The respiratory membrane-bound nitrate reductase enzyme complex plays a role in generation of metabolic energy by using nitrate as a terminal electron acceptor during anaerobic conditions. Proposed membrane-embedded heme-iron subunit involved in a protonmotive Q-cycle mechanism-based electron transfer to the beta subunit. Binds 2 heme groups per subunit. Probable multiprotein complex; a catalytic heterodimer of an alpha and beta chain is proposed to associate with additional subunits involved in membrane attachment and electron transfer. Proposed to confer antimycin sensitivity to the respiratory nitrate reductase. Belongs to the cytochrome b family.