This is a second nitrate reductase enzyme which can substitute for the NRA enzyme and allows E.coli to use nitrate as an electron acceptor during anaerobic growth. The gamma chain is a membrane-embedded heme-iron unit resembling cytochrome b, which transfers electrons from quinones to the beta subunit. a quinol + nitrate = a quinone + H2O + nitrite Binds 2 heme groups per subunit. Heme 1 is located at the cytoplasmic interface, heme 2 is located at the periplasmic interface. Electrons are transferred from the periplasmic to the cytoplasmic heme. 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.