-!- The reaction is catalyzed by two types of enzymes, found in the perimplasm of denitrifying bacteria. -!- One type comprises proteins containing multiple copper centers, 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 catalyze the reaction of EC 1.7.99.1 since this is a well- known activity of cytochrome cd1. -!- Formerly EC 1.6.6.5, EC 1.7.99.3 and EC 1.9.3.2.