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. Component of the periplasmic nitrate reductase NapAB complex composed of NapA and NapB. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family. NasA/NapA/NarB subfamily.