Nitritoxidoreduktase

Nitritoxidoreduktase, or nitrite oxidoreductase (NXR), is a multi-subunit enzyme complex in nitrite-oxidizing bacteria that catalyzes the oxidation of nitrite to nitrate, the second step of aerobic nitrification. The overall reaction is NO2- + H2O → NO3- + 2 H+ + 2 e-, with the released electrons feeding the respiratory chain and ultimately reducing O2 to water.

In many nitrite-oxidizing bacteria, NXR is located in the periplasm and functions as a membrane-associated complex. Core components encoded by nxrA and nxrB harbor the molybdenum-containing active site that performs nitrite oxidation, and a cytochrome-containing subunit (nxrC) or an associated electron-transfer chain accepts electrons from the active site and directs them into the quinone pool or to periplasmic electron carriers.

Genetic and structural analyses show that nxrA, nxrB, and nxrC form the canonical gene cluster for nitrite oxidation in diverse nitrite-oxidizing bacteria such as Nitrospira, Nitrobacter, Nitrococcus, and related genera. The presence of nxr genes is widely used as a molecular marker for nitrite-oxidizing potential in environmental samples.

Ecologically, NXR enables chemolithoautotrophic growth by coupling energy from nitrite oxidation to carbon fixation, supporting nitrification in soils, sediments, and aquatic systems. It also plays a central role in wastewater treatment and global nitrogen cycling, influencing nitrate production and nitrous oxide emissions under varying environmental conditions.