Hydrogenases are subclassified into three different types based on the active site metal content: iron–iron hydrogenase, nickel–iron hydrogenase, and iron hydrogenase.
All hydrogenases catalyze reversible H uptake, but while the [FeFe] and [NiFe] hydrogenases are true redox catalysts, driving H oxidation and H reduction
:H  2 H + 2 e
the [Fe] hydrogenases catalyze the reversible heterolytic cleavage of H.
:H  H + H