Electron Transfer Pathway Coupled to the Oxidation of Ammonia

3 Electron Transfer Pathway Coupled to the Oxidation of Ammonia 

As mentioned above, ammonia is oxidized to nitrous acid via hydroxylamine in N. europaea first ammonia is oxidized to hydroxylamine by the catalysis of ammonia monooxygenase, and hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase. Molecular oxygen is not necessary to the reaction itself of NH2OH — HN02 (Yamanaka and Sakano, 1980) but it is required for the consumption of electrons liberated from the reaction, NH2OH + H20 — HN02 + 4H+ + 4c. Electrons thus liberated are transferred first to cytochrome c-554, then to cytochrome c-552, and finally oxidized with molecular oxygen by the catalysis of cytochrome c oxidase. Based on the results described above, the electron transfer pathway in the oxidation of ammonia to nitrite or nitrous acid by N. europaea will be presented as shown in Fig. 3.3. 

Cytochrome b-560 has been obtained from the bacterium (Fukumori et al., 1988a), and the occurrence of two membrane-bound fr-type cytochromes has been observed (Miller and Wood, 1983). As no cytochrome is involved in the system of the oxidation of ammonia and hydroxylamine, the b-type cytochrome(s) will function in the reduction system of NAD(P)+ in which ubiquinone-8 seems to be involved (Hooper et al., 1972). Although an electron transfer pathway in which ubiquinone-8 mediates electrons between cytochrome c-554 and cytochrome c-552 

It seems very difficult to reconstitute the hydroxylamine oxidase system by mixing purified components, because the optimal pHs of hydroxylamine oxidore-ductase and cytochrome c oxidase are 9.6 and 5.6, respectively. However, as the optimal pH of the oxidoreductase is lowered to around 7 in the presence of ubiquinone-8 and oleic acid, the reconstitution of the system seems possible (Numata, 1989). When hydroxylamine and the oxidoreductase are added to the mixture of cytochromes c-554 and c-552 supplemented with ubiquinone-8 and oleic acid, cytochrome c-552 is reduced at pH 7.0, and ferrocytochrome c-552 thus formed is oxidized on addition of cytochrome c oxidase. Although these reactions are spectrally observed, the formation of nitrite is not observed, unfortunately, in the above system. 

The ammonia-oxidizing bacteria biosynthesizes the cellular materials from carbon dioxide. For this purpose, they need NAD(P)H. Electrons to reduce NAD(P)+ seem to come from ferrocytochrome c-552 by the supply of energy, because Aleem (1966) reported that he had demonstrated that NAD(P)+ was anaerobically reduced with horse ferrocytochrome c on addition of ATP using the cell-free extracts of N. europaea, though the enzymatic system participating in the reduction of NAD(P)+ has not been known. However, every attempt by the author and his colleagues to reproduce his results has been unsuccessful to date. 

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