Use of Fungal Laccases and Peroxidases for Enzymatic Treatment of Wastewater Containing Synthetic Dyes - Green Chemistry for Dyes Removal from Wastewater - page 225

where AH 2 and AH· represent a substrate and a free radical product,
respectively. The oxidized substrate (AH·) undergoes a variety of sponta-
neous reactions. Each of the nonspecific peroxidase enzymes has different
substrate specificity and unique characteristics.

6.5.1.1 Lignin Peroxidase (LiP) and Manganese-Dependent

Peroxidase (MnP)

Two archetypical ligninolytic fungal peroxidases, LiP [1,2-bis(3,4-dime-
thoxyphenyl)propane-1,3-diol:hydrogen-peroxide oxidoreductase; EC
1.11.1.14] and MnP [Mn(II):hydrogen-peroxide oxidoreductase; EC
1.11.1.13], are class II peroxidases in the non-animal peroxidase group
that have been discovered and isolated from extracellular culture medium
of P. chrysosporium [101-103] and many other white rot fungi. Six iso-
zymes of LiP and four isozymes of MnP were purified and sequenced from
the extracellular fluid of P. chrysosporium . hese P. chrysosporium LiP and
MnP isozymes share 43% amino acid sequence identity and have very
similar active site structures around the heme [104,105]. The published
crystal structures of LiP and MnP from P. chrysosporium are presented in
Figure 6.12 [104,106].
The characteristic reactions catalyzed by LiP include nonstereospecific
C α -C β cleavage and β- O -4 cleavage in lignin model dimers, aromatic ring
opening, oxidation of benzyl alcohols such as veratryl alcohol to corre-
sponding aldehydes or ketones and hydroxylation of benzylic methylene
groups [103,107,108]. It is known that veratryl alcohol, a secondary metab-
olite of white rot fungi, acts as a mediator between LiP and polymeric sub-
stances that are less accessible to the enzyme's active site [109,110].
As the name suggests, MnP requires Mn(II) ion, as well as hydrogen
peroxide, for its catalytic activity. The enzyme catalyzes the oxidation of
Mn(II) to Mn(III) which in turn oxidizes various lignin-related organic
compounds, including vanillylacetone, 2,6-dimethyloxyphenol, curcumin,
syringic acid, guaiacol, syringaldazine, divanillyacetone, and coniferyl
alcohol [111-113]. Glenn et al. [111] noted that the complex of Mn(III)
with α-hydroxy acid, such as lactate, worked as a diffusive intermediate in
the oxidative reactions. It is also known that MnP also has an oxidase activ-
ity to generate hydrogen peroxide [113].

6.5.1.2 Versatile Peroxidase and Dye-Decolorizing

Peroxidase (DyP)

Since the discovery of LiP and MnP in P. chrysosporium in the early
1980s, the activity of these ligninolytic enzymes has been found in the