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- Main article: Magnesium
Magnesium ions have important biological roles. 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. 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.
Magnesium transport[edit | edit source]
- Main article: 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. 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. 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). 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, 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. However, only a small selection of Mg2+ transporters have been characterised at the molecular level.
Ligand ion channel blockade[edit | edit source]
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.
See also[edit | edit source]
Notes[edit | edit source]
- Leroy, J. (1926). Necessite du magnesium pour la croissance de la souris. Comptes Rendus de Seances de la Societe de Biologie 94: 431–433.
- Lusk, J.E., Williams, R.J.P., and Kennedy, E.P. (1968). Magnesium and the growth of Escherichia coli. Journal of Biological Chemistry 243 (10): 2618–2624.
- Marschner, H. (1995). Mineral Nutrition in Higher Plants, San Diego: Academic Press.
- "Magnesium" Centre for Cancer Education, University of Newcastle upon Tyne. http://cancerweb.ncl.ac.uk/cgi-bin/omd?magnesium
- Hille, 1992. Chapter 11
- 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 (24): 15717–15722.
- Warren, M.A., Kucharski, L.M., Veenstra, A., Shi, L., Grulich, P.F., and Maguire, M.E. (2004). The CorA Mg2+ transporter is a homotetramer. Journal of Bacteriology 186 (14): 4605–4612.
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- Gardner, R.C. (2003). Genes for magnesium transport. Current Opinion in Plant Biology 6 (3): 263–267.