Electrocatalytic mechanism of reversible hydrogen cycling by enzymes and distinctions between the major classes of hydrogenases.

The extraordinary ability of Fe- and Ni-containing enzymes to catalyze rapid and efficient H(+)/H(2) interconversion--a property otherwise exclusive to platinum metals--has been investigated in a series of experiments combining variable-temperature protein film voltammetry with mathematical modeling. The results highlight important differences between the catalytic performance of [FeFe]-hydrogenases and [NiFe]-hydrogenases and justify a simple model for reversible catalytic electron flow in enzymes and electrocatalysts that should be widely applicable in fields as diverse as electrochemistry, catalysis, and bioenergetics. The active site of [FeFe]-hydrogenases, an intricate Fe-carbonyl complex known as the "H cluster," emerges as a supreme catalyst.