Page 101 - Advances in Textile Biotechnology

82 Advances in textile biotechnology

Laccase
Laccase (EC 1.10.3.2) is a copper-containing oxireductase found in white

rot fungi. Laccases are non-specific enzymes, oxidizing primarily diphenol
groups using molecular oxygen as an electron acceptor. The application of
laccases has been extensively studied to increase strength of paper and
board via lignin bonding, but it also has applications in the textile industry
in the bleaching of denim and in wastewater decoloration. Laccases are
often used in combination with a mediator such as 1-hydroxybenzotriazole
(HBT) or 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS).
The purpose of the mediator is twofold: (i) owing to their size (65–70 kDa)
laccases cannot penetrate deeply into fi bres, smaller mediators do not nec-
essarily have this limitation, and (ii) because of their low redox potential
(0.5–0.8 V) laccases are not able to oxidize compounds with high redox
potentials (larger than 1.5 V). Several studies (e.g. Miettinen-Oinonen et al.,
2002, 2004) reported increased hydrophilicity of PET treated with laccases
that caused surface oxidation of the material. However, Liu and co-workers
hypothesised that the improved hydrophilicity when treating PET with a
laccase from Streptomyces coelicolor in combination with a mediator
(Denilite II Assist) and a non-ionic surfactant (Triton X-100) might to a
large extent be caused by protein adsorption on the surface. After a pro-
tease treatment to hydrolyse adsorbed laccase, and a thorough rinsing proc-
esses to remove the proteins (laccase) from the surface, the hydrophilicity
decreased dramatically (wetting time using a water drop test was 165 s for
the laccase treated sample compared with 170 s for the sample treated with
surfactant only, before protease treatment and rinsing the wetting time was
20 s) and almost lost the gain in hydrophilicity resulting from the treatment.
The marginal improvement in hydrophilicity and the relatively high laccase

concentration are significant hurdles in the actual application of laccases in

modification of PET. Laccase is able to modify surfaces with aromatic
groups; however, no real evidence for successful surface modifi cation of
PET has been presented so far.

Cutinase
In contrast to laccases that oxidize the PET surface, lipases, polyesterases,
and cutinases increase hydrophilicity by actual hydrolysis of PET (Fig. 4.1).
Lipases, polyesterases, and cutinases are all carboxylic ester hydrolases.
Cutinases from Fusarium solani pisi, Fusarium oxysporum and Thermobi-
fi da fusca are most frequently studied in enzymatic hydrolysis or modifi ca-
tion of PET. The structure and properties of cutinases are well described
(Carvalho et al., 1998, 1999). In contrast to lipases, cutinases do not require
interfacial activation, the active site is accessible because it does not have

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