Role of oxidizing mediators and tryptophan 172 in the decoloration of industrial dyes by the versatile peroxidase from Bjerkandera adusta

A purified preparation of the versatile peroxidase from Bjerkandera adusta was tested for industrial dye decoloration. This ligninolytic enzyme efficiently oxidizes Mn(II) to Mn(III) and also exhibits Mn(II)-independent activity in the oxidation of aromatic substrates. It was able to decolorize 27 of the 41 industrial dyes tested. The presence of manganese in the reaction mixture did not enhance the decoloration rate, in fact for some dyes the transformation rate was inhibited. On the other hand, the presence of mediator molecules in the reaction mixture generally enhanced the decoloration of dyes that showed low activity without mediators, while the dyes that were decolorized rapidly without mediator were unaffected by the presence of veratryl alcohol, acetosyringone or TEMPO as oxidizing mediators. The role of tryptophan 172 on dye decoloration was investigated. Chemical modification of tryptophan residues using N-bromosuccinimide drastically reduced Mn(II)-independent activity and dye decoloration, while the Mn(II)-dependent activity was maintained. The loss of Mn(II)-independent and decoloration activities after tryptophan modification was not reversed by the addition of mediators. These results strongly suggest that the tryptophan 172 is involved in both mediator oxidation and dye decoloration activities, and in Mn(II)-independent activity.