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 beta and gamma subunits form the sulfur reducing component that catalyzes the cytoplasmic production of hydrogen sulfide in the presence of elemental sulfur. Not active in the presence of sodium sulfate, sodium sulfite, sodium thiosulfate or cysteine. H2 + n sulfur = H(+) + hydrogen sulfide + (n-1) sulfur Binds 1 FAD per subunit. Binds 1 [2Fe-2S] cluster. Stimulated by rubredoxin at pH 7.6 but not ferredoxin. Measured for the whole complex. Optimum pH is 8.4 for sulfur reductase activity at 80 degrees Celsius. Optimum temperature is 80 degrees Celsius for maximal sulfur reductase activity. Activity increases linearly from above 30 degrees Celsius to reach maximal levels at 80 degrees Celsius and then decreases to 15% of activity at 90 degrees Celsius. 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. Truncated N-terminus.