The Enzyme Database

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Accepted name: hydroxylamine oxidase (cytochrome)
Reaction: hydroxylamine + O2 = nitrite + H2O + H+ (overall reaction)
(1a) hydroxylamine + 2 ferricytochrome c = nitroxyl + 2 ferrocytochrome c + 2 H+
(1b) nitroxyl + 2 ferrocytochrome c + O2 + H+ = nitrite + 2 ferricytochrome c + H2O (spontaneous)
Other name(s): HAO (ambiguous); hydroxylamine oxidoreductase (ambiguous); hydroxylamine oxidase (misleading)
Systematic name: hydroxylamine:oxygen oxidoreductase
Comments: The enzyme from the heterotrophic nitrifying bacterium Paracoccus denitrificans contains three to five non-heme, non-iron-sulfur iron atoms and interacts with cytochrome c556 and pseudoazurin [2,3]. Under anaerobic conditions in vitro only nitrous oxide is formed [3]. Presumably nitroxyl is released and combines with a second nitroxyl to give nitrous oxide and water. When oxygen is present, nitrite is formed.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 9075-43-8
1.  Kurokawa, M, Fukumori, Y and Yamanaka, T A hydroxylamine - cytochrome c reductase occurs in the heterotrophic nitrifier Arthrobacter globiformis. Plant Cell Physiol. 26 (1985) 1439–1442.
2.  Wehrfritz, J.M., Reilly, A., Spiro, S. and Richardson, D.J. Purification of hydroxylamine oxidase from Thiosphaera pantotropha. Identification of electron acceptors that couple heterotrophic nitrification to aerobic denitrification. FEBS Lett. 335 (1993) 246–250. [DOI] [PMID: 8253206]
3.  Moir, J.W., Wehrfritz, J.M., Spiro, S. and Richardson, D.J. The biochemical characterization of a novel non-haem-iron hydroxylamine oxidase from Paracoccus denitrificans GB17. Biochem. J. 319 (1996) 823–827. [PMID: 8920986]
4.  Wehrfritz, J., Carter, J.P., Spiro, S. and Richardson, D.J. Hydroxylamine oxidation in heterotrophic nitrate-reducing soil bacteria and purification of a hydroxylamine-cytochrome c oxidoreductase from a Pseudomonas species. Arch. Microbiol. 166 (1996) 421–424. [PMID: 9082922]
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