Page 164 - Materials Chemistry, Second Edition

Fungal Treatment of Pharmaceuticals in Effluents 145

bacteria typically break down the organic compounds needed for carbon and/or
energy sources using enzymes with relatively high substrate specificity, fungi are
often more flexible in their ability to act on pollutants. The typically smaller size
of the bacterial cells also makes them less sensitive to mechanical disturbance and
shear stress than fungal hyphae (Harms et al., 2011).
Most of the fungi that have been documented to either metabolize or bind phar-
maceutical pollutants are from two fungal phyla, Ascomycota (ascomycetes) and
Basidiomycota (basidiomycetes) (Harms et al., 2011). This includes a few single-
celled yeast genera in the Ascomycota, but mostly involves filamentous fungi form-
ing hyphae. The ability of cell walls or other components of some fungi to biosorb
pollutants such as heavy metals (Bishnoi, 2005; Harms et al., 2011; Siddiquee et al.,
2015) or dyes (Kabbout and Taha, 2014; Yagub et al., 2014; Rybczyńska-Tkaczyk
and Korniłłowicz-Kowalska, 2016; Lu et al., 2017) makes them useful in biosorp-
tion remediation of some pharmaceutical wastes. Mycoremediation of many phar-
maceutical wastes depends on fungal enzymes. The WRF in the Basidiomycota have
received perhaps the most attention due to the non-specificity and efficacy of their
extracellular enzymes to break down a variety of organic compounds (Harms et al.,
2011; Kües, 2015; Tortella et al., 2015).

8.3.2 fungal enzyMes
Some fungi produce a variety of enzymes that can chemically alter and some-
times even mineralize a variety of naturally occurring recalcitrant molecules,
such as lignin, and many synthetic xenobiotics. Fungal groups that have evolved
to interact with lignocellulose, soil humus, and/or the defense mechanisms of
green plants seem to be most equipped with these enzymes (Harms et al., 2011;
Kües, 2015). Many of them produce a variety of extracellular oxidoreductases
that are relatively nonspecific regarding which organic substrates they can bind
and act on. WRF have received much attention (Tišma et al., 2010; Marco-Urrea
and Reddy, 2012; Asif et al., 2017) and are especially well equipped, producing
a variety of extracellular enzymes, including several lignin-modifying class II
heme peroxidases (lignin peroxidase [LiP], manganese peroxidase [MnP], and
versatile peroxidase [VP]), as well as dye-decolorizing peroxidases, all being
active at acidic pH. Peroxidases require hydrogen peroxide (H O ) for enzymatic
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activity (Harms et al., 2011; Kües, 2015).
The lignin peroxidases, ranging in molecular mass from 37 to 50 kDa (Hirai
et al., 2005; Asgher et al., 2007, 2008, 2006), are secreted during the secondary
metabolism of WRF, and are capable of mineralizing a variety of recalcitrant sub-
stances, including aromatic compounds and lignin (Shrivastava et al., 2005). The pH
and temperature activity profiles of LiPs depend on the enzyme’s source; however,
optimum activities have been reported between pH 2 and 5 and between 35 and 55°C
(Yang, 2004; Asgher et al., 2007). Veratryl alcohol (VA) and 2-chloro-1,4-dime-
thoxybenzene (fungal secondary metabolites) stimulate the LiP-catalyzed oxidation
of substrates, acting as redox mediators. The LiP oxidation rate is dependent on the
molar ratio of H O to the pollutant. While low concentrations of H O activate the
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Phanerochaete chrysosporium LiP (Pc-LiP), higher concentrations rapidly inhibit