Nitrite/sulphite reductase 4Fe-4S domain (IPR006067)

Short name: NO2/SO3_Rdtase_4Fe4S_dom

Overlapping homologous superfamilies


Domain relationships



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 [PMID: 10984484]. 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 [PMID: 7569952]. In the E. coli SiR-HP enzyme (EC:, the iron is bound to cysteine residues at positions 433, 439, 478 and 482, the latter also forming the siroheme ligand.

GO terms

Biological Process

GO:0055114 oxidation-reduction process

Molecular Function

GO:0020037 heme binding
GO:0051536 iron-sulfur cluster binding
GO:0016491 oxidoreductase activity

Cellular Component

No terms assigned in this category.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.