Nitrification is a process of nitrogen compound oxidation (effectively, loss of electrons from the nitrogen atom to the oxygen atoms), and is catalyzed step-wise by a series of enzymes.
: (Nitrosomonas, Comammox)
: (Nitrobacter, Nitrospira, Comammox)
OR
In Nitrosomonas europaea, the first step of oxidation (ammonia to hydroxylamine) is carried out by the enzyme ammonia monooxygenase (AMO).
The second step (hydroxylamine to nitrite) is catalyzed by two enzymes. Hydroxylamine oxidoreductase (HAO), converts hydroxylamine to nitric oxide.
Another currently unknown enzyme converts nitric oxide to nitrite.
The third step (nitrite to nitrate) is completed in a distinct organism.