a menaquinone + H2 = a menaquinol Binds 1 [3Fe-4S] cluster. Binds 2 [4Fe-4S] clusters. Heterodimer of a large and a small subunit. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has been experimentally proven. Belongs to the [NiFe]/[NiFeSe] hydrogenase small subunit family.