Nitrite oxidoreductase is an enzyme involved in nitrification

Nitrification

Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil...

. It is the last step in the process of aerobic ammonia oxidation, which is carried out by two groups of nitrifying bacteria

Nitrifying bacteria

Nitrifying bacteria are chemoautotrophic or chemolithotrophs depending on the genera bacteria that grow by consuming inorganic nitrogen compounds...

: ammonia oxidizers such as Nitrosospira, Nitrosomonas

Nitrosomonas

Nitrosomonas is a genus comprising rod shaped chemoautotrophic bacteria.This rare bacteria oxidizes ammonia into nitrite as a metabolic process. Nitrosomonas are useful in treatment of industrial and sewage waste and in the process of bioremediation. They are important in the nitrogen cycle by...

and Nitrosococcus convert ammonia to nitrite, while nitrite oxidizers such as Nitrobacter

Nitrobacter

Nitrobacter is genus of mostly rod-shaped, gram-negative, and chemoautotrophic bacteria.Nitrobacter plays an important role in the nitrogen cycle by oxidizing nitrite into nitrate in soil...

and Nitrospira oxidize nitrite to nitrate.

The enzyme is bound to the inner cytoplasmic surface of the bacterial membrane and contains multiple subunits, iron-sulfur centers and a molybdenum

Molybdenum

Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

cofactor.