Cytochrome c554 as a possible electron donor in the hydroxylation of ammonia and carbon monoxide in Nitrosomonas europaea.

Mechanism of ammonia oxidation was studied in the reconstituted system of Nitrosomonas membrane fraction plus the Nitrosomonas cytochrome c554. The cytochrome c554 was reduced by hydroxylamine, hydrazine, and ammonia and the reduced cytochrome was oxidized upon the addition of ammonia or carbon monoxide. The oxidation of carbon monoxide in the presence of hydroxylamine or hydrazine was studied as a possible assay method for ammonia hydroxylase where hydroxylamine or hydrazine was supplying the reducing power required for the hydroxylation of carbon monoxide. The stoichiometry of the reaction, Km values for substrates, and effects of pH and inhibitors were investigated. It is concluded that carbon monoxide, a competitive inhibitor for ammonia oxidation, is an alternate substrate for ammonia hydroxylase using the reduced cytochrome c554 as the reducing power.