NxrAB

NxrAB is the two-subunit nitrite oxidoreductase complex found in nitrite-oxidizing bacteria. It catalyzes the oxidation of nitrite to nitrate, a key step in aerobic nitrification, and transfers electrons from the nitrite oxidation reaction into the bacterial electron transport chain, contributing to the proton motive force used for energy conservation.

The complex is composed of two main subunits, NxrA and NxrB. NxrA is the catalytic subunit and contains a molybdenum-containing cofactor at its active site, which facilitates the oxidation of nitrite. NxrB serves as the electron-transfer subunit, typically containing iron-sulfur clusters and heme groups that relay electrons from NxrA to downstream electron carriers such as cytochromes and quinones in the membrane.

Localization and organization can vary among nitrite-oxidizing bacteria, but NXR is commonly associated with the membrane or periplasmic space, enabling interaction with the electron transport chain. In some species, nxrA and nxrB are found in an operon that may include additional subunits (such as nxrC) or accessory components that support assembly and electron transfer.

Ecologically, nxrAB genes are widespread among nitrite-oxidizing bacteria, including genera such as Nitrobacter, Nitrococcus, and Nitrospira. Regulation of nxrAB expression is generally linked to nitrite availability, with transcription upregulated in the presence of nitrite to meet the cellular demand for nitrite oxidation. The NXR complex is a central component of the microbial nitrogen cycle, influencing nitrite and nitrate fluxes in soils, sediments, and aquatic systems.