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 −8Fe + 8S, 4Fe + 4S and 2Fe + 2S. All three have been found in bacteria while the 2Fe and some 8Fe ferredoxins have been found in plants and animals possibly representing an evolutionary sequence. The 8Fe ferredoxin may all be composed of two 4Fe units. We have proposed that because of the simplicity of the 8Fe 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 non-enzymatically 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 4Fe or 2Fe 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.