W., Reis, A., and Konrad, M. (2000). "Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis maps to chromosome 3q27 and is associated with mutations in the PCLN-1 gene". European Journal of Human Genetics 8: 414-422.</ref><ref name="Weber 2001">Weber, S., Schneider, L., Peters, M., Misselwitz, J., Roennefarth, G., Boeswald, M., Bonzel, K.E., Seeman, T., Sulakova, T., Kuwertz-Broeking, E., Gregoric, A., Palcoux, J.-B., Tasic, V., Manz, F., Schaerer, K., Seyberth, H.W., and Konrad, M. (2001). "Novel paracellin-1 mutations in 25 families with familial hypomagnesemia with hypercalciuria and nephrocalcinosis". Journal of the American Society of Nephrology 12: 1872-1881.</ref><ref name="Chubanov 2004">Chubanov, V., Waldegger, S., Mederos y Schnitzler, M., Vitzthum, H., Sassen, M.C., Seyberth, H.W., Konrad, M., and Gudermann, T. (2004). "Disruption of TRPM6/TRPM7 complex formation by a mutation in the TRPM6 gene causes hypomagnesemia with secondary hypocalcemia". Proceedings of the National Academy of Sciences of the United States of America 101: 2894-2899.</ref>
Over-accumulation of magnesium may lead to toxic effects on the cell. These effects have been much more difficult to show experimentally in single cells. In humans magnesium overload (hypermagnesemia) is well documented, though usually caused by loss of function in the kidneys, since otherwise excess magnesium is rapidly excreted in the urine (Harrison’s Principles of Internal Medicine, Online Edition). Magnesium can also be toxic to plants, although accompanying drought stress is generally required.<ref name="Kaiser 1987">Kaiser, W.M. (1987). "Effects of water deficit on photosynthetic capacity". Physiologia Plantarum 71: 142-149.</ref><ref name="Rao 1987">Rao, I.M., Sharp, R.E., and Boyer, J.S. (1987). "Leaf magnesium alters photosynthetic response to low water potentials in sunflower". Plant Physiology 92: 29-36.</ref>
[edit] 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. 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.<ref name="Cowan 1995">Cowan, J.A. (1995). J.A. Cowan: Introduction to the biological chemistry of magnesium. New York: VCH.</ref><ref name="Romani 2002">Romani, A.M.P., Maguire, M.E. (2002). "Hormonal regulation of Mg2+ transport and homeostasis in eukaryotic cells". BioMetals 15: 271-283.</ref><ref name="Shaul 2002"/> In nucleotides, the triple phosphate moiety of the compound is invariably stabilized by association with Mg2+ in all enzymic processes.
In photosynthetic organisms Mg2+ has the additional vital role of being the coordinating ion in the chlorophyll molecule. This role was discovered by R. M. Willstätter, who received the Nobel Prize in Chemistry 1915 for the purification and structure of chlorophyll.
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.<ref name="Black 1995a">Black, C.B., Cowan, J.A. (1995). "Magnesium-dependent enzymes in nucleic acid biochemistry".</ref><ref name="Black 1995b">Black, C.B., Cowan, J.A. (1995). "Magnesium-dependent enzymes in general metabolism".</ref><ref name="Cowan 1995"/><ref name="Cowan 2002">Cowan, J.A. (2002). "Structural and catalytic chemistry of magnesium-dependent enzymes". BioMetals 15: 225-235.</ref> 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. 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, 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.
Nucleic acids have an important range of interactions with Mg2+. 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+.<ref name="Cowan 1995"/> Additionally, ribosomes contain large amounts of Mg2+ and the stabilisation provided is essential to the complexation of this ribo-protein.<ref name="Sperazza 1983">Sperazza, J.M., Spremulli, L.L. (1983). "Quantitation of cation binding to wheat grem ribosomes: influences on subunit association equlibria and ribosome activity". Nucleic Acids Research 11: 2665-2679.</ref> 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<ref name="Smith 1995">Smith, R.L., Thompson, L.J., and Maguire, M.E. (1995). "Cloning and characterization of MgtE, a putative new class of Mg2+ transporter from Bacillus firmus OF4". Journal of Bacteriology 177: 1233-1238.</ref>).
