Degradation of Azo Dyes by White-Rot Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 319

Fig. 7 Catalytic cycle of laccase enzyme (Kurniawati and Nicell 2007 ; reprinted with permission
from Elsevier Ltd., UK.)

Laccases oxidise different compounds, such as phenols, polyphenols and aromatic
amines with the concomitant reduction of molecular oxygen to water. Their broad
substrate spectrum can be additionally extended by the presence of the so-called redox
mediators (Bourbonnais et al. 1995 ; Johannes and Majcherczyk 2000 ). The basis of
the laccase mediator system (LMS) is the use of low-molecular weight compounds
that are oxidised by laccase to organic radicals or positively charged intermediates,
which, in turn, act as redox mediators (Morozova et al. 2007 ). These radicals or
charged intermediates are capable of oxidising compounds that are not substrates of
laccases because of either their large size or their particular high redox potential.
The catalytic cycle of laccase has been shown in Fig. 7 . In this cycle, the copper
centre at the catalytic site of laccase donates electrons to oxygen, leaving the
laccase enzyme in an oxidised state. In typical interactions of laccase with a sub-
strate, the catalytic site of laccase abstracts electrons from the substrate and releases
an oxidised product. When a mediator is present, the mediator is oxidised by
laccase and further oxidises another compound that is either a substrate or a non-
substrate of laccase, resulting in the formation of oxidised product(s) and the
mediator regeneration (Banci et al. 1999 ). Examples of laccase mediators are 2,2 -

azinobis-(3-ethylbenzthiazoline-6-sulphonate (ABTS), 1-hydroxybenzotriazole
(HBT), and 3-hydroxyanthranilate (3-HAA), of which the last one is found to occur
naturally in the cultures of Pycnoporus cinnabarinus (Eggert et al. 1996 ; Morozova
et al. 2007 ). Also some phenolic lignin precursors and degradation products might
act as laccase mediators in nature (Camarero et al. 2005 ).

′

5 Microbial Toxicity of Dyes and Their Degradation
Products

The acute toxicity of azo dyes, according to the criteria of the European Union for
the classi

cation of dangerous substances, is low and the values of 50 % lethal dose
(LD 50 ) are 250

2,000 mg kg − 1 body weight (Clarke and Anliker 1980 ). However,

-