Identification of a Catalytic Iron-Hydride at the H-Cluster of FeFe-Hydrogenase

Hydrogenases couple electrochemical potential to the reversible chemical transformation of H 2 and protons, yet the reaction mechanism and composition of intermediates are not fully understood. In this Communication we describe the biophysical properties of a hydride-bound state (H hyd ) of the FeFe-hydrogenase from Chlamydomonas reinhardtii . The catalytic H-cluster of FeFe-hydrogenase consists of a 4Fe-4S subcluster (4Fe-4S H ) linked by a cysteine thiol to an azadithiolate-bridged 2Fe subcluster (2Fe H ) with CO and CN – ligands. Mössbauer analysis and density functional theory (DFT) calculations show that H hyd consists of a reduced 4Fe-4S H + coupled to a diferrous 2Fe H with a terminally bound Fe-hydride. The existence of the Fe-hydride in H hyd was demonstrated by an unusually low Mössbauer isomer shift of the distal Fe of the 2Fe H subcluster. A DFT model of H hyd shows that the Fe-hydride is part of a H-bonding network with the nearby bridging azadithiolate to facilitate fast proton exchange and catalytic turnover.