Members of this superfamily share the structure of the dinitrogenase iron-molybdenum cofactor biosynthesis domain which comprises several Nif (B, X and Y) proteins. These are involved in the biosynthesis of the iron-molybdenum cofactor (FeMo-co) present in the dinitrogenase enzyme of the nitrogenase complex in nitrogen-fixing bacteria. These complex catalyses the reduction of atmospheric dinitrogen to ammonia, and is composed of an iron metalloprotein (dinitrogenase reductase; homodimer of NifH) and a Fe-Mo metalloprotein (dinitrogenase; heterotetramer of NifD and NifK).