Page 223 - Advances in Textile Biotechnology

204 Advances in textile biotechnology

So far, most of the work on wool fibres has been done by using the micro-
bial TGase from S. mobaraensis. A new stable form of microbial TGase
from Streptomyces hygroscopicus WSH03-13 was recently obtained (Cui
et al., 2006). This enzyme was used for wool treatment, the aim being to
monitor its effect on shrink resistance, tensile strength (Du et al., 2007) and
dyeing properties with natural dyes (Cui et al., 2008). In contrast to the S.
mobaraensis TGase, this novel microbial TGase was almost ineffective

towards intact wool fibres because properties such as tensile strength and

area shrinkage were not influenced by enzyme treatment. This result was
attributed to the fact that the external hydrophobic layer of intact wool
fibres limited the accessibility of target reactive sites. On the other hand,

oxidative, reductive, or proteolytic pretreatments opened up the fi bres for
the penetration of the enzyme, which could catalyse the formation of cova-
lent crosslinks. As a result, tensile strength and area shrinkage were
improved to different extents, depending on the kind of pre-treatment, thus
confirming the ability of microbial TGase to repair chemical or biological

damages of wool fibres. Crosslinking of gelatine onto the surface of wool
fibres via microbial TGase-mediated reaction resulted in further improve-

ment of mechanical and antifelting properties of wool (Cui et al., 2009).

Good antimicrobial properties were also imparted on wool fibres by graft-
ing ε-polylysine through microbial TGase-catalysed reaction (Jin et al.,
2009).

The good reactivity of TGases towards wool fibres can be explained in
terms of amount and accessibility of Gln residues at the fi bre surface
(cuticle) and within the fibre texture (CMC and cortex). The microbial

TGase from S. mobaraensis has a relatively small size (about 38 kDa), not
too far from that of bacterial proteases, whose ability to diffuse inside wool
fibres has been reported (Silva et al., 2005). The results of TGase-catalysed

functionalisation of wool suggest that the enzyme is able to diffuse inside
the fibre matrix and to reach the Gln residues, whose accessibility can be

further enhanced by treatments able to lower the hydrophobic barrier of

intact wool fibres. In fact, cuticle cells consist of lamellar components, whose
thin outermost layer (epicuticle) is resistant to acids, alkalis, oxidising
agents, and to enzymatic attack. The epicuticle is formed by heavily cross-
linked proteins covalently bound to 18-methyleicosanoic acid through a
thioester bond (Negri et al., 1993). This assembly makes the fi bre surface
strongly hydrophobic and hardly accessible to reagents and enzymes. Thus,
accessibility of target amino acid residues to enzyme molecules sometimes
might be a real challenge if suitable pre-treatment aimed at decreasing the

hydrophobic barrier and opening the fibre surface are not performed.
No results have yet been published on the modifi cation and/or function-

alisation of silk fibres with TGase. The total amount of Gln+Glu in silk
−1

fibroin is about 180 μmol g (∼1.2 mol%) and the amount of Gln can be

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