Laccase-catalyzed decolorization of synthetic dyes

Commercial dyes are not uniformly susceptible to microbial attack in conventional aerobic treatment because of their unique and stable chemical structures. Three synthetic dyes with typical chromophores (anthraquinone, azo and indigo) were decolorized by a white-rot fungus Trametes versicolor. The responsible enzyme for dye decomposition was laccase, an extracellular oxidase released by the fungus under the conditions of slow growth or in its stationary phase. The mechanism of laccase-catalyzed dye decomposition, however, was different depending on dye structures. Anthraquinone dye was an enzyme substrate that was directly oxidized by laccase while decolorization of azo and indigo dyes involved some small molecule (<8 kDa) metabolites. It was demonstrated that azo and indigo dyes were not the substrates of laccase and the small molecule metabolites mediated the interaction between the dyes and the enzyme. The decolorization rate of the nonsubstrate dyes was actually limited by the concentration of mediating compounds rather than laccase activity in the solutions. Some synthetic compounds such as 2,2′-azino-bis(3-ethylthiazoline-6-sulfonate) or ABTS and anthraquinone dye could also mediate the decolorization of azo and indigo dyes. The mediating function of ABTS and anthraquinone dye was quantitatively compared in the decomposition of two nonsubstrate dyes. This fact implies that the laccase-substrate dyes in an industrial effluent can promote the decolorization of those nonsubstrate dyes. Effluent decolorization, therefore, may not be limited by the small molecule metabolites which are not produced in large amount by fungus in most industrial effluents. A laccase-catalyzed and mediator-involved dye degradation mechanism is proposed for further kinetic studies.