Page 241 - Advances in Textile Biotechnology

222 Advances in textile biotechnology

could be achieved owing to the high affinity of ABTS oxidation products
for keratins and to improved mass transfer within the reaction system
(Munteanu et al., 2007). These results show the potential of the enzymatic
approach to wool dyeing and how the process can be intensifi ed