Heme P460 of Hydroxylamine Oxidoreductase of Nitrosomonas

Abstract

Hydroxylamine oxidoreductase (HAO) of Nitrosomonas catalyzes the dehydrogenation of NH2OH and subsequent addition of O2 to form nitrite. HAO contains c hemes and the CO-binding heme P460 in a 7:1 ratio; dehydrogenation of NH2OH involves passage of electrons to P460 and then c hemes. Electrons rapidly pass from c hemes of HAO to the P460 center and then to H2O2. This conclusion is supported by inhibition of c heme oxidation with CO and loss of H2O2-oxidizability of ferrous c hemes following specific destruction of heme P460. Reaction of ferrous P460 with H2O2 is rate-limiting. Activation of dioxygen for N-oxidation by ferrous HAO may involve the 2-electron reduction of O2 by P460. The reaction of ferrous HAO with H2O2 was studied as it may reveal aspects of the mechanism of activation of dioxygen. Reaction of ferrous heme P460 with CO is slow and with low affinity as compared with other hemoproteins. Values for reaction of CO with enzyme were: k1, 1.1 .times. 10-3 M-1 s-1 and Kd, 12 .mu.M.