The iron-sulphur proteins: Evolution of a ubiquitous protein from model systems to higher organisms

Abstract

Ferredoxins are Fe−S proteins with low molecular weight (6–12000) which act as electron carriers at very low redox potentials eg. −300 to −500 mV, in diverse biochemical processes such as bacterial and plant photosynthesis, N₂ fixation, carbon metabolism, oxidative phosphorylation and steroid hydroxylation. They are found in a wide range of organisms from the ‘primitive’ obligate anaerobic bacteria, through photosynthetic bacteria, blue-green and green algae, to all higher plants and animals. Three types of ferredoxins are known −8 Fe+8 S, 4 Fe+4 S and 2 Fe+2 S. All three have been found in bacteria while the 2 Fe and some 8 Fe ferredoxins have been found in plants and animals possibly representing an evolutionary sequence. The 8 Fe ferredoxin may all be composed of two 4 Fe units. We have proposed that because of the simplicity of the 8 Fe ferredoxins (only 9 common simple amino acids in clostridia, 6 of which have been detected in the Murchison meteorite) they may have been amongst the earliest proteins formed during the origin of life. A simple peptide of about 27 amino acids could incorporate inorganic Fe+S (or possibly an existing Fe−S complex) into it nonenzymatically under anaerobic conditions to form a protein carrying one or two electrons at the potential of the H₂ electrode. More than ten Fe−S model compounds have been proposed as analogues of the 4 Fe or 2 Fe containing active centres; inorganic, organometallic and peptide complexes have been synthesized. A few have many of the properties of ferredoxins but none as yet fulfills a sufficient number of criteria to substitute for ferredoxins chemically and biologically — a goal which will provide many clues to primitive peptide systems undergoing biological electron transfer reactions.