Enzymatic Mechanism of Fe-Only Hydrogenase:  Density Functional Study on H−H Making/Breaking at the Diiron Cluster with Concerted Proton and Electron Transfers

The mechanism of the enzymatic hydrogen bond forming/breaking (2H⁺ + 2e H₂) and the plausible charge and spin states of the catalytic diiron subcluster [FeFe]_H of the H cluster in Fe-only hydrogenases are probed computationally by the density functional theory. It is found that the active center [FeFe]_H can be rationally simulated as {[H](CH₃S)(CO)(CN^-)Fe_p(CO_b)(μ-SRS)Fe_d(CO)(CN^-)L}, where the monovalence [H] stands for the [4Fe4S]_H²⁺ subcluster bridged to the [FeFe]_H moiety, (CH₃S) represents a Cys−S, and (CO_b) represents a bridging CO. L could be a CO, H₂O, H^-, H₂, or a vacant coordination site on Fe_d. Model structures of possible redox states are optimized and compared with the X-ray crystallographic structures and FTIR experimental data. On the basis of the optimal structures, we study the most favorable path of concerted proton transfer and electron transfer in H₂-forming/breaking reactions at [FeFe]_H. Previous mechanisms derived from quantum chemical computations of Fe-only hydrogenases (Cao, Z.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734; Fan, H.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3828) involved an unidentified bridging residue (μ-SRS), which is either a propanedithiolate or dithiomethylamine. Our proposed mechanism, however, does not require such a ligand but makes use of a shuttle of oxidation states of the iron atoms and a reaction site between the two iron atoms. Therefore, the hydride H_b^- (bridged to Fe_p and Fe_d) and η²-H₂ at Fe_p or Fe_d most possibly play key roles in the dihydrogen reversible oxidation at the [FeFe]_H active center. This suggested way of H₂ formation/splitting is reminiscent of the mechanism of [NiFe] hydrogenases and therefore would unify the mechanisms of the two related enzymes.