This entry represent the hydroxylamine reductases (Hcp, also known as Prismane) and the Ni-containing CO dehydrogenases (CODH) . Hydroxylamine reductases have been identified in bacteria, archaea and eukaryotic protozoa. They contain two Fe/S centres - a [4Fe-4S] cubane cluster, and a hybrid [4Fe-2S-2O] cluster. The physiological role of this protein is as yet unknown, although a role in nitrate/nitrite respiration has been suggested. The Hcp protein from Escherichia coli was shown to contain hydroxylamine reductase activity (NH2OH + 2e + 2 H+ -> NH3 + H2O). This activity is rather low. Hydroxylamine reductase activity was also found in CO-dehydrogenase in which the active site Ni was replaced by Fe. The CO dehydrogenase contains a Ni-3Fe-2S-3O centre. Ni-containing CO dehydrogenases allows bacterial growth in a CO-dependent manner in the dark. It oxidizes carbon monoxide coupled, via CooF, to the reduction of a hydrogen cation by a hydrogenase (possibly CooH).