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.<ref name="Cowan 1995"/> 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 (ΔΨ) across the membrane, 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.<ref name="Berkowitz 1993"/>
The Mg2+ ion tends to bind only weakly to proteins (Ka ≤ 105<ref name="Cowan 1995"/>) 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, the total Mg2+ content of animal cells is 30 mmol/L<ref name="Ebel 1980">Ebel, H., Gunther, T. (1980). "Magnesium metabolism: a review". Journal of Clinical Chemistry and Clinical Biochemistry 18: 257-270.</ref> 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), 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.
In biological systems, only manganese (Mn2+) is readily capable of replacing Mg2+, and 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. Mn2+ can also replace Mg2+ as the activating ion for a number of Mg2+-dependent enzymes, although some enzyme activity is usually lost.<ref name="Cowan 1995"/> Sometimes such enzyme metal preferences vary among closely related species: for example is that 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+.
[edit] Measuring magnesium in biological samples
[edit] 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. 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. Additionally, since 1990 no facility has routinely produced 28Mg and the price per mCi is now predicted to be approximately US$30,000.<ref name="Maguire 2002">Maguire, M.E., Cowan, J.A. (2002). "Magnesium chemistry and biochemistry". BioMetals 15: 203-210.</ref> The chemical nature of Mg2+ is such that it is closely approximated by few other cations.<ref name="Tevelev 1995">Tevelev, A., Cowan, J.A. (1995). J.A. Cowan: Metal substitution as a probe of the biological chemistry of magnesium ion. New York: VCH.</ref> However, Co2+, Mn2+ and Ni2+ have been used successfully to mimic the properties of Mg2+ in some enzyme reactions, 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.<ref name="Tevelev 1995"/>
[edit] By fluorescent indicators
A number of chelators of divalent cations have different fluorescence spectra in the bound and unbound states.<ref name="Drakenberg 1995">Drakenberg, T. (1995). J.A. Cowan: Physical methods for studying the biological chemistry of magnesium. New York: VCH.</ref> 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<ref name="Raju 1989">Raju, B., Murphy, E., Levy, L.A., Hall, R.D., and London, R.E. (1989). "A fluorescent indicator for measuring cytosolic free magnesium". Am J Physiol Cell Physiol 256: C540-548.</ref>) actually has a higher affinity for Ca2+.<ref name="Grubbs 2002">Grubbs, R.D. (2002). "Intracellular magnesium and magnesium buffering". BioMetals 15: 251-259.</ref> This limits the application of this dye to cell types where the resting level of Ca2+ is < 1 μM 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+.<ref name="Otten 2001">Otten, P.A., London, R.E., and Levy, L.A. (2001). "4-Oxo-4H-quinolizine-3-carboxylic acids as Mg2+ selective, fluorescent indicators". Bioconjugate Chemistry 12: 203-212.</ref> 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.
[edit] By electrophysiology
First, ion-specific microelectrodes can be used to measure the internal free ion concentration of cells and organelles. 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.<ref name="Gunzel 2002">Gunzel, D., Schlue, W.-R. (2002). "Determination of [Mg2+]i - an update on the use of Mg2+-selective electrodes". BioMetals 15: 237-249.</ref>
Second, the technique of two-electrode voltage-clamp allows the direct measurement of the ion flux across the membrane of a cell.<ref name="Hille 1992 ch2">Hille, B. (1992). “2”, Ionic channels of excitable membranes. Sunderland: Sinauer Associates Inc..</ref> 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. Under ideal conditions the conductance of individual channels can be quantified. This methodology gives the most direct measurement of the action of ion channels.<ref name="Hille 1992 ch2"/>
[edit] By absorption spectrography
Flame atomic absorption spectroscopy (AAS) determines the total magnesium content of a biological sample.<ref name="Drakenberg 1995"/> 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. 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.<ref name="Dean 1997">See Chapters 5 and 6 in Dean, J.R. (1997). Atomic Absorption and Plasma Spectroscopy. Chichester: John Wiley & Sons. for descriptions of the methodology as applied to analytical chemistry.</ref> 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.
