Subunits alpha and gamma of HoxS constitute an NADH--oxidoreductase. H2 + NAD(+) = H(+) + NADH Binds 1 FMN. Binds 1 [4Fe-4S] cluster. Tetramer of an alpha and a gamma subunits (flavin-containing dimer), and a delta and a nickel-containing beta subunit (hydrogenase dimer). Belongs to the complex I 51 kDa subunit family.