The nitrate reductase enzyme complex allows S.flexneri to use nitrate as an electron acceptor during anaerobic growth. The beta chain is an electron transfer unit containing four cysteine clusters involved in the formation of iron-sulfur centers. Electrons are transferred from the gamma chain to the molybdenum cofactor of the alpha subunit. a quinol + nitrate = a quinone + H2O + nitrite Binds 3 [4Fe-4S] clusters per subunit. Binds 1 [3Fe-4S] cluster per subunit. Dimer of heterotrimers each composed of an alpha, a beta and a gamma chain. Alpha and beta are catalytic chains; gamma chains are involved in binding the enzyme complex to the cytoplasmic membrane (By similarity).