Chemical evolution of iron containing enzymes: mixed ligand complexes of iron as intermediary steps

Activities of the iron complexes of evolutionary importance like K4[Fe(CN)6], K4[Fe(CN)5(gly)], and K4[Fe(CN)5(trigly)] have been tested towards some redox reactions of biological significance, namely, decomposition of hydrogen peroxide, dehydrogenation of NADH and ascorbic acid both coupled with reduction of methylene blue. It has been observed that the catalytic activities of iron (II) complexes towards the redox reactions studied at pH 9.18 followed the order, K4[Fe(CN)6] less than K4[Fe(CN)5(gly)] less than K4[Fe(CN)5(trigly)]. Decomposition of H2O2 catalysed by cyanocomplexes of iron (II) has been discussed through the formation of an innersphere complex in which loosly bound ligands like, glycine and triglycine are replaced by hydroperoxide ion. A tentative mechanism for the catalysed decomposition of H2O2 has been discussed. Based upon the experimental observations a hypothesis on the evolution of iron containing enzymes has been envisaged as: iron(II) ion----iron(II) cyanide complexes----mixed ligand iron(II) cyanide and amino acid complexes----iron(II) complexes of macromolecules----pŕoenzyme or early enzyme containing iron(II).