InterPro: IPR006066 Nitrite/sulphite reductase iron-sulphur/siroheam-binding site

Nitrite reductases and bacterial sulphite reductases catalyse the 6-electron reduction of nitrite (sulphite) to ammonia (sulphide) [1, 2]. On the basis of physiological function, 2 types of nitrite reductase 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 nitrite reductases contain a prosthetic group termed sirohaem (an iron tetra-hydroporphyrin of the isobacteriochlorin type, with 8 carboxylic acid-containing peripheral sidechains), and an iron-sulphur cluster. Similarly, there are 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 of reductase contain sirohaem and iron-sulphur clusters [1]. A region of sequence similarity, about 80 amino acids long, is shared by assimilatory nitrite [2] and sulphite reductases [3, 4]. Four conserved Cys residues are suggested to be involved in binding the sirohaem group and/or the iron-sulphur centre [4].