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. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family. NasA/NapA/NarB subfamily.