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. May transfer electrons to the iron-sulfur centers of the catalytic beta subunit. Binds 1 heme b (iron(II)-protoporphyrin IX) group 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.