The NAD+-reducing hydrogenase (soluble hydrogenase, SH) creates a NADH-reducing equivalence by oxidizing hydrogen gas. The SH is a heterohexameric protein with two subunits making up the large and small subunits of the hydrogenase and the other two subunits comprising a reductase module similar to the one of Complex I. The [Ni-Fe active site oxidized hydrogen gas which transfers electrons to a FMN-a cofactor, then to a [Fe-S] cluster relay of the small hydrogenase subunit and the reductase module, then to another FMN-b cofactor and finally to NAD+. The reducing equivalences are then used for fixing carbon dioxide when "C. necator" is growing autotrophically.