Part of a bifunctional enzyme complex that functions as an NADPH-dependent hydrogen-evolving hydrogenase with sulfur reducing activity. May play a role in hydrogen cycling during fermentative growth. Activity not exhibited with NAD. The alpha and delta subunits form the hydrogenase component that catalyzes the reduction of protons to evolve hydrogen. H2 + NADP(+) = H(+) + NADPH Binds 1 nickel ion per tetramer. Binds 2 [4Fe-4S] clusters. Binds 1 [3Fe-4S] cluster. Measured for the whole complex. Optimum pH is 8 for hydrogenase activity at >95 degrees Celsius. Optimum temperature is greater than 95 degrees Celsius for hydrogenase activity. Stable up to 100 degrees Celsius. Loses 50% of activity at 80 degrees Celsius after 21 hour incubation. Heterotetramer of alpha, beta, gamma and delta subunits. The nickel-containing alpha and delta subunits constitute the hydrogenase activity. The beta and gamma subunits (flavin-containing dimer) constitute the sulfur reductase activity. Belongs to the [NiFe]/[NiFeSe] hydrogenase small subunit family.