Page 213 - Advances in Textile Biotechnology

194 Advances in textile biotechnology

fibre weight, respectively. Fibroin, the main constituent of silk fi bres, consists
of three polypeptides: the heavy (391.6 kDa) and light (27.7 kDa) fi broin

chains, linked by one disulfide bridge, and a low molecular weight protein

called P25 (25.2 kDa), also known as fibrohexamerin, which are present in
a mass ratio of 93 : 6 : 1, respectively. The heavy fibroin chain, the real fi brous

component, is rich in glycine (Gly), alanine (Ala), and serine (Ser) amino
acids (about 85 mol%) in the molar ratio 3 : 2 : 1. The primary structure
consists of eleven repetitive hydrophobic sequences (R) regularly inter-
rupted by ten hydrophilic spacer sequences (A), in addition to two hydro-
philic N- and C-terminus sequences (Zhou et al., 2000). The R sequences
are characterised by repeating -(Gly-Ala) n - motifs which form the well

known β-sheet crystalline regions present in the fibre as well as in regener-

ated silk fibroin materials like films and gels. Sericin, the silk gum holding
together the fi broin filaments, is a complex mixture of 5–6 polypeptides

widely differing in size (40–400 kDa), chemical composition, structure, and
properties. The amino acid composition is characterised by an extremely
high concentration of Ser, which ranges from 16 to 38 mol%. Sericin is
usually removed during textile processing by a wet treatment called
degumming.

As proteins, keratins, fibroin, and sericin are potential substrates for
various enzymatic reactions. The use of proteases as environmentally
friendly processes for wool antifelting and softening or for silk degumming
is reviewed in chapter 8. Here, the use of non-proteolytic enzymes able to
modify the chemistry of amino acid residues without breaking the peptide
bond is considered. In particular, transferases (e.g. transglutaminase) and
oxidoreductases (e.g. tyrosinase, laccase, peroxidase) have the capability of

modifying the properties of wool and silk protein fibres by targeting reac-
tive side groups of amino acid residues, thus resulting in activation, func-

tionalisation, or crosslinking. The specificity and selectivity of these enzymes
may allow wool and silk to be processed under milder reaction conditions
and their properties to be better tailored to end-use requirements, thus
offering cleaner and safer alternatives to current processing practices which
make use of harsh chemicals.
The most recent studies on the biotechnological functionalisation of

protein-based textile fibres with transglutaminase and tyrosinase are
reviewed in sections 9.3 and 9.5, respectively. Biotechnology is typically a
cross-sectoral technology able to drive product and process innovation in
a range of textile and non-textile industrial sectors. In order to provide a
wider overview of the great potentiality of these enzymes, not only textile,
but also other biotechnological applications falling in different scientifi c
and technological sectors are reviewed here. To outline the state of the art
of non-proteolytic enzymes available for protein fi bre modifi cation, other

enzymes having displayed some activity on protein fibre substrates, such as

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