This family includes both hybrid-cluster proteins and the beta chain of carbon monoxide dehydrogenase. The hybrid-cluster proteins contain two Fe/S centers - 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 prismane 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.