title: Enzymatic redox chemistry: a proposed reaction pathway for the six-electron reduction of SO3(2-) to S2- by the assimilatory-type sulfite reductase from Desulfovibrio vulgaris (Hildenborough).
abstract: A detailed reaction pathway for the six-electron reduction of SO3(2-) to S2- by the assimilatory-type sulfite reductase (SiR) from Desulfovibrio vulgaris (Hildenborough) has been deduced from experiments with 35S-labeled enzyme and the relative reaction rates of nitrogenous substrates. The ligand bridging the prosthetic [Fe4S4]-siroheme center is apparently exchanged by 35S2- in both oxidized and reduced enzyme. This 35S2- label was retained in the course of SO3(2-) reduction, implicating substrate binding to the nonbridging axial site of the siroheme. A reaction mechanism is proposed in which SO3(2-) binds to Fe2+ through the sulfur atom, followed by a series of two-electron reductive cleavages of S-O bonds. Protonation of oxygen facilitates bond cleavage, giving hydroxide as leaving group. The bridge remains intact throughout the course of the reaction, providing an efficient coupling pathway for electron transfer between the cluster and siroheme.
authors.affiliation: Evans Laboratory of Chemistry, Ohio State University, Columbus 43210.
authors.forename: J
authors.initials: J
authors.lastname: Tan
authors.forename: J A
authors.initials: JA
authors.lastname: Cowan
mesh_terms: D001665:Binding Sites; D003901:Desulfovibrio / Q000201:enzymology*; D004579:Electron Transport; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000378:metabolism*; D013379:Substrate Specificity; D013440:Sulfides / Q000737:chemistry*; D013447:Sulfites / Q000737:chemistry*; D013462:Sulfur Radioisotopes / Q000378:metabolism
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D013440:Sulfides; D013447:Sulfites; D013462:Sulfur Radioisotopes; C000499:ferric oxide; D050862:Oxidoreductases Acting on Sulfur Group Donors; C022597:ferrous sulfide