Direct Electrochemistry

The more common solution tests of enzyme activity provide a simple explanation of the direct electrochemistry approach. In homogeneous enzyme kinetics, the enzyme, its substrate, and a redox partner—whose absorbance depends on the redox state of the substrate and serves as a source or sink of electrons for the substrate's redox transformation—can all be combined in a cuvette (note that we call the substrate the molecule that the enzyme transforms into a product, and not a solid material, as in the language of surface science). By monitoring the change in the solution's absorbance, it is possible to calculate the rates of substrate and cosubstrate transformations in the steady state, which are equivalent to the enzyme's turnover rate. The homogeneous catalytic process that takes place in the majority of the electrochemical cell in mediated electrochemistry is fundamentally the same as that in solution assays. The consumption of the redox partner is detected as a current wave as a result of its electrochemical recycling on the electrode. When the electrode potential is correct, electrons go from the substrate to the electrode via the enzyme's active site, and the amount of current flow is just proportional to how quickly the substrate is being turned over. Interfacial ET should ideally be quick and the electrode should rotate quickly to eliminate mass transport control, allowing the current response to directly reflect the inherent features of the enzyme.