 The large subunit is where the [Ni-Fe] active site resides and the small subunit is composed of [Fe-S] clusters.However, the hydrogenases of "C. necator" are different from typical [Ni-Fe] hydrogenases because they are tolerant to oxygen and are not inhibited by CO.While the four hydrogenases perform the same reaction in the cell, each hydrogenase is linked to a different cellular process.