The reaction is catalysed by two types of enzymes, found in the perimplasm of denitrifying bacteria. One type comprises proteins containing multiple copper centres, the other a heme protein, cytochrome cd1. Acceptors include c-type cytochromes such as cytochrome c-550 or cytochrome c-551 from Paracoccus denitrificans or Pseudomonas aeruginosa, and small blue copper proteins such as azurin and pseudoazurin. Cytochrome cd1 also has oxidase and hydroxylamine reductase activities. May also catalyse the reaction of hydroxylamine reductase (EC 18.104.22.168) since this is a well-known activity of cytochrome cd1.
deletion of cytochrome cd1-type nitrite reductase NirS gene or gene NirN results in impaired growth and smaller, fewer, and aberrantly shaped magnetite crystals during nitrate reduction. Nitrite reduction is completely abolished in the DELTAnirS mutant. NirN is required for full reductase activity of NirS by maintaining a proper form of d1 heme for holo-cytochrome cd1 assembly