Effect of a Dispersion of Interfacial Electron Transfer Rates on Steady State Catalytic Electron Transport in NiFe-hydrogenase and Other Enzymes

Redox enzymes can be adsorbed onto electrode surfaces such that there is a rapid and efficient direct electron transfer (ET) between the electrode and the enzyme's active site, along with high catalytic activity. In an idealized way, this may be analogous to protein−protein ET or, more significantly, the nonrigid interface between different domains of membrane-bound enzymes. The catalytic current that is obtained when substrate is added to the solution is directly proportional to the enzyme's turnover rate and its dependence on the electrode potential reports on the energetics and kinetics of the entire catalytic cycle. Although the current is expected to reach a limiting value as the electrode potential is varied to increase the driving force, a residual slope in voltammograms is often observed. This slope is significant, as it is unexpected from all simple considerations of electrochemical kinetics. A particularly remarkable result is obtained in experiments carried out with the NiFe-hydrogenase from Allochromatium vinosum: this enzyme displays high catalytic activity for hydrogen oxidation and is easily studied up to 60 °C, at which temperature the current−potential response becomes completely linear over a range of more than 0.5 V. The explanation for this effect is that the enzyme molecules are not adsorbed in a homogeneous manner but vary in their degree of ET coupling with the electrode, i.e., through there being many slightly different orientations. Under conditions in which interfacial ET becomes rate-limiting, i.e., when turnover number is high at elevated temperatures, the current−potential response reflects the superposition of numerous electrochemical rate constants. This is highly relevant in the interpretation of the catalytic electrochemistry of enzymes.