Iron–sulfur proteins

Iron–sulfur (Fe–S) proteins contain clusters of iron and sulfide, usually bound to sulfur atoms of cysteine residues or imidazole nitrogens of histidine. The simpler Fe–S proteins such as ferredoxins contain clusters of 2, 3, or 4 atoms of iron and sulfide; more complex ?superclusters' occur in enzymes such as nitrogenase and hydrogenase. Fe–S proteins are involved in oxidation–reduction (electron transfer) processes, including photosynthetic primary reactions and the mitochondrial respiratory chain, where their clusters undergo one-electron changes in oxidation state. Other Fe–S clusters are involved in metalloenzyme catalysis and in gene regulation. Fe–S clusters can readily undergo ligand-exchange reactions, which facilitate their assembly, a process that is essential for living cells. In bacteria there are three different systems: the NIF and ISC systems, which in eukaryotes occur primarily in the mitochondria and cytosol, and the SUF system, which occurs in chloroplasts. Fe–S cluster assembly involves a scaffolding protein, systems for supply of iron and sulfide, and chaperones for insertion of the clusters into apo-proteins.