IUBMB Enzyme Nomenclature


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. [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]

[EC created 1972 as EC, part transferred 2013 to EC, modified 2015]

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