X-ray Crystal Structure of the Fe-Only Hydrogenase (CpI) from Clostridium pasteurianum to 1.8Angstrom Resolution

A three-dimensional structure for the monomeric iron-containing hydrogenase (CpI) from Clostridium pasteurianum was determined to 1.8 angstrom resolution by x-ray crystallography using multiwavelength anomalous dispersion (MAD) phasing. CpI, an enzyme that catalyzes the two-electron reduction of two protons to yield dihydrogen, was found to contain 20 gram atoms of iron per mole of protein, arranged into five distinct Fe-S clusters. The probable active-site cluster, previously termed the H-cluster, was found to be an unexpected arrangement of six iron atoms existing as a 4Fe-4S cubane subcluster covalently bridged by a cysteinate thiol to a 2Fe subcluster. The iron atoms of the 2Fe subcluster both exist with an octahedral coordination geometry and are bridged to each other by three non-protein atoms, assigned as two sulfide atoms and one carbonyl or cyanide molecule. This structure provides insights into the mechanism of biological hydrogen activation and has broader implications for Fe-S cluster structure and function in biological systems.