Degradation of Anthroquinone Dyes Stimulated by Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 336

1 mM HBT was found to be suitable to decolorize RB-5 (50 mg l − 1 )by62and
77.4 % within 1 and 2 h, respectively by the crude laccase (25 Um l − 1 ) where RBBR
(50 mg l − l ) was decolorized by 90 % within 20 h. However, laccase was found to be
more ef cient in presence of HBT, showing 92 % decolorization within 2 h.

Hadibarata et al. ( 2011 ) have also reported decolorization of anthraquinone
RBBR dye by laccase enzyme extracted from white rot fungus Polyporus sp., both
in the presence and absence of a redox mediator HBT. It was observed that laccase
individually could also decolorize dye but in the presence of HBT, the decolor-
ization was increased by 20 %.
Several workers have reported use of HBT as a laccase mediator and the reason
for selection is that the dyes with NH 2 and OH groups in their structure create steric
hinderance and reduce the accessibility of laccases. Thus, these dyes are more
degradable by laccase attack (Soares et al. 2001 ; Camarero et al. 2005 ; Rodriguez-
Couto 2007 ). The af nity of the oxidized mediator for the substrate reaction is an

important factor for the ef ciency of a laccase-mediator system.

3.4 Dye-Decolorizing Peroxidase (DyP)

Dye decolorizing peroxidase, a heme-containing enzyme, is broadly present in
plants, microorganisms and animals (Duarte-Vazquez et al. 2003 ). The metabolism
of aromatic dyes is mediated either by precipitation or by opening the aromatic ring
structure. So, this group of enzyme has opened new prospects for the development of
biotechnological processes aimed at the degradation of xenobiotic compounds (Field
et al. 1993 ), ef uent decolorization (Banat et al. 1996 ) and bio bleaching of Kraft

pulp (Moreira et al. 1997 ). DyP, isolated from fungus Thanatephorus cucumeris,isa
glycoprotein novel extracellular peroxidase and contains one heme as a co-factor.
This enzyme has molecular mass of 58 kDa and essentially requires H 2 O 2 for all
enzyme reactions. This clearly indicates that it works as a peroxidase. It functions
well under lower pH (3-3.2) conditions and shows no homology to other peroxi-
dases. Besides, it has broad substrate speci city, as DyP degrades not only the

typical peroxidase substrates, but also degrades hydroxyl-free anthraquinone, which
is not a substrate of other peroxidases (Kim and Shoda 1999 ; Sugano et al. 2000 ,
2006 ). Most of the synthetic dyes are derived from anthraquinone compounds.
Hence, DyP is a promising enzyme for the treatment of the dye-contaminated water,
because it degrades synthetic dyes very ef ciently and effectively (Kim and Shoda

1999 ; Sugano et al. 2000 ; Shakeri et al. 2007 ).
So far, degradation of anthraquinone by peroxidases is found to be dif cult, as it

contains no hydrogen atoms that can be withdrawn by peroxidase. In fact, no model
related to in vitro degradation of anthraquinone dyes by peroxidase has been
published. Two peroxidases, namely pseudoperoxidase and horseradish peroxidase
have been found which directly or indirectly mediate the degradation of anthra-
quinone. Oxidative degradation of a hydroxyl anthraquinone compound is mediated
by pseudoperoxidase activity offerrylmyoglobin (Cartoni et al. 2004 ), while