<p>Autotrophic ammonia, such as <taxon tax_id="915">Nitrosomonas europaea</taxon> oxidising bacteria acquire energy from the oxidation of ammonia to nitrite, and fix carbon dioxide to obtain biomass [<cite idref="PUB00016931"/>]. The respiratory chain in these organisms consists of ammonia monooxygenase (AMO), which oxidises ammonia to hyroxylamine, hydroxylamine oxidoreductase (HAO), which oxidises hydroxylamine to nitrite, and several c-type cytochromes involved in the electron transport. HAO is a key enzyme within this chain as the electrons generated by its reaction are partitioned to AMO, which needs them to generate hydroxylamine, and to the electron transport chain for energy generation.</p> <p>The HAO enzyme from N. europaea (<db_xref db="SWISSPROT" dbkey="Q50925"/>) is a homotrimer which binds a remarkable total of 24 haem groups, aligned to form a ring that has inlet and outlet sites [<cite idref="PUB00024877"/>]. Each monomer binds eight haems; seven c-type haems, and a novel haem designated P460 which has a tyrosine residue attached to the tetrapyrrole ring. The c-type haem irons are all coordinated by two histidine residues, while that of P460 is coordinated by one, with the extra site on the iron being available for substrate binding. The oxidation of hydroxylamine is thought to occur in two, two-electron steps, with the tyrosine attached to P460 allowing it to transiently accept two electrons. Trimerisation is necessary for catalysis, allowing a tyrosine residue from one subunit to cross-link to P460 from a neighbouring subunit, and providing a hydrophobic environment for the haem groups where stable electron transfer can occur.</p> Hydroxylamine oxidase