Hydrogen would be an ideal reductant, especially from an environmental point of view as it does not yield any waste-products. The NAD + reducing hydrogenase from Ralstonia eutropha (soluble hydrogenase, SH) uses molecular hydrogen (H2) to reduce NAD + to NADH and consists of a hydrogenase module (HoxHY) with the H2 converting active site and a diaphorase module (HoxFUI2) with the NAD + binding site (Fig. ). H2 is oxidized at the NiFe active site with three biological untypical CN -and CO ligands. The electrons are transferred via the FMN-a close to active site within the hydrogenase module and Fe-S cluster chain to the second FMN-b at the NAD + binding site. FMN-b serves here as a mediator between the one electron centers (Fe-S clusters) and the two-electron acceptor NAD + . The flavins of the SH are non-covalently bound.