Formate dehydrogenase allows the bacterium to use formate as major electron donor during anaerobic respiration, when nitrate is used as electron acceptor. Subunit gamma is the cytochrome b556 component of the formate dehydrogenase-N, and also contains a menaquinone reduction site that receives electrons from the beta subunit (FdnH), through its hemes. Formate dehydrogenase-N is part of a system that generates proton motive force, together with the dissimilatory nitrate reductase (Nar) (By similarity). 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. Trimer of heterotrimers, consisting of subunits alpha, beta and gamma. Belongs to the formate dehydrogenase gamma subunit family.