This iron-iron protein is part of the nitrogenase complex that catalyzes the key enzymatic reactions in nitrogen fixation. Other nitrogenase complexes utilize a molybdenum-iron protein or a vanadium-iron protein. 16 ATP + 16 H2O + N2 + 8 reduced [2Fe-2S]-[ferredoxin] = 16 ADP + 6 H(+) + H2 + 2 NH4(+) + 8 oxidized [2Fe-2S]-[ferredoxin] + 16 phosphate Binds 1 [8Fe-7S] cluster per heterodimer. Binds 1 [8Fe-9S-C-homocitryl] cluster per subunit. Hexamer of two alpha, two beta, and two delta chains. The structure of the 8Fe-9S-C-homocitryl cluster is assumed to be analogous to the 7Fe-Mo-9S-C-homocitryl cluster. Belongs to the NifD/NifK/NifE/NifN family.