Reliable estimation of the kinetic parameters of redox enzymes by taking into account mass transport towards rotating electrodes in protein film voltammetry experiments

Abstract

In Protein Film Voltammetry, a redox enzyme is immobilized on a rotating electrode in a configuration allowing fast, direct electron transfer. This technique is used to probe the mechanism of enzymes by quantitatively interpreting the response in current as a function of the experimental parameters. Limitation by mass transport of the substrate towards the electrode may obscure important features and complicate the analysis of the enzymatic response, all the more that the enzyme has high activity. In this work, we derive equations taking into account mass transport of substrate, for the steady-state current generated by an enzyme following Michaelis-Menten kinetics and immobilized onto a hydrodynamic (e.g. rotating) electrode. We use these equations to model the current response of films of CO-dehydrogenase, a metalloenzyme that catalyzes the oxidation of CO, to transient exposures to its gaseous substrate. We show that neglecting transport yields poor fits and overestimated, unreliable, values of Km (even when using the Koutecky-Levich approximation), whereas taking into account transport yields much better fits and more reliable parameters. We interpret new and previously published data by taking into account transport limitations. We propose a simple method using QSoas (http://qsoas.org) to analyze data.