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), 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 sirohaem 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]. This entry describes the ferrodoxin-like (alpha/beta sandwich) domain. Two copies of this domain are found in Nitrite and Sulfite reductases and form a single structural domain.

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

Sulphite reductases (SiRs) and related nitrite reductases (NiRs) catalyse the six-electron reduct
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