Catalytic subunit of the periplasmic nitrate reductase complex NapAB. Receives electrons from NapB and catalyzes the reduction of nitrate to nitrite. 2 Fe(II)-[cytochrome] + 2 H(+) + nitrate = 2 Fe(III)-[cytochrome] + H2O + nitrite Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Interaction in the cytoplasm with the NapD chaperone prevents premature export. Component of the periplasmic nitrate reductase NapAB complex composed of NapA and NapB (PubMed:10234835, PubMed:17130127). Before export to the periplasm, the NapA twin-arginine signal sequence interacts with NapD and NapF (PubMed:17074894, PubMed:17901208, PubMed:22329966, PubMed:24314029, Ref.13). The twin-arginine signal peptide of NapA is structured in its unbound form and undergoes a small but significant conformational change upon interaction with NapD. Exported by the Tat system (PubMed:17218314). The position of the signal peptide cleavage has been experimentally proven (PubMed:10234835). Belongs to the prokaryotic molybdopterin-containing oxidoreductase family. NasA/NapA/NarB subfamily.