Nitrate reductases, NapA (Nitrate-R-NapA), NasA, and NarB catalyze the reduction of nitrate to nitrite. Monomeric Nas is located in the cytoplasm and participates in nitrogen assimilation. Dimeric Nap is located in the periplasm and is coupled to quinol oxidation via a membrane-anchored tetraheme cytochrome. Members of the MopB_Nitrate-R-NapA CD belong to the molybdopterin_binding (MopB) superfamily of proteins.