Degradation of Anthroquinone Dyes Stimulated by Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 339

Resting peroxidase þ H 2 O 2 ! Compound I þ H 2 O

AH : Compound I þ AH 2 ! Compound II þ

AH : Compound II þ AH 2 ! Resting peroxidase þ

2AH : ! A 2 H 2 or AH 2 þ A non - enzymatic reaction ð Þ

The above reactions summarize the general catalytic cycle of peroxidases. The
term resting peroxidase indicates the peroxidase resting state and AH 2 and AH are
substrate molecules and the corresponding radical species, respectively.
This is the key step in heme peroxidase catalysis. After proton rearrangement by
the distal base catalytic residue, two electrons are transferred from heme to
hydrogen peroxide, which turns enzyme active, called Compound I (Poulos and
Kraut 1980 ). B indicates a basic residue, i.e., a histidine or aspartate/glutamate in
peroxidases. L indicates the proximal heme ligand, usually a histidine residue
(Strittmatter et al. 2013 ).

4 Mechanisms of Fungal Dye Degradation
and Decolorization

In comparison to physical and chemical methods, biological treatment is often found
to be economical and eco-friendly alternative. Biological methods, such as microbial
decolorization and degradation, adsorption by living or dead microbial biomass, are
commonly used for the treatment of industrial ef

uents (Fu and Viraraghavan 2001 ;

McMullan et al. 2001 ).
In the decolorization and degradation of dyes by fungal strains, the chemicals are
adsorbed on the microbial biomass. This process is known as biosorption which
takes place on both living and dead biomass. Dead microbial biomass contains a
natural polysaccharide chitin and its derivative chitosan in their cell walls which has
a unique molecular structure with a high af nity for many classes of dyes. Hence,

microbial biomass can be used as an ef cient absorbent for synthetic dyes (Joshi

et al. 2004 ).
During decolorization and degradation of dyes by fungal strains, at least two or
three highly non-speci

c enzymes, like lignin (LiP), manganese peroxidase (MnP)
and laccase (Lac), are induced which generate free radicals in a variety of reactions
(Knapp et al. 2001 ; Pointing 2001 ). The degradation of dyes generally depends on
their chemical structure which strongly in uences their degradability by pure cul-

tures and isolated enzymes.
The fungal action rarely leads to the mineralization of dyes, depending on their
chemical structures. It was also observed that dyes containing substituted aromatic
rings in their structure were mineralized faster than unsubstituted rings. Nitrogen-
limited conditions also mediate faster mineralization of dyes. A few researchers
have also reported that the dyes can be utilized as a carbon source by the fungal