Page 243 - Advances in Textile Biotechnology

224 Advances in textile biotechnology

only be able to reach the polypeptide chain segments where the target
amino acidic groups are located, but must also be able to bind with them.
As demonstrated by the results reported for the TGase-catalysed modifi ca-
tion of globular proteins in aqueous solution, even exposed Gln residues
are not attacked by the enzyme if other requirements are not fulfi lled. In
particular, the pattern of surrounding amino acidic sequences, as well as the
local structure of the polypeptide chain segments where the amino acidic

groups are located must be endowed with sufficiently high mobility and

flexibility to allow the active site of the enzymes to come into close contact
with the substrate and to bind to it. Consequently, it is reasonable to suppose
that a certain number of potentially reactive amino acid residues of wool

and silk, either at the surface or inside the fibre matrix, do not match the
above requirements, being involved in hydrogen and/or covalently bonded
secondary and higher order structures that are not flexible enough to inter-

act dynamically with the enzyme molecules. However, various enzymatic or
chemical treatments can modify protein fi bres not only at the surface level
but also in the bulk, thus enhancing the accessibility of amino acid residues
by removing barrier elements, by changing their chemical environment, or
by inducing conformational transitions of the backbone peptide chains
where they are located.
Far from being considered a limitation, the above morphological, molec-
ular, and structural features must be viewed as a way to address selectivity

and specificity in the enzymatic modification of wool and silk fi bres. To fully
exploit the great potential of enzymatic tools, it is important to gain wider
understanding of the accessibility and reactivity of the target amino acid
residues in wool and silk. Building a database of accessible amino acid
residues in protein fibres is still a challenge for textile biotechnology scien-

tists. However, unravelling fundamental aspects of the enzyme/substrate
interactions would make it possible to design suitable enzymatic function-

alisation strategies able to address specific physicochemical and functional
properties of wool and silk fi bres.
TGases and tyrosinases are both protein crosslinking enzymes. As
described in the previous paragraphs, inter- or intramolecular crosslinking
may directly result from the TGase-catalysed reaction between Gln and Lys
or may be the outcome of a non-enzymatic reaction step between tyrosi-
nase-activated chemical species, typically quinine–quinone or quinone–
nucleophile coupling. To allow crosslinking, the amino acid groups involved
should be in the right position to meet the steric requirements of the
enzyme active site or must be close enough to reach the reactive counter-
parts. It is reasonable to suppose that these conditions are not always ful-
filled when the amino acid residues belong to polypeptide chains that are

blocked into closely packed three-dimensional fibrous structures such as

wool and silk. Crosslinking of wool fibres by TGase-catalysed reaction has

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