ABSTRACT

The large majority of known iron-sulfur clusters in proteins is involved in elec­ tron transfer processes that exploit the chemical properties of these clusters for elec­ tron uptake, storage, donation, and exchange, both intra-and intermoleeular. All the small iron-sulfur proteins described in Secs. 2.1 to 2.5 (also reviewed in [8] and [9]) and most iron-sulfur clusters in redox enzymes such as trimethylamine dehydrogen­ ase, fumarate reductase, pyruvaterferredoxin oxidoreductase and hydrogenase have such a function. The best studied example of a non-redox enzyme with an iron-sulfur cluster at its catalytic site is mitochondrial aconitase, which catalyzes the citrateisocitrate isomerization in the tricarboxylic acid cycle. Both Fe-only hydrogenase and sulfite reductase are redox enzymes in which a Fe4S4 cubane is bridged to another bioinorganic component of the active site through a common cysteinyl sulfur ligand. These special cofactors are involved in the biological production or consumption of hydrogen and the six-electron reductions of sulfite to sulfide and nitrite to ammonia, respectively. The Fe4 “ hybrid” cluster of Fepr (“ putative prismane” ) protein is likely to be an active site as well; this is suggested by the interesting crystal structure of this protein whose function is not known yet (Sec. 2.12).