InterPro domain: IPR006066

Nitrite reductases and bacterial sulphite reductases catalyse the 6-electron reduction of nitrite (sulphite) toammonia (sulphide) [ 1 ]. On the basis of physiological function, 2 types of nitritereductase can be defined: the assimilatory type, which is involved in nitrate assimilation (denitrification);and the dissimilatory type, which is responsible for nitrate respiration function. Assimilatory nitritereductases contain a prosthetic group termed sirohaem (an iron tetra-hydroporphyrin of the isobacteriochlorintype, with 8 carboxylic acid-containing peripheral sidechains), and an iron-sulphur cluster. Similarly, thereare 2 types of sulphite reductase: the assimilatory type, which participate in the synthesis of sulphur-containing compounds; and the dissimilatory type, which are terminal reductases in the reduction of sulphate.Assimilatory sulphite reductases can catalyse 6-electron reduction without the formation of free intermediates,while dissimilatory reductases can produce trithionate and thiosulphate in addition to sulphide. Both types ofreductase contain sirohaem and iron-sulphur clusters. A region of sequence similarity, about 80amino acids long, is shared by assimilatory nitrite [ 2 ] and sulphite reductases [ 2 , 3 ]. Four conserved Cys residues are suggested to be involved in binding the sirohaem group and/or theiron-sulphur centre [ 4 ].

1. Functional domains of assimilatory nitrate reductases and nitrite reductases. Trends Biochem. Sci. 15, 315-9
2. Sequence analysis and expression of the Salmonella typhimurium asr operon encoding production of hydrogen sulfide from sulfite. J. Bacteriol. 173, 1544-53
3. Characterization of the cysJIH regions of Salmonella typhimurium and Escherichia coli B. DNA sequences of cysI and cysH and a model for the siroheme-Fe4S4 active center of sulfite reductase hemoprotein based on amino acid homology with spinach nitrite reductase. J. Biol. Chem. 264, 15726-37