Nitrite produced in the first step of autotrophic nitrification is oxidized to nitrate by nitrite oxidoreductase (NXR) (). It is a membrane-associated iron-sulfur molybdo protein and is part of an electron transfer chain which channels electrons from nitrite to molecular oxygen. The enzymatic mechanisms involved in nitrite-oxidizing bacteria are less described than that of ammonium oxidation. Recent research (e.g. Woźnica A. et al., 2013) proposes a new hypothetical model of NOB electron transport chain and NXR mechanisms. Here, in contrast to earlier models, the NXR would act on the outside of the plasma membrane and directly contribute to a mechanism of proton gradient generation as postulated by Spieck  and coworkers. Nevertheless, the molecular mechanism of nitrite oxidation is an open question.