Page 233 - Advances in Textile Biotechnology

214 Advances in textile biotechnology

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polypeptides). The amount of Tyr in whole wool fibres is 366 μmol g
−1
(∼4 mol%). The Tyr content of cuticle, CMC, and cortex is 292 μmol g
−1
−1
(∼2.9 mol%), 571 μmol g (∼6.2 mol%), and 389 μmol g (∼4.2 mol%),
respectively (Lindley, 1977). The relatively higher content of Tyr in CMC is
mainly caused by the presence of the high glycine–tyrosine keratin fraction.
Considering the relative proportion of cuticle, CMC, and cortex in the wool
fibre, it can be calculated that about 30 μmol of Tyr are present in the cuticle,

and almost the same amount is contained in the CMC. Cuticle can be rea-
sonably considered to be the morphological fraction more exposed to enzy-
matic attack, but also CMC is a potential path for enzyme diffusion because
it is a lightly crosslinked continuous phase that provides channels by which
various molecular species can diffuse into and within wool fi bres (Brady,
1992). The remaining Tyr residues (about 300 μmol) are contained in the
cortical cells.
The amount of Tyr residues does not seem to be a limitation for the
reactivity of tyrosinase towards wool, because when wool hydrolysates were
incubated with tyrosinase under homogeneous reaction conditions, the
tyrosinase-catalysed oxidation proceeded steadily (Jus et al., 2009; Lantto
et al., 2005b). However, under heterogeneous conditions, by using wool

fibres as substrate, a range of chemical, morphological, and structural factors
are likely to impose restrictions to the effective mass transfer of the enzyme

from solution to fibres and to its diffusion inside the fibre matrix. Provided
that adsorption and diffusion occur, the target keratin-bound Tyr residues
must be located in exposed chain segments endowed with suffi cient fl exibil-
ity to properly fit the enzyme’s active site. For wool, enzymes should fi rst

overcome the external hydrophobic barrier characteristic of intact fi bres
(epicuticle) before reaching the cuticle- and/or CMC-bound Tyr residues.
The possibility of modifying the Tyr residue in untreated wool fi bres with
tyrosinase and other oxidative enzymes was first investigated by Lantto

et al. (2005b). Upon incubation with tyrosinase, no detectable activation of

fibres was observed by oxygen consumption measurements. However, XPS
measurements allowed small chemical changes to be detected at the fi bre
surface attributable to the formation of oxidised amino acid species. Intact
wool fibres were used in this work and their slight reactivity under the reac-

tion conditions used was attributed to limited accessibility of Tyr residues.
Jus et al. (2008) reported the tyrosinase-catalysed grafting of antioxidant
compounds (caffeic acid, chlorogenic acid) onto wool fi bres. Interestingly,
wool fibres incubated with A. bisporus tyrosinase displayed a continuous

decrease of oxygen concentration in the reaction system over 24 h of the
reaction, in apparent contrast to the results reported by Lantto et al. (2005b).
However, it is not reported whether intact or surface pre-treated wool fi bres
were used. Wool fibres enzymatically grafted with caffeic or chlorogenic

acid showed antioxidant activity, which was detectable even after a strong

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