[edit] Magnesium transport
The chemical and biochemical properties of Mg2+ present the cellular system with a significant challenge when transporting the ion across biological membranes. 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.<ref name="Hille 1992 ch11">Hille, 1992. Chapter 11</ref> 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.<ref name="Haynes 2002">Haynes, W.J., Kung, C., Saimi, Y., and Preston, R.R. (2002). "An exchanger-like protein underlies the large Mg2+ current in Paramecium". PNAS 99: 15717-15722.</ref> The mechanisms of Mg2+ transport by the remaining proteins are unknown, and no three dimensional structure of any Mg2+ transport complex has been solved.
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). 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.<ref name="Maguire 2002"/> Due to the strength of the inner sphere complexation between Mg2+ and any ligand, 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.
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.<ref name="Gardner 2003">Gardner, R.C. (2003). "Genes for magnesium transport". Current Opinion in Plant Biology 6: 263-267.</ref> However, only a small selection of Mg2+ transporters have been characterised at the molecular level.
[edit] 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.
[edit] Nutritional requirements and interactions
Mg2+ is essential for plant growth and is present in higher plants in amounts on the order of 80 μmol g-1 dry weight.<ref name="Marschner 1995"/> 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;<ref name="Shaul 1999"/> and in times of starvation Mg2+ is redistributed, in many plants, from older to newer leaves.<ref name="Marschner 1995"/><ref name="Laing 2000">Laing, W., Greer, D., Sun, O., Beets, P., Lowe, A., and Payn, T. (2000). "Physiological impacts of Mg deficiency in Pinus radiata: growth and photosynthesis". New Phytol 146: 47-57.</ref>
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;<ref name="Heenan 1981">Heenan, D.P., Campbell, L.C. (1981). "Influence of potassium and manganese on growth and uptake of magnesium by soybeans (Glycine max (L.) Merr. cv Bragg". Plant Soil 61: 447-456.</ref> 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.<ref name="Hope 1952">Hope, A.B., Stevens, P.G. (1952). "Electrical potential differences in bean roots on their relation to salt uptake". Australian Journal of Scientific Research, Series B 5: 335-343.</ref> 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; 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. 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.<ref>Section 8.5.2 in Marschner, 1995</ref> The requirement for Mg2+ in mitochondria has been demonstrated in yeast<ref name="Bui 1999">Bui, D.M., Gregan, J., Jarosch, E., Ragnini, A., and Schweyen, R.J. (1999). "The bacterial magnesium transporter CorA can functionally substitute for its putative homologue Mrs2p in the yeast inner mitochondrial membrane". Journal of Biological Chemistry 274: 20438-20443.</ref> 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+.<ref name="Demmig 1979">Demmig, B., Gimmler, H. (1979). "Effect of divalent cations on cation fluxes across the chloroplast envelope and on photosynthesis of intact chloroplasts". Zeitschrift fur Naturforschung 24C: 233-241.</ref><ref name="Huber 1980">Huber, S.C., Maury, W.J. (1980). "Effects of magnesium on intact chloroplasts". Plant Physiology 65: 350-354.</ref> In addition, it seems likely that the other subcellular organelles (e.g. Golgi, endoplasmic reticulum, etc) also require Mg2+.
[edit] 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.<ref>Section 2.7 in Marschner, 1995</ref> 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,<ref>Section 3.3 in Marschner, 1995</ref> the ions will be transported to the top of the plant and back down again in a continuous cycle of replenishment. 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).
