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, called the proximal or heme Bp in, is located at the cytoplasmic interface, heme 2, called the distal or heme Bd, is located at the periplasmic interface. Electrons are transferred from the periplasmic to the cytoplasmic heme.