Hydrogenase: A hydrogen-metabolizing enzyme. What do the crystal structures tell us about its mode of action?

Hydrogenases are proteins which metabolize the most simple of chemical compounds, molecular hydrogen, according to the reaction H-2 <--> 2H(+) + 2e(-). These enzymes are found in many microorganisms of great biotechnological interest such as methanogenic, a acetogenic, nitrogen fixing, photosynthetic or sulfate-reducing bacteria. The X-ray structure of a dimeric [NiFe] hydrogenase together with a wealth of biophysical, biochemical and genetic studies have revealed that the large subunit contains the bimetallic [Ni-Fe] active site, with biologically uncommon CO and CN ligands to the iron, whereas the small subunit contains three iron-sulfur clusters. During catalysis, the nickel atom is most likely responsible for a base-assisted heterolytic cleavage of the hydrogen molecule whereas the iron atom could be redox active. Specific channels are probably required for the transfer of the chemical reaction partners (H-2, H+ and e(-)) between the active site, deeply buried inside the protein, and the molecular surface. The generation of a functional enzyme, including the assembly of the complex catalytic center, requires maturation and involves a large number of auxiliary proteins which have been partly characterized by molecular biology.