Catalytic Electron Transport in Chromatium vinosum [NiFe]-Hydrogenase: Application of Voltammetry in Detecting Redox-Active Centers and Establishing That Hydrogen Oxidation Is Very Fast Even at Potentials Close to the Reversible H+/H2 Value

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Abstract
The nickel−iron hydrogenase from Chromatium vinosum adsorbs at a pyrolytic graphite edge-plane (PGE) electrode and catalyzes rapid interconversion of H+(aq) and H2 at potentials expected for the half-cell reaction 2H+ ⇄ H2, i.e., without the need for overpotentials. The voltammetry mirrors characteristics determined by conventional methods, while affording the capabilities for exquisite control and measurement of potential-dependent activities and substrate−product mass transport. Oxidation of H2 is extremely rapid; at 10% partial pressure H2, mass transport control persists even at the highest electrode rotation rates. The turnover number for H2 oxidation lies in the range of 1500−9000 s-1 at 30 °C (pH 5−8), which is significantly higher than that observed using methylene blue as the electron acceptor. By contrast, proton reduction is slower and controlled by processes occurring in the enzyme. Carbon monoxide, which binds reversibly to the NiFe site in the active form, inhibits electrocatalysis and allows improved definition of signals that can be attributed to the reversible (non-turnover) oxidation and reduction of redox centers. One signal, at −30 mV vs SHE (pH 7.0, 30 °C), is assigned to the [3Fe-4S]+/0 cluster on the basis of potentiometric measurements. The second, at −301 mV and having a 1.5−2.5-fold greater amplitude, is tentatively assigned to the two [4Fe-4S]2+/+ clusters with similar reduction potentials. No other redox couples are observed, suggesting that these two sets of centers are the only ones in CO-inhibited hydrogenase capable of undergoing simple rapid cycling of their redox states. With the buried NiFe active site very unlikely to undergo direct electron exchange with the electrode, at least one and more likely each of the three iron−sulfur clusters must serve as relay sites. The fact that H2 oxidation is rapid even at potentials nearly 300 mV more negative than the reduction potential of the [3Fe-4S]+/0 cluster shows that its singularly high equilibrium reduction potential does not compromise catalytic efficiency.

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