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Hydroxylamine oxidoreductase
From Wikipedia, the free encyclopedia
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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 (), 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]
![{\displaystyle {\mathrm {NH} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {OH} }}](http://wikimedia.org/api/rest_v1/media/math/render/svg/65571fd607792c1ecbac0dd871633642ea63a5de)
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Structural studies
Crystallographic methods show that HAO (PDB code: 1FGJ) is a cross-linked trimer of polypeptides containing 24 heme cofactors.[2][3]
Reactivity
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For many decades the enzyme was thought to catalyze the following reaction:[4]
![{\displaystyle {\mathrm {NH} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {OH} {}+{}\mathrm {H} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {O} {}\mathrel {\longrightarrow } {}\mathrm {NO} {\vphantom {A}}_{\smash[{t}]{2}}{\vphantom {A}}^{-}{}+{}5\,\mathrm {H} {\vphantom {A}}^{+}{}+{}4\,\mathrm {e} {\vphantom {A}}^{-}}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/07d7e8a5eaa1bc1dd6bf7787170e89a9c5b6bc14)
Recent work in the field, however, reveals that this enzyme catalyzes an entirely different reaction:[1]
![{\displaystyle {\mathrm {NH} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {OH} {}\mathrel {\longrightarrow } {}\mathrm {NO} {}+{}3\,\mathrm {H} {\vphantom {A}}^{+}{}+{}3\,\mathrm {e} {\vphantom {A}}^{-}}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/f4901376128c81306fd806e0662f5f569eb1a9e4)
Subsequent oxidation of the nitric oxide to nitrite caused by reaction with oxygen accounts for the reactivity previous described by Hooper et al.
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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
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