Part of a bifunctional enzyme complex that functions as a hydrogen-evolving hydrogenase with sulfur-reducing activity. May play a role in hydrogen cycling during fermentative growth. Activity exhibited with NAD in addition to NADPH. The alpha and delta subunits form the hydrogenase component that catalyzes the reduction of protons to evolve hydrogen. H2 + NADP(+) = H(+) + NADPH H2 + NAD(+) = H(+) + NADH Binds 1 nickel ion per subunit. 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 90 degrees Celsius. Activity increases with increasing temperature from 30 degrees Celsius to 90 degrees Celsius. Has a half-life of 6 hours at 95 degrees Celsius. Dimer of 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.