Cytochrome cd1 (nitrite reductase) catalyses the conversion of nitrite to nitric oxide in the nitrogen cycle. This family represents the d1 haem-binding domain of cytochrome cd1, in which His/Tyr side chains ligate the d1 haem iron of the active site in the oxidized state [<cite idref="PUB00006385"/>]. Cytochrome d1, haem domain