Electrochemical investigations of the multicopper oxidase from Aquifex aeolicus under direct electron transfer with carbon electrodes

The study of redox proteins confined over an electrode surface is a powerful approach enabling precise modulation of the driving force for interfacial electron transfer through the control of the electrode potential. In this work, we present the investigation of a protein variant of the multicopper oxidase from Aquifex aeolicus obtained by directed evolution. Motivated by the enhanced performance of the protein variant for the oxidation of organic compounds in solution, the enzyme is studied under direct electron transfer with the electrode. A marked dependence of the redox potential with the pH of the electrolyte solution is observed, which is further confirmed by studies under non-turnover conditions. Further investigations show the need for an initial reductive activation to attain maximum bioelectrocatalytic activity for O2 reduction, in agreement with similar observations made for other multicopper oxidases (MCOs). Moreover, the influence of chloride ions on the bioelectrocatalytic reaction is described. The O2-reduction activity is barely affected at KCl concentrations as high as 100 mM. However, a shift in the O2-reduction potential towards more negative values is observed at low pH, explaining the apparent loss in activity for the oxidation of organic substrates in the presence of KCl. The obtained results contribute to a more detailed understanding of the electrochemical properties of metallo-oxidases and other MCOs.