Evolution of metal ions in biological systems
Metal ions have been associated with biological systems for billions of years, but only in the last century have scientists began to truly appreciate the scale of their influence.
Vanadium, molybdenum, cobalt, copper, chromium, iron, manganese, nickel, and zinc are deemed essential because without them biological function is impaired.
The Great Oxygenation Event occurred approximately 2.4 Ga (billion years ago) as cyanobacteria and photosynthetic life induced the presence of dioxygen in the planet's atmosphere.
The different anaerobic, autocatalysed, reductive, metabolic pathways seen in the earliest known cells developed in separate energised vesicles, protocells, where they were produced cooperatively with certain bases of the nucleic acids.
Magnesium also has many functions in prokaryotes such as glycolysis, all kinases, NTP reaction, signalling, DNA/RNA structures and light capture.
[9] Magnesium in ATP hydrolysis acts as a co-factor to stabilize the high negative charge transition state.
Evidence suggests that manganese (Mn) was first incorporated into biological systems roughly 3.2–2.8 billion years ago, during the Archean Period.
Together with calcium, it formed the manganese-calcium oxide complex (determined by X-ray diffraction) which consisted of a manganese cluster, essentially an inorganic cubane (cubical) structure.
The incorporation of a manganese center in photosystem II was highly significant, as it allowed for photosynthetic oxygen evolution of plants.
The oxygen-evolving complex (OEC) is a critical component of photosystem II contained in the thylakoid membranes of chloroplasts; it is responsible for terminal photooxidation of water during light reactions.
[11] The incorporation of Mn in proteins allowed the complexes the ability to reduce reactive oxygen species in Mn-superoxide dismutase (MnSOD) and catalase, in electron transfer-dependent catalysis (for instance in certain class I ribonucleotide reductases) and in the oxidation of water by photosystem II (PSII), where the production of thiobarbituric acid-reactive substances is decreased.
It played a larger role in the geological past in marine geochemistry, as evidenced by the deposits of Precambrian iron-rich sediments.
Iron is frequently found in mononuclear sites in the reduced Fe(II) form, and functions in dioxygen activation; this function is used as a major mechanism adopted by living organisms to avoid the kinetic barrier hindering the transformation of organic compounds by O2.
They ultimately ended up making chlorophyll from Mg(II), as is found in cyanobacteria and plants, leading to modern photosynthesis.
The process starts with uroporphyrin, a primitive precursor to the porphyrin ring which may be biotic or abiotic in origin, which is then modified in cells differently to make Mg, Fe, nickel (Ni), and cobalt (Co) complexes.
One billion years ago, after the great oxidation event the oxygen pressure rose sufficiently to oxidise Cu+ to Cu2+, increasing its solubility in water.
The three elements, copper, iron and manganese, can all catalyze superoxide to ordinary molecular oxygen or hydrogen peroxide.
It is believed that the later addition of ions such as zinc and copper allowed them to displace iron and manganese from the enzyme superoxide dismutase (SOD).
This could only have occurred due to the long life of eukaryotes, which allowed time for zinc to exchange and hence become an internal messenger coordinating the action of other transcription factors during growth.
Mo is found in the active sites of metalloenzymes that perform key transformations in the metabolism of carbon, nitrogen, arsenic, selenium, sulfur, and chlorine compounds.
They belong to a class of enzymes with a mononuclear Mo center and they catalyze the metabolism reaction of C, N, S, etc., in bacteria, plants, animals, and humans.
This reaction made the highly soluble molybdate ion available for incorporation into critical metalloenzymes, and may have thus allowed life to thrive.
These enzymes are used by bacteria that usually live in a symbiotic relationship with plants; their role is nitrogen fixation, which is vital for sustaining life on earth.
Mo enzymes also play important roles in sulfur metabolism of organisms ranging from bacteria to humans.
[18] Tungsten is one of the oldest metal ions to be incorporated in biological systems, preceding the Great Oxygenation Event.
Before the abundance of oxygen in Earth's atmosphere, oceans teemed with sulfur and tungsten, while molybdenum, a metal that is highly similar chemically, was inaccessible in solid form.
[19] Although research into the specific enzyme complexes in which tungsten is incorporated is relatively recent (1970s), natural tungstoenzymes are abundantly found in a large number of prokaryotic microorganisms.
The first crystal structure of a tungsten- or pterin-containing enzyme, that of aldehyde ferredoxin oxidoreductase from P. furiosus, has revealed a catalytic site with one W atom coordinated to two pterin molecules which are themselves bridged by a magnesium ion.
