Page 238 - Advances in Textile Biotechnology

Functionalisation of wool and silk fi bres using enzymes 219

lignins and aryl diamines as well as some inorganic ions. In the presence of
small molecules capable of acting as electron transfer mediators, laccases
are also able to oxidise non-phenolic structures or substrates otherwise not
accessible to the enzyme, thus expanding the range of compounds that can
be oxidised by these enzymes. Laccases have received much attention for
their application in several biotechnological processes. Such applications

include the detoxification of industrial effluents, bioremediation of soil,
medical diagnostics, manufacture of anti-cancer drugs, and as ingredients in
cosmetics (Rodríguez Couto and Toca Herrera, 2006).
Peroxidases (hydrogen-peroxide oxidoreductase, EC 1.11.1.7) have been
discovered in various plants, microbes, and mammals (Floché and Ursini,
2008). The plant peroxidase from horseradish (HRP) is one of the best
characterised enzymes in this group of oxidoreductases, which has found a
wide range of biosensing and biotechnological applications, including the
peroxidase-catalysed polyphenol synthesis (Ayyagari et al., 1996). HRP is a
hemoprotein of about 44 kDa containing an atom of iron in the protopor-
phyrin IX prosthetic group. It uses hydrogen peroxide (H 2 O 2 ) as electron
acceptor for the oxidation of a wide range of substrates. The generally
accepted reaction mechanism of HRP involves the formation of two oxida-
tion states of the Fe-containing prosthetic group, i.e. compound I and II
(Fig. 9.4). The catalytic cycle is initiated by the rapid oxygen transfer from
a peroxide to the resting ferric state of the enzyme (two electron oxidation),
resulting in the formation of compound I intermediate state. The enzyme
regains its resting state by passing through two consecutive one-electron
reduction steps, with the formation of the intermediate compound II (one
electron reduction of compound I). In the presence of excess H 2 O 2 , HRP
is reversibly transformed into compound III, an inactive form of the enzyme
(Bagger and Williams, 1971). Electrons are extracted from aromatic
substrates, which form reactive radical species that usually undergo non-
enzymatic radical–radical coupling. Formation of dityrosine by in vitro
HRP-catalysed oxidation of free Tyr and Tyr-containing peptides and pro-
teins in the presence of H 2 O 2 , has been reported (Michon et al., 1997).
Current applications of laccases and peroxidases in textile processing
mostly refer to laccase-mediated indigo decolorisation in denim fi nishing
and to removal of reactive dyes by means of a chemo-enzymatic peroxi-
dase-based technology (Schäfer et al., 2007). Both enzymatic systems are
commercially available. Several enzymatic techniques for dyeing based on
the use of laccase and peroxidase redox enzymes have been tested, but not
yet commercialised. Worth mentioning are the studies on decolorisation of
textile and other industrially important dyes from wastewater as an alterna-
tive strategy to conventional chemical, physical and biological treatments
(Husain, 2006). The potential of peroxidases, laccases, and other oxidases is
under investigation. The addition of certain redox mediators enhances the

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