<P> Around 4 - 3 Ga, anaerobic prokaryotes began developing metal and organic cofactors for light absorption . They ultimately ended up making chlorophyll from Mg (II), as is found in cyanobacteria and plants, leading to modern photosynthesis . However, chlorophyll synthesis requires numerous steps . 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 . The centers of these rings are not selective, thus allowing the variety of metal ions to be incorporated . Mg porphyrin gives rise to chlorophyll, Fe porphyrin to heme proteins, Ni porphyrin yields factor F - 430, and Co porphyrin Coenzyme B12 . </P> <P> Before the Great Oxygenation Event, copper was not readily available for living organisms . Most early copper was Cu and Cu . This oxidation state of copper is not very soluble in water . One billion years ago, after the great oxidation event the oxygen pressure rose sufficiently to oxidise Cu to Cu, increasing its solubility in water . As a result, the copper became much more available for living organisms . </P> <P> Most copper - containing proteins and enzymes can be found in eukaryotes . Only a handful of prokaryotes such as aerobic bacteria and cyanobacteria contain copper enzymes or proteins . Copper can be found in both prokaryotes and eukaryotes superoxide dismutase (SOD) enzyme . There are three distinct types of SOD, containing Mn, Fe and Cu respectively . Mn - SOD and Fe - SOD are found in most prokaryotes and mitochondria of the eukaryotic cell . Cu - SOD can be found in the cytoplasmic fraction of the eukaryotic cells . The three elements, copper, iron and manganese, can all catalyze superoxide to ordinary molecular oxygen or hydrogen peroxide . However, Cu - SOD is more efficient than Fe - SOD and Mn - SOD . Most prokaryotes only utilize Fe - SOD or Mn - SOD due to the lack of copper in the environment . Some organisms did not develop Cu - SOD due to the lack of a gene pool for the Cu - SOD adoption . </P> <P> Zinc (Zn) was incorporated into living cells in two waves . Four to three Ga, anaerobic prokaryotes arose, and the atmosphere was full of H S and highly reductive . Thus most zinc was in the form of insoluble ZnS . However, because seawater at the time was slightly acidic, some Zn (II) was available in its ionic form and became part of early anaerobic prokaryotes' external proteases, external nucleases, internal synthetases and dehydrogenases . </P>

Metal ions involved in electron transport in biological system