Direct Electrochemistry

The more common solution tests of enzyme activity make it simple to understand how direct electrochemistry works. In homogeneous enzyme kinetics, the enzyme, its substrate, and a redox partner may all be combined in a cuvette. The redox partner's absorbance is dependent on the redox state of the substrate and acts as either a source or a sink of electrons for the substrate's redox transformation (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). The rates of substrate and co*substrate transformations are equal to the enzyme turnover rate in the steady state, therefore they may be calculated by monitoring the solution's change in absorbance. Only the mediator interacts with the electrode in mediated electrochemistry, and the homogeneous catalytic process that takes place in the majority of the electrochemical cell is fundamentally the same as that in solution assays. The consumption of the redox partner is detected as a current wave resulting from 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 the rate of turnover. The electrode should spin quickly during interfacial ET in order to eliminate mass transport control and have the current response directly reflect the intrinsic features of the enzyme.