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Functionalisation of wool and silk fi bres using enzymes 223

protein folding but also because PDIs can catalyse the rearrangement of

wrongly formed disulfides, which often occur during folding and prevent
the formation of the native structure.
The biotechnological potential of these enzymes is still under scrutiny.
Research efforts dealing with screening of micro-organisms for the identi-

fication, isolation, and purification of SOX and PDI activities suitable for
laboratory-scale trials are in progress. Approaches aimed at inducing over-
production or heterologous production of these enzymes are also being
investigated. No results have been reported yet on their application on

protein fibres. Preliminary studies on model peptides and proteins are
needed to disclose the real potential of these enzymes for the functionalisa-
tion of wool and silk fi bres.

9.7 Conclusions and future trends

Enzymatic reactions on protein fibres typically occur under heterogeneous

conditions, with fibres in the form of solid substrate. Most of the studies
reviewed in this chapter, as well as others not considered here, demonstrate
how complicated these systems are and how many chemical, physical, struc-

tural, and morphological parameters can influence the outcome of the
enzymatic reactions. The experimental results obtained do not always
comply with the initial hypotheses. A deeper understanding of the enzyme/
substrate interactions, as well as a more detailed knowledge of the effects
of various experimental variables on reaction kinetics and yield would be
very helpful for devising proper strategies for the enzyme-mediated modi-

fication of protein fi bres.
The individual polypeptide chains forming wool and silk fibres are held

together by a range of weak-to-strong molecular interactions (from hydro-
phobic interactions to electrostatic, hydrogen, and covalent bonds) which
result in the formation of compact and tough three-dimensional fi brous
structures, whose main primary function in nature is to resist external
chemical, physical, mechanical, and biological stresses. When immersed in

aqueous solution, these fibres may undergo more or less extensive swelling
thus allowing penetration and diffusion of reagents inside the fi bre matrix.
Owing to morphological, chemical, and structural factors, enzymes can pen-
etrate more easily inside wool than silk fibres. This suggests that only amino

acid residues located at the surface of the fibres can be potentially exploited
for the enzymatic functionalisation of silk, whereas for wool, enzymes can
also use a certain amount of the amino acid residues confined in the fi bre

matrix.
Although the total amount of potentially reactive amino acid residues in
a fibre substrate is important, the effective accessibility of these amino acid

residues is much more important for a reaction to occur. Enzymes must not

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