<p>This entry represents the large subunit of a family of nitrate reductases found in proteobacteria which are localised to the periplasm. This subunit binds molybdopterin and contains a twin-arginine motif at the N terminus. The protein associates with NapB, a soluble haem-containing protein and NapC, a membrane-bound cytochrome c. The periplasmic nitrate reductases are not involved in the assimilation of nitrogen, and are not directly involved in the formation of electrochemical gradients (i.e. respiration) either. Rather, the purpose of this enzyme is either dissimilatory (i.e. to dispose of excess reductive equivalents) or indirectly respiratory by virtue of the consumption of electrons derived from NADH via the proton translocating NADH dehydrogenase. The enzymes from <taxon tax_id="106590">Ralstonia eutropha</taxon> (Alcaligenes eutrophus) [<cite idref="PUB00013852"/>] and <taxon tax_id="82367">Paracoccus pantotrophus</taxon> (Thiosphaera pantotropha) [<cite idref="PUB00013806"/>] have been characterised. In <taxon tax_id="562">Escherichia coli</taxon> [<cite idref="PUB00013835"/>] (as well as other organisms) this gene is part of a large nitrate reduction operon (napFDAGHBC).</p>