Effects of axial ligand mutation of the type I copper site in bilirubin oxidase on direct electron transfer-type bioelectrocatalytic reduction of dioxygen

Direct electron transfer (DET)-type electrocatalytic properties were examined for a site-directed mutant (M467Q) of Myrothecium verrucaria bilirubin oxidase (BOD), in which Met467 as the axial ligand of the type I copper site is replaced by Gln. Although the enzymatic activity of the mutant is very low toward bilirubin [A. Shimizu, et al., J. Biochem. 125 (1999) 662–668], it works as a good catalyst in DET-type bioelectrocatalytic reduction of dioxygen into water. The wave appears around the formal potential of the type I Cu site, which is about 0.23 V more negative than that of the wild-type recombinant (rBOD), and the catalytic limiting current density is larger than that of rBOD. Detailed analysis of the voltammograms and the enzyme kinetics suggests that the electrons are transferred from electrodes to the type I Cu site and that the mutation does not injure the kinetics of the three processes: the heterogeneous electron transfer, the intramolecular electron transfer from the type I Cu site to the trinuclear type II–III Cu cluster, and the catalytic process to reduce dioxygen at the type II–III Cu cluster. The overall catalytic constant is increased by the mutation. This result can be interpreted in terms of the increased Gibbs energy difference in the electron transfer reaction from the type I Cu site to the type II–III Cu cluster.