Enzyme Kinetics via Open Circuit Potentiometry

We demonstrate the application of open circuit potentiometry (OCP) to measure enzyme turnover kinetics, kappa(turn). The electrode surface will become poised by the addition of a well-behaved redox pair, such as ferrocenemethanol/ferrocenium methanol (FcMeOH/FcMeOH(+)), which acts as the cosubstrate for the enzymatic process. A measurable change in potential results when an enzyme consumes the one-electron transfer mediator. Glucose oxidase was studied as a test-case, but the method is generalizable across oxidoreductase enzymes that rely on electron transfer mediators. In the presence of glucose and FcMeOH(+), glucose oxidase delivers electrons to FcMeOH(+), and the potential changes with respect to the Nernst equation. A theoretical model incorporating enzymatic rate expressions into the Nernst equation was derived to explain the observed potential transients, and experimental data fit theory well. A similar experiment was performed using amperometry on ultramicroelectrodes (UMEs). Here, the same enzymatic rate expression may be incorporated into the equation for steady-state flux to an UME to obtain kappa(turn). While similar kinetic information was obtained from the potentiometric and amperometric responses, potentiometry is independent of electrode size and mass transfer effects. Finally, we show how kappa(turn) changes as a function of one-electron mediator. Our results may eventually find applications to biosensors, where electrode fouling plagues long-term sensor performance.