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 reversible arrow H-2) 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(COb)(mu-SRS)Fe-d(CO)(CN-)L}, where the monovalence [H] stands for the [4Fe4S](H)(2+) subcluster bridged to the [FeFe](H) moiety, (CH3S) represents a Cys-S, and (COb) represents a bridging CO. L could be a CO, H2O, H-, H-2, 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 H-2-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 (mu-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 eta(2)-H-2 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 H-2 formation/splitting is reminiscent of the mechanism of [NiFe] hydrogenases and therefore would unify the mechanisms of the two related enzymes.