Kinetic control by the substrate and/or the cosubstrate in electrochemically monitored redox enzymatic homogeneous systems. Catalytic responses in cyclic voltammetry

The catalytic response of a homogeneous redox enzymatic system connected to the electrode by a freely diffusing mediator (cosubstrate) may be kinetically controlled by the substrate and/or the cosubstrate. How the electrochemical responses are related to the rate constants, to the concentrations of enzyme, substrate and cosubstrate and to the scan rate, is systematically analyzed in the framework of cyclic voltammetry. Because of its frequent occurrence in practice, emphasis is put on the case of a fast enzymatic process, as compared to the diffusion of the cosubstrate, provision being made for Michaelis–Menten behavior for both substrate and cosubstrate. Within this framework, two situations of particular interest are discussed, namely the case of a negligible consumption of the substrate in the reaction layer and the opposite case where the consumption of the substrate is so important that its diffusion toward the electrode controls the current. In the first case, plateau-shaped responses, independent of scan rate, are obtained. The plateau current is not proportional to the substrate concentration but, at best, to its square root. In the second case, a peak-shaped curve is obtained with a peak current proportional to the substrate concentration and square root of the scan rate. The combination of these two regimes accounts for a biphasic variation of the electrochemical signal with the substrate concentration. The relationships between the electrochemical responses and the kinetic characteristics of the enzymatic reaction form the bases of procedures for ascertaining the mechanism and measuring the key rate constants. In this connection, strategies for determining Michaelis–Menten characteristics of both the substrate and cosubstrate reactions are discussed.