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 bi otechnological interest such as methanogenic, a acetogenic, nitrogen f ixing, photosynthetic or sulfate-reducing bacteria. The X-ray structur e of a dimeric [NiFe] hydrogenase together with a wealth of biophysica l, biochemical and genetic studies have revealed that the large subuni t contains the bimetallic [Ni-Fe] active site, with biologically uncom mon 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 hyd rogen molecule whereas the iron atom could be redox active. Specific c hannels are probably required for the transfer of the chemical reactio n partners (H-2, H+ and e(-)) between the active site, deeply buried i nside the protein, and the molecular surface. The generation of a func tional 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.