[NiFe]-hydrogenases: spectroscopic and electrochemical definition of reactions and intermediates

Production and usage of di-hydrogen, H-2, in micro-organisms is catalysed by highly active, 'ancient' metalloenzymes known as hydrogenases. Based on the number and identity of metal atoms in their active sites, hydrogenases fall into three main classes, [NiFe]-, [FeFe]- and [Fe]-. All contain the unusual ligand CO (and in most cases CN- as well) making them intriguing examples of 'organometallic' cofactors. These ligands render the active sites superbly 'visible' using infrared spectroscopy, which complements the use of electron paramagnetic resonance spectroscopy in studying mechanisms and identifying intermediates. Hydrogenases are becoming a focus of attention for research into future energy technologies, not only H-2 production but also H-2 oxidation in fuel cells. Hydrogenases immobilized on electrodes exhibit high electrocatalytic activity, providing not only an important new technique for their investigation, but also a basis for novel fuel cells either using the enzyme itself, or inspired synthetic catalysts. Favourable comparisons have been made with platinum electrocatalysts, an advantage of enzymes being their specificity for H-2 and tolerance of CO. A challenge for exploiting hydrogenases is their sensitivity to O-2, but some organisms are known to produce enzymes that overcome this problem by subtle alterations of the active site and gas access channels.