Proton Inventory and Dynamics in the Nia-S to Nia-C Transition of a [NiFe] Hydrogenase
2016
Greene, Brandon L. | Wu, Chang-Hao | Vansuch, Gregory E. | Adams, Michael W. W. | Dyer, R Brian
Hydrogenases (H₂ases) represent one of the most striking examples of biological proton-coupled electron transfer (PCET) chemistry, functioning in facile proton reduction and H₂ oxidation involving long-range proton and electron transport. Spectroscopic and electrochemical studies of the [NiFe] H₂ases have identified several catalytic intermediates, but the details of their interconversion are still a matter of debate. Here we use steady state and time-resolved infrared spectroscopy, sensitive to the CO ligand of the active site iron, as a probe of the proton inventory as well as electron and proton transfer dynamics in the soluble hydrogenase I from Pyrococcus furiosus. Subtle shifts in infrared signatures associated with the Niₐ-C and Niₐ-S states as a function of pH revealed an acid–base equilibrium associated with an ionizable amino acid near the active site. Protonation of this residue was found to correlate with the photoproduct distribution that results from hydride photolysis of the Niₐ-C state, in which one of the two photoproduct states becomes inaccessible at low pH. Additionally, the ability to generate Niₐ-S via PCET from Niₐ-C was weakened at low pH, suggesting prior protonation of the proton acceptor. Kinetic and thermodynamic analysis of electron and proton transfer with respect to the various proton inventories was utilized to develop a chemical model for reversible hydride oxidation involving two intermediates differing in their hydrogen bonding character.
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