Proton Transport in <i>Clostridium pasteurianum</i> [FeFe] Hydrogenase I: A Computational Study

To better understand the proton transport through the H₂ production catalysts, the [FeFe] hydrogenases, we have undertaken a modeling and simulation study of the proton transfer processes mediated by amino acid side-chain residues in hydrogenase I from <i>Clostridium pasteurianum</i>. Free-energy calculation studies show that the side chains of two conserved glutamate residues, Glu-279 and Glu-282, each possess two stable conformations with energies that are sensitive to protonation state. Coordinated conformational changes of these residues can form a proton shuttle between the surface Glu-282 and Cys-299, which is the penultimate proton donor to the catalytic H-cluster. Calculated acid dissociation constants are consistent with a proton relay connecting the H-cluster to the bulk solution. The complete proton-transport process from the surface-disposed Glu-282 to Cys-299 is studied using coupled semiempirical quantum-mechanical/classical-mechanical dynamics. Two-dimensional free-energy maps show the mechanisms of proton transport, which involve Glu-279, Ser-319, and a short internal water relay to connect functionally Glu-282 with the H-cluster. The findings of conformational bistability, PT event coupling with p<i>K</i>_a mismatch, and water participation have implications in the design of artificial water reduction or general electrocatalytic H₂-production catalysts