Cytochrome cd1 (nitrite reductase) catalyses the conversion of nitrite to nitric oxide in the nitrogen cycle. This family represents the d1 heme binding domain of cytochrome cd1, in which His/Tyr side chains ligate the d1 heme iron of the active site in the oxidised state [1].

1: Haem-ligand switching during catalysis in crystals of a nitrogen-cycle enzyme. Williams PA, Fulop V, Garman EF, Saunders NF, Ferguson SJ, Hajdu J; Nature 1997;389:406-412. PMID:9311786