Laccases are multicopper phenol oxidases that oxidize phenols and aromatic amines. Rather than H2O2, these enzymes utilize O2 as an oxidant, reducing it by four electrons to H2O (Call and Mucke 1997). LACs are generally larger than peroxidases, having molecular weights of approximately 60kDa and above (Call and Mucke 1997). As with other extracellular enzymes, LACs are glycosylated.
Laccase oxidizes phenolic lignin model compounds directly (Kawai et al. 1988). Although it is a phenoloxidase and its redox potential is too low to directly oxidize the nonphenolic components of lignin, it has been shown to degrade lignin efficiently in the white rot Pycnoporus cinnabarinus, which does not produce MnP or LIP (Eggert et al. 1997). To overcome the redox potential barrier, P. cinnabarinus produces a metabolite, 3-hydroxyanthranilate that can mediate the oxidation of nonphenolic substrates by LAC (Eggert et al. 1996). It is believed that natural mediators, such as 3-hydroxyanthranillic acid in the white rot P. cinnabarinus act as diffusable lignin-oxidizing agents. In the presence of the mediators 3-hydroxyanthranillic acid, hydroxybenzotria-zole, or 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS), LAC has been shown to oxidize nonphenolic lignin, VA, and PAH (Bourbonnais et al. 1995; Collins et al. 1996; Eggert et al. 1996; Majcherczyk et al. 1998; 1999).
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