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Hydroxylamine oxidoreductase (HAO) is an enzyme found in the prokaryotic genus Nitrosomonas. It plays a critically important role in the biogeochemical nitrogen cycle as part of the metabolism of ammonia-oxidizing bacteria.

The substrate is hydroxylamine (${\displaystyle {\ce {NH2OH}}}$), a chemical produced biologically by the enzyme Ammonia monooxygenase. The products of the catalyzed reaction are debated, but recent work shows compelling evidence for the production of nitric oxide.

Structural studies

Crystallographic methods show that HAO (PDB code: 1FGJ) is a cross-linked trimer of polypeptides containing 24 heme cofactors.

Reactivity

For many decades the enzyme was thought to catalyze the following reaction:

${\displaystyle {\ce {NH2OH + H2O -> NO2^- + 5 H+ + 4e^-}}}$

Recent work in the field, however, reveals that this enzyme catalyzes an entirely different reaction:

${\displaystyle {\ce {NH2OH -> NO + 3H+ + 3e^-}}}$

Subsequent oxidation of the nitric oxide to nitrite caused by reaction with oxygen accounts for the reactivity previous described by Hooper et al.

Environmental Impact

Nitric oxide, the product of HAO catalysis, is a potent greenhouse gas. Additionally, the oxidized product of nitric oxide in the presence of oxygen is nitrite - a common pollutant in agricultural run-off.

References

1. ^ Caranto, Jonathan D.; Lancaster, Kyle M. (2017-07-17). "Nitric oxide is an obligate bacterial nitrification intermediate produced by hydroxylamine oxidoreductase". Proceedings of the National Academy of Sciences. 114 (31): 8217–8222. Bibcode:2017PNAS..114.8217C. doi:10.1073/pnas.1704504114. ISSN 0027-8424. PMC 5547625. PMID 28716929.
2. Cedervall, Peder; Hooper, Alan B.; Wilmot, Carrie M. (2013-09-10). "Structural Studies of Hydroxylamine Oxidoreductase Reveal a Unique Heme Cofactor and a Previously Unidentified Interaction Partner". Biochemistry. 52 (36): 6211–6218. doi:10.1021/bi400960w. ISSN 0006-2960. PMID 23952581.
3. Igarashi, N.; Moriyama, H.; Fujiwara, T.; Fukumori, Y.; Tanaka, N. (April 1997). "The 2.8 A structure of hydroxylamine oxidoreductase from a nitrifying chemoautotrophic bacterium, Nitrosomonas europaea". Nature Structural Biology. 4 (4): 276–284. doi:10.1038/nsb0497-276. ISSN 1072-8368. PMID 9095195. S2CID 1028628.
4. Hendrich, Michael P.; Logan, Michael; Andersson, Kristoffer K.; Arciero, Dave M.; Lipscomb, John D.; Hooper, Alan B. (1994-12-01). "The Active Site of Hydroxylamine Oxidoreductase from Nitrosomonas: Evidence for a New Metal Cluster in Enzymes". Journal of the American Chemical Society. 116 (26): 11961–11968. doi:10.1021/ja00105a041. ISSN 0002-7863.
5. Montzka, S. A.; Dlugokencky, E. J.; Butler, J. H. (2011). "Non-CO2 greenhouse gases and climate change". Nature. 476 (7358): 43–50. doi:10.1038/nature10322. PMID 21814274. S2CID 205225911.

• Hooper AB, Balny C (1982). "Reaction of oxygen with hydroxylamine oxidoreductase of Nitrosomonas: fast kinetics". FEBS Lett. 144 (2): 299–303. Bibcode:1982FEBSL.144..299H. doi:10.1016/0014-5793(82)80658-3. PMID 7117545. S2CID 9726167.
• Lipscomb JD, Hooper AB (1982). "Resolution of multiple heme centers of hydroxylamine oxidoreductase from Nitrosomonas. 1. Electron paramagnetic resonance spectroscopy". Biochemistry. 21 (17): 3965–72. doi:10.1021/bi00260a010. PMID 6289867.
• Rees MK (1968). "Studies of the hydroxylamine metabolism of Nitrosomonas europaea. I Purification of hydroxylamine oxidase". Biochemistry. 7 (1): 353–66. doi:10.1021/bi00841a045. PMID 5758552.

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