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.