Direct Electrochemistry of CueO and Its Mutants at Residues to and Near Type I Cu for Oxygen-Reducing Biocathode

CueO, a multi-copper oxidase (MCO) occurring in Escherichia coli, catalyses a four-electron reduction of O-2 in a direct electron transfer (DET) mechanism with very high electrocatalytic activity on carbon aerogel electrodes. However, the overpotential of CueO is greater than that in other MCOs. By understanding the redox properties of CueO, we attempted to reduce this overpotential. Direct electrochemistry of CueO on carbon aerogel electrodes showed a pair of redox waves derived from the type I (T1) Cu site with a redox potential (E-T1(o)') of 0.28 V versus Ag vertical bar AgCl at pH 5.0. Dependence of E-T1(o)', on pH suggests the participation of proton transfer and acid-base equilibrium of some amino acid residue. The shape of the catalytic current is consistent with the T1 site being an inlet of electrons in the DET bioelectrocatalysis of O-2, in which case the overpotential could be reduced by shifting E-T1(o)' towards the positive potential. To achieve this, we created mutants of CueO at M510, which is the axial ligand of the T1 Cu, and at D439, which forms a hydrogen bond with His443 coordinated with the T1 Cu. Two mutants, M510L and D439A, successfully reduced the overpotential.