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Figure 6.13 Crystal structure of fungal laccase from Trametes versicolor [124] (RCSB PDB
Entry: 1GYC).
redox potential of laccase enzymes did not seem to be suitable for the oxi-
dation of non-phenolic lignin structures [125]. In addition, not all white
rot fungi produce laccases. Various mechanisms have been discovered
to mediate the lignin degradation by laccases, such as quinone reduc-
tion by glucose oxidase [126], the laccase-MnP system in Rigidoporus
lignosus [127], and the laccase-veratryl alcohol oxidase interaction in
P.   o s t r e a t u s [128]. Bourbonnais and Paice [129] discovered the small-
molecule redox mediator system, such as [2,2'-azino-bis(3-ethylbenzothi-
azoline-6-sulfonic acid); ABTS], that can overcome the barrier of redox
potential for oxidizing non-phenolic lignin structures. Some white rot
fungi also produce metabolites that act as a mediator like ABTS, such as
3-hydroxyanthranilate produced by Pycnoporus cinnabarinus [130]. he
use of this mediator system (see Figure 6.14 for the chemical structures of
common mediators) has become a very popular technique for the degra-
dation of xenobotics, including PAHs, pesticides, and synthetic dyes, by
fungal laccases [15,131-135].
6.6
Enzymatic Treatment of Synthetic Dyes
Virtually all of the fungal decolorization studies reviewed above clearly
indicate the importance of ligninolytic enzymes, especially laccase and
MnP, in decolorization of synthetic dyes. Thus, it is natural to consider
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