All the elements present on the Earth today were created gradually only after the Big Bang explosion; lighter elements like hydrogen and helium forming first and the heavier ones later. Heavier elements were formed inside the stars by the process called nucleosynthesis. As the universe expanded and cooled down, protons, neutrons and electrons (which were formed just after big bang explosion) combined to form H, He, Li, Be. All the other elements were produced by fusion of these lighter elements: First hydrogen fused to form He, then He into C, N, O and ultimately carbon fusions resulted in O, Ne, Na, Mg.
Over the course of evolution, metal ions have been incorporated into biological systems as they lose electrons facilitating redox reactions, which are necessary for all biochemical reactions occurring in biological systems. Not all elements could evolve to fit in the requirements of living systems and are today considered to be toxic while many others have proved to be beneficial and have been selected under the selection pressure over time and are said to be conserved.
One such essential electrolyte is Magnesium which is found in all living systems starting from kingdom Monera till Animalia. It is the 8th most abundant element in the universe and 7th most abundant element in earth’s crust. It is not found in the free form, but can be extracted from minerals dolomite and carnallite. Mostly, it is obtained from seawater. In plants and cyanobacteria, it is present in the center of porphyrin ring of chlorophyll thereby aiding in photosynthesis.
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Olivine ([Mg, Fe]2SiO4) was the first predominant mineral on earth’s crust, which served as a source of ionic magnesium to soil and water. Magnesium by inorganic condensation reactions resulted in organic compounds from where anaerobic bacteria emerged (the first form of life). This was followed with emergence of chlorophyll with Mg as central atom in anoxygenic purple photosynthetic bacteria which allowed production of oxygen and carbohydrates in the presence of sunlight. Evolution of ATP as energy storehouse resulted in the need of incorporation of Mg into biological machinery. Then evolved aerobic bacteria, multicellular plants and animals.
Bacterial species have been found to possess transporters to sense and take up magnesium from sea water, which can possibly be considered as one of the ways of incorporation of Mg first in the cells which later remained conserved because of its ability to aid various reactions and because of its comparatively higher available concentration.
Reasons for incorporation of Mg in living systems during early evolution can be:
Metal ion like magnesium could bind reversibly and stabilize O-donors like HPO2- which were abundant in prebiotic atmosphere thereby helping to synthesize polymers like DNA.
Approximately 4 billion years ago, seawater had high amount of H2S due to which there was a reducing environment. Thus, elements like magnesium having negative reduction potential with respect to the environment (-2.372 V) existed as free ions and thus could easily be incorporated in the cells
Magnesium was also readily available due to its high solubility and high concentration, therefore is today found in nearly all cell types.
Magnesium, and not calcium, was selected during evolution, despite both being divalent cations of same group in periodic table, because of comparatively higher binding constant of Mg than Ca. Also, smaller Mg ion could form complexes with 2-3 oxygen atoms of different phosphate groups than larger calcium ion.
Magnesium’s small size, high charge density and its ability to coordinate 6 O atoms in its first coordination shell allows it to play a special role in biochemical reactions.
Inorganic chemistry of Mg plays a key role in the first chemical processes, which led to the origin of life i.e. ribozymes and the early evolution of life. According to reports, Mg helped in self cleavage reactions.
Magnesium is the second most abundant cation in mammalian cells and is essential for numerous cellular processes. It plays a special role in biochemistry; it serves as a cofactor for more than 300 enzymatic reactions including energy metabolism, protein synthesis and signal transduction.
Being a divalent cation, it contributes to genome stability via following mechanisms:
Its role as a cofactor for DNA replication and repair mechanism related enzyme.
It acts as a competitive inhibitor of DNA damaging factor by binding to DNA.
Its positive charge interacts with the negative charges of phosphate group of DNA and thus stabilizes the secondary and tertiary structures of DNA.
Disorder of Mg2+ homeostasis has been linked to alterations in blood lipids, hypertension, atherosclerosis, myocardial infarction, type 2 diabetes and neurodegenerative diseases . It acts as natural calcium antagonist and blocks the N-methyl-D-aspartate (NMDA) receptor thereby causing muscle relaxation and blood vessel dilation. It maintains nerve impulse conduction by regulating active transport of calcium and potassium ions across plasma membrane. Further, it maintains bone integrity and prevents diabetes by increasing insulin release.
Interestingly, Mg2+-implicated nucleosome self-assembly was reported in 2013. Nucleosomes were found to have homology sensing ability and those with identical DNA sequences preferentially associate in the presence of Mg2+ ions. Thus, it seems to be a key regulator of chromatin based biological processes.
Magnesium has been an essential element for living systems since the time of origin of life. It had a key role in evolution of life by coupling to various biochemical reactions and molecules which evolved to be a necessity of living system. It then continued to be conserved over the course of evolution as the living systems tuned themselves according to magnesium.
