Enzyme electrokinetics: using protein film voltammetry to investigate redox enzymes and their mechanisms

Protein film voltammetry is a relatively new approach to studying redox enzymes, the concept being that a sample of a redox protein is configured as a film on an electrode and probed by a variety of electrochemical techniques. The enzyme molecules are bound at the electrode surface in such a way that there is fast electron transfer and complete retention of the chemistry of the active site that is observed in more conventional experiments. Modulations of the electrode potential or catalytic turnover result in the movement of electrons to, from, and within the enzyme; this is detected as a current that varies in characteristic ways with time and potential. Henceforth, the potential dimension is introduced into enzyme kinetics. The presence of additional intrinsic redox centers for providing fast intramolecular electron transfer between a buried active site and the protein surface is an important factor. Centers which carry out cooperative two-electron transfer, most obviously flavins, produce a particularly sharp signal that allows them to be observed, even as transient states, when spectroscopic methods are not useful. High catalytic activity produces a large amplification of the current, and useful information can be obtained even if the coverage on the electrode is low. Certain enzymes display optimum activity at a particular potential, and this can be both mechanistically informative and physiologically relevant. This paper outlines the principles of protein film voltammetry by discussing some recent results from this laboratory.