Page 234 - Advances in Textile Biotechnology

Functionalisation of wool and silk fi bres using enzymes 215

washing cycle. This finding led to the suggestion that the antioxidant mol-

ecule was effectively grafted onto the fibres. The same enzymatic approach
was further extended to grafting molecules of biological interest (collagen,
gelatine, elastin) onto the surface of wool fi bres (Jus et al., 2009). The use

of FITC-labelled collagen confirmed that the protein was grafted onto the

surface of wool fibres and that it was resistant to severe washing.
Mattinen et al. (2008) studied the reactivity of A. bisporus tyrosinase and
of the novel T. reesei tyrosinase towards wool fibres. Because the close

packing of keratins into hierarchical three dimensional fibrous structures is

likely to restrict the chain mobility and to make direct crosslinking of
enzyme-activated species difficult, small molecular weight freely diffusible

mono- and diphenols were used as coupling agents. Moreover, wool fi bres
were pre-treated with an oxidising agent in order to facilitate the penetra-

tion of the enzyme into the wool fibres and to enhance the accessibility of
wool-bound Tyr residues. The T. reesei tyrosinase/l-dopa system was the
best performing one in terms of high fluorescence intensity of wool fi bres,

whereas A. bisporus tyrosinase was almost unable to activate wool fi bres
under the same reaction conditions.

The silk proteins, sericin and fibroin, have been extensively investigated
as substrates for the tyrosinase-catalysed oxidation of tyrosyl residues and
subsequent grafting of chitosan under both homogeneous and heteroge-
neous reaction conditions (Anghileri et al., 2007; Freddi et al., 2006; Kang
et al., 2004; Sampaio et al., 2005). The amount of Tyr residues in fi broin and
sericin is about 5.1 and 3.7 mol%, respectively.
Sericin and fibroin polypeptides dissolved in aqueous solution were effec-

tively oxidised by A. bisporus tyrosinase. An average amount of 56–58% of
the available Tyr residues were oxidised as determined by oxygen consump-
tion measurements (Anghileri et al., 2007). The behaviour of the FTIR and
Raman bands characteristic of Tyr confirmed that only a fraction of the

tyrosyl residues was oxidised by the enzyme (Taddei et al., 2007). The

random coil conformation taken by both sericin and fibroin in aqueous

solution allowed sufficient chain flexibility for the Tyr residues belonging
to the more hydrophilic polypeptide sequences exposed to the solvent to
be accommodated into the tyrosinase active site and to be oxidised. On the
other hand, the tyrosyl residues buried into the apolar micro-environments
from which the solvent was excluded were less or not accessible to the
enzyme for steric reasons.

The yield of oxidation decreased significantly when the enzymatic reac-

tion was carried out under heterogeneous condition, with fibroin in the
form of gel. About 10–12% of the tyrosyl residues were oxidised based on
the amino acid analysis (Freddi et al., 2006). This result can be easily
explained if the primary and higher order structures of silk fi broin are
considered. About 80% of the Tyr residues belong to the repetitive

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