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.
), a chemical produced biologically by the enzyme Ammonia monooxygenase.
[1] Crystallographic methods show that HAO (PDB code: 1FGJ) is a cross-linked trimer of polypeptides containing 24 heme cofactors.
[2][3] For many decades the enzyme was thought to catalyze the following reaction:[4] Recent work in the field, however, reveals that this enzyme catalyzes an entirely different reaction:[1] Subsequent oxidation of the nitric oxide to nitrite caused by reaction with oxygen accounts for the reactivity previous described by Hooper et al. Nitric oxide, the product of HAO catalysis, is a potent greenhouse gas.