Steady-state kinetics of cyclic conversions of substrate in amperometric bienzyme sensors

A mathematical model has been developed for amperometric bienzyme sensors in whose coating the assayed substrate undergoes cyclic conversions according to the Michaelis-Menten kinetic mechanism, the sensors being able to detect the product of substrate or intermediate regeneration. When the rate of enzymatic conversions exceeds the rate of external mass transfer (this is the most favorable situation with respect to practical application of biosensors), the steady-state kinetics of the biosensors are satisfactorily described by the Michaelis-Menten equation. Equations are given for the dependences of the effective maximum current density and the effective Michaelis constant on the physicochemical characteristics of the layer of immobilized enzymes and the external membrane of the biosensors. The dependences are used for the description of the behavior of the biosensors in a solution of intermediate. It is shown that the value of the dimensionless diffusion modulus for one of the enzymes can be calculated from the measurement of the maximum sensitivity of biosensor to substrate and intermediate.