The heme-based enzyme nitrite reductase catalyzes the conversion of nitrite to ammonia. The cycle begins with reduction of an iron-nitrite complex to a metal nitrosyl complex.
The copper-containing enzyme nitrite reductase (CuNIR) catalyzes the 1-electron reduction of nitrite to nitric oxide. The proposed mechanism entails the protonation of a κ-NO-Cu(I) complex. This protonation induces cleavage of an N–O bond, giving a HO–Cu–ON center, which features a nitric oxide ligand O-bonded to Cu(II) (an isonitrosyl).