Hydroxylamine oxidoreductase explained

hydroxylamine oxidase
Ec Number:	1.7.3.4
Cas Number:	9075-43-8
Go Code:	0047991
Hydroxylamine Oxidoreductase

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 (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.^[1]

Structural studies

Crystallographic methods show that HAO (PDB code:) is a cross-linked trimer of polypeptides containing 24 heme cofactors.^{[2] [3]}

Reactivity

For many decades the enzyme was thought to catalyze the following reaction:^[4]

NH2OH + H2O -> NO2^- + 5 H+ + 4e^-

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

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.^[5] Additionally, the oxidized product of nitric oxide in the presence of oxygen is nitrite - a common pollutant in agricultural run-off.

References

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

Notes and 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. en. 8217–8222. 10.1073/pnas.1704504114. 0027-8424. 28716929. 5547625. free.
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. 10.1021/bi400960w. 23952581. 0006-2960.
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. 1072-8368. 9095195. 10.1038/nsb0497-276. 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. 10.1021/ja00105a041. 0002-7863.
5. Montzka. S. A.. Dlugokencky. E. J.. Butler. J. H.. Non-CO2 greenhouse gases and climate change. Nature. 476. 7358. 43–50. 10.1038/nature10322. 21814274. 2011. 205225911.