Subunits alpha and gamma of HoxS constitute an NADH--oxidoreductase. H2 + NAD(+) = H(+) + NADH Binds 1 [2Fe-2S] cluster. Binds 2 [4Fe-4S] clusters per subunit. Tetramer of an alpha and a gamma subunits (flavin-containing dimer), and a delta and a nickel-containing beta subunits (hydrogenase dimer). Belongs to the complex I 75 kDa subunit family.