Concerted electron/proton transfer mechanism in the oxidation of phenols by laccase

This study aimed to assess structural requirements in the enzyme/substrate interactions that are responsible for tuning the enzymatic reactivity. To better assess the role of the aspartic residue in the substrate-binding pocket of basidiomycetetype laccases, we compared the catalytic efficiency of wildtype enzymes to that of a mutant in which carboxylic acid residue Asp206 was changed to alanine. Oxidation efficiency towards phenolic substrates by laccases of Trametes villosa, Trametes versicolor and a T. versicolor D206A mutant was studied at two pH values. By the Hammett approach and Marcus analysis, we obtained unambiguous evidence that the oxidation takes place by a concerted electron/proton transfer (EPT) mechanism, and that at pH 5 (optimum pH for enzyme activity) the phenolic proton is transferred to Asp206 during the concerted electron/proton transfer process.