InterPro domain: IPR006067

Sulphite reductases (SiRs) and related nitrite reductases (NiRs) catalyse the six-electron reduction reactions of sulphite to sulphide, and nitrite to ammonia, respectively. The Escherichia coli SiR enzyme is a complex composed of two proteins, a flavoprotein alpha-component (SiR-FP) and a hemoprotein beta-component (SiR-HP) ( IPR005117 ), and has an alpha(8)beta(4) quaternary structure [ 1 ]. SiR-FP contains both FAD and FMN, while SiR-HP contains a Fe(4)S(4) cluster coupled to a siroheme through a cysteine bridge. Electrons are transferred from NADPH to FAD, and on to FMN in SiR-FP, from which they are transferred to the metal centre of SiR-HP, where they reduce the siroheme-bound sulphite.

SiR-HP has a two-fold symmetry, which generates a distinctive three-domain alpha/beta fold that controls assembly and reactivity [ 2 ]. In the E. coli SiR-HP enzyme ( 1.8.1.2 ), the iron is bound to cysteine residues at positions 433, 439, 478 and 482, the latter also forming the siroheme ligand.

1. A simplifed functional version of the Escherichia coli sulfite reductase. J. Biol. Chem. 275, 37651-6
2. Sulfite reductase structure at 1.6 A: evolution and catalysis for reduction of inorganic anions. Science 270, 59-67