Formate dehydrogenase allows the bacterium to use formate as major electron donor during anaerobic respiration, when nitrate is used as electron acceptor. The beta subunit FdnH is an electron transfer unit containing 4 iron-sulfur clusters; it serves as a conduit for electrons that are transferred from the formate oxidation site in the alpha subunit (FdnG) to the menaquinone associated with the gamma subunit (FdnI) of formate dehydrogenase-N. Formate dehydrogenase-N is part of a system that generates proton motive force, together with the dissimilatory nitrate reductase (Nar) (By similarity). Binds 4 [4Fe-4S] clusters per subunit. Trimer of heterotrimers, consisting of subunits alpha, beta and gamma. By nitrate under anaerobic conditions.