Figure 5.

Ammonia (NH₃) oxidation by pMMO and fate of the reaction products. pMMO fortuitously oxidises ammonia to hydroxylamine. Subsequently, hydroxylamine oxidoreductase (HAO) oxidises hydroxylamine to nitric oxide, donating electrons to an unknown cytochrome c protein (indicated by the red diamond). Nitric oxide can be reduced to inert N₂O by the nitric oxide reductase (NOR), using an unknown electron donor. Alternatively, nitric oxide is converted chemically to nitrite in the presence of oxygen, or by an unknown enzyme (red arrow). Under anaerobic conditions, nitrite could be utilised as alternative electron acceptor. The nitrite reductase (NIR) could then reduce nitrite to nitric oxide, which is subsequently reduced to N₂O. P: periplasm; C: cytoplasm.