Environmental Engineering Reference
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100
80
60
40
20
0
AQ
dyes
Azo
dyes
Fig. 1 Decolorization of several of anthraquinonic (AQ) and azo dyes after 24 h of reaction in the
absence of redox mediators by using CotA-laccase (adapted from Pereira et al. 2009b )
Two major differences were observed when compared to dye degradation using
fungal laccases: (1) the non requirement of redox mediators and (2) a maximal
activity at the neutral to alkaline range of pH. The lack of a strict requirement for
redox mediators exhibited by bacterial CotA-laccase constitutes a signi
cant
advantage over fungal enzymes from a technological perspective. Low-molecular
weight compounds are expensive and a large quantity in relation to the substrate is
often required. Moreover, some mediators give rise to highly unstable compounds
that can lead to enzyme inactivation and are toxic upon release into natural envi-
ronments. The requirement of redox mediators, acting as electron shuttles, is usu-
ally justi
ed to overcome the steric hindrance of substrates that impairs its proper
approach to the enzyme
s catalytic center or the high redox potential of the sub-
strates in comparison to the enzyme (Bourbonnais and Paice 1990 ). Interestingly, it
was observed that CotA, a low redox laccase (E
'
= 525 mV vs. NHE), is able to
degrade high redox compounds, e.g. the azo dye reactive black 5 (E
º
= 742 mV) to
a higher extent in the absence of redox mediators, in contrast to what was observed
with high-redox potential fungal laccases (E
º
780 mV vs. NHE) which requires
the presence of redox mediators (Abadulla et al. 2000 ; Zille et al. 2004 ; Camarero
et al. 2005 ; Tauber et al. 2005 ). This indicates that redox potential is not the only or
the most important parameter to be considered in what concerns substrate oxidation
by laccases (Dur
º *
ã
o et al. 2006 ).
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