2FRV

Hydrogenases are enzymes that catalyse the reversible heterolytic dissociation of molecular hydrogen H₂ to two protons and two electrons. The largest class of hydrogenases contains a NiFe center that is believed to be the catalytic site for hydrogen activation.The active site comprises a heterobimetallic cluster of Ni and Fe atoms. The bridging ligand X is proposed to be sulphur species due to the release of H₂S during the course of the reaction.

Several alternative mechanisms exist for hydrogen activation by transition metal complexes. Generally these reactions proceed through either oxidative addition or sigma-bond metathesis (which is usually described as homolytic and without strong charge polarisation unless R is a polar group). Hydrogen isotope exchange and computational experiments for this enzyme indicate that the H-H cleavage reaction is heterolytic [PMID:11703120]. Cytochrome c3, the physiological electron acceptor for this particular hydrogenase, is a tetraheme cytochrome. The hemes all have different potentials, and probably only one or two are likely to accept electrons from the enzyme under physiological conditions.