The Mechanism of Laccase‐Catalyzed Oxidations: Kinetic Evidence for the Involvement of Several Electron‐Accepting Sites in the Enzyme

Abstract

The anaerobic reduction of fungal laccase by quinol and ferrocyanide has been studied in a stopped‐flow apparatus by measurements of the change in the absorbancy at 610 mμ associated with the reduction of the so‐called Type 1 Cu 2+ . With ferrocyanide the rate of formation of product was also measured. The reoxidation of reduced laccase by molecular oxygen has been studied by the same technique. The kinetics of reduction and reoxidation as well as of product formation has also been investigated in a series of experiments with both the reducing substrate and oxygen present. In addition, the effect of an inhibitor, fluoride, on the kinetics has been studied. The results confirm earlier static experiments showing that laccase contains two electronaccepting sites other than Type 1 and Type 2 Cu 2+ . Type 1 Cu 2+ is reduced rapidly enough to involve it in the catalytic reaction. However, it appears to be rapidly reoxidized only when the other electron‐accepting sites are also occupied. A mechanism involving the cooperation of several electron‐accepting sites is consistent with the kinetic results.