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.<ref name="Marschner 1995"/> Mg2+ is taken up into the root cell wall space (1) and interacts with the negative charges associated with the cell walls and membranes. 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. 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). 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.<ref name="Marschner 1995"/> 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. Additionally, when Mg2+ in the environment is limiting some species are able to mobilise Mg2+ from older tissues.<ref name="Laing 2000"/> 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. In times of growth and development Mg2+ is also remobilised within the plant as source and sink relationships change.<ref name="Marschner 1995"/>
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 ΔpH.<ref>Section 2.4 in Marschner, 1995</ref> 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, via channels using the ΔΨ (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.
A schematic of a plant cell is shown including the four major compartments currently recognised as interacting with Mg2+. H+-ATPases maintain a constant ΔpH across the plasma membrane and the vacuole membrane. Mg2+ is transported into the vacuole using the energy of ΔpH (in A. thaliana by AtMHX). Transport of Mg2+ into cells may use either the negative ΔΨ or the ΔpH. The transport of Mg2+ into mitochondria probably uses ΔΨ 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. 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.
[edit] 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.<ref name="Marschner 1995"/><ref name="Scott 1990a">Scott, B.J., Robson, A.D. (1990). "Distribution of magnesium in subterranean clover (Trifolium subterranean L.) in relation to supply". Australian Journal of Agricultural Research 41: 499-510.</ref><ref name="Scott 1990b">Scott, B.J., Robson, A.D. (1990b). "Changes in the content and form of magnesium in the first trifoliate leaf of subterranean clover under altered or constant root supply". Australian Journal of Agricultural Research 41: 511-519.</ref> Thylakoid stacking is stabilised by Mg2+ and is important for the efficiency of photosynthesis, allowing phase transitions to occur.<ref name="Fork 1986">Fork, D.C. (1986). "The control by state transitions of the distribution of excitation energy in photosynthesis". Annual Review of Plant Physiology and Plant Molecular Biology 37: 335-361.</ref>
Mg2+ is probably taken up into chloroplasts to the greatest extent during the light induced development from proplastid to chloroplast or etioplast to chloroplast. At these times the synthesis of chlorophyll and the biogenesis of the thylakoid membrane stacks absolutely require the divalent cation.<ref name="Gregory 1989">Gregory, R.P.F. (1989). Structure and function of the photosynthesising cell. New York: John Wiley and Sons.</ref><ref name="Lu 1995">Lu, Y.-K., Chen, Y.-R., Yang, C.-M., and Ifuku, K. (1995). "Influence of Fe- and Mg-deficiency on the thylakoid membranes of a chlorophyll-deficient ch5 mutant of Arabidopsis thaliana". Botanical Bulletin of Academia Sinica 36.</ref>
Whether Mg2+ is able to move into and out of chloroplasts after this initial developmental phase has been the subject of several conflicting reports. Deshaies et al. (1984) found that Mg2+ did move in and out of isolated chloroplasts from young pea plants,<ref name="Deshaies 1984">Deshaies, R.J., Fish, L.E., and Jagendorf, A.T. (1984). "Permeability of chloroplast envelopes to Mg2+". Plant Physiology 74: 956-961.</ref> but Gupta and Berkowitz (1989) were unable to reproduce the result using older spinach chloroplasts.<ref name="Gupta 1989">Gupta, A.S., Berkowitz, G.A. (1989). "Development and use of chlorotetracycline fluorescence as a measurement assay of chloroplast envelope-bound Mg2+". Plant Physiology 89: 753-761.</ref> 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. 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. 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.<ref name="Heldt 1973">Heldt, H.W., Werdan, K., Milovancev, M., and Geller, G. (1973). "Alkalization of the chloroplast stroma caused by light-dependent proton flux into the thylakoid space". Biochimica et Biophysica Acta 314: 224-241.</ref><ref name="Hind 1974">Hind, G., Nakatani, H.Y., and Izawa, S. (1974). "Light-dependent redistribution of ions in suspensions of chloroplast thylakoid membranes". Proceedings of the National Academy of Sciences of the United States of America 71: 1484-1488.</ref> Mg2+ (along with K+) is released from the lumen into the stroma, in an electroneutralisation process to balance the flow of H+.<ref name="Bulychev 1976">Bulychev, A.A., Vredenberg, W.J. (1976). "Effect of ionophores A-23187 and nigericin on the light induced redistribution of magnesium potassium and hydrogen ions across the thylakoid membrane". Biochimica et Biophysica Acta 449: 48-58.</ref><ref name="Krause 1977">Krause, G.H. (1977). "Light-induced movement of magnesium ions in intact chloroplasts. Spectroscopic determination with Eriochrome Blue SE". Biochimica et Biophysica Acta 460: 500-510.</ref><ref name="Portis 1981">Portis, A.R. (1981). "Evidence of a low stromal Mg2+ concentration in intact chloroplasts in the dark". Plant Physiology 67: 985-989.</ref><ref name="Ishijima 2003">Ishijima, S., Uchibori, A., Takagi, H., Maki, R., and Ohnishi, M. (2003). "Light-induced increase in free Mg2+ concentration in spinach chloroplasts: Measurement of free Mg2+ by using a fluorescent probe and intensity of stromal alkalinization". Archives of Biochemistry and Biophysics 412: 126-132.</ref> Finally, thiol groups on enzymes are reduced by a change in the redox state of the stroma.<ref name="Sharkey 1998">Sharkey, T.D. (1998). "Photosynthetic carbon reduction": 111-122.</ref> Examples of enzymes activated in response to these changes are fructose 1,6-bisphosphatase, sedoheptulose bisphosphatase and ribulose-1,5-bisphosphate carboxylase.<ref name="Black 1995b"/><ref name="Marschner 1995"/><ref name="Sharkey 1998"/> 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.<ref name="Black 1995b"/> Firstly, enzymes in the glycolytic pathway most often interact with two atoms of Mg2+. 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. Mg2+ may also serve in a structural maintenance role in these enzymes (e.g. enolase).
[edit] Magnesium stress
Plant stress responses can be observed in plants that are under or over supplied with Mg2+. 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.<ref name="Laing 2000"/> In Mg2+ deficiency, reported secondary effects include carbohydrate immobility, loss of RNA transcription and loss of protein synthesis.<ref>Section 8.5.6 of Marschner, 1995</ref> However, due to the mobility of Mg2+ within the plant, the deficiency phenotype may be present only in the older parts of the plant. 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.<ref name="Laing 2000"/>
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.<ref name="Marschner 1995"/> 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;<ref name="Heenan 1981"/> In acid soils Al3+ is a particularly strong inhibitor of Mg2+ uptake.<ref name="Rengel 1989">Rengel, Z., Robinson, D.L. (1989). "Competitive Al3+ inhibition of net Mg2+ uptake by intact Lolium multiflorum roots. I. Kinetics". Plant Physiology 91: 1407-1413.</ref><ref name="Marschner 1991">Marschner, H. (1991). Y. Waisel, A. Eshel, and U. Kafikfai: Root-induced changes in the avaibility of micronutrients in the rhizosphere. New York: Marcel Dekker.</ref> 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.<ref name="Marschner 1995"/> In bacteria and yeast, such binding by Mn2+ has already been observed. Stress responses in the plant develop as cellular processes halt due to a lack of Mg2+ (e.g. maintenance of ΔpH 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%.<ref name="Dorenstouter 1985">Dorenstouter, H., Pieters, G.A., and Findenegg, G.R. (1985). "Distribution of magnesium between chloroplhyll and other photosynthetic functions in magnesium deficient 'sun' and 'shade' leaves of poplar". Journal of Plant Nutrition 8: 1088-1101.</ref> Presumably, this imbalance has detrimental effects on other cellular processes.
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. 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.<ref name="Wu 1991">Wu, W., Peters, J., and Berkowitz, G.A. (1991). "Surface charge-mediated effects of Mg2+ on K+ flux across the chloroplast envelope membrane are associated with the regulation of stromal pH and photosynthesis". Plant Physiology 97: 580-587.</ref>
[edit] See also
[edit] References
<references/>
