Hydroxylamine oxidoreductase (HAO) is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

found in the prokaryote ''

Nitrosomonas europaea ''Nitrosomonas europaea'' is a Gram-negative obligate chemolithoautotroph that can derive all its energy and reductant for growth from the oxidation of ammonia to nitrite and lives in several places such as soil, sewage, freshwater, the walls of ...

.'' It plays a critically important role in the biogeochemical nitrogen cycle as part of the metabolism of

ammonia-oxidizing bacteria Nitrifying bacteria are chemolithotrophic organisms that include species of genera such as ''Nitrosomonas'', '' Nitrosococcus'', ''Nitrobacter'', '' Nitrospina'', ''Nitrospira'' and '' Nitrococcus''. These bacteria get their energy from the oxidati ...

. The substrate is

hydroxylamine Hydroxylamine is an inorganic compound with the formula . The material is a white crystalline, hygroscopic compound.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Professional Publishing Ltd. pp. 431–43 ...

(NH2OH), a chemical produced biologically by the enzyme Ammonia monooxygenase. The products of the

catalyzed Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

reaction are debated, but recent work shows compelling evidence for the production of

nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...

Structural studies

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

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consis ...

cofactors.

Reactivity

For many decades the enzyme was thought to catalyze the following reaction: :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 A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...

. Additionally, the oxidized product of nitric oxide in the presence of oxygen is

nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

- a common pollutant in agricultural run-off.

References

* * * EC 1.7.3 Heme enzymes Enzymes of known structure {{1.7-enzyme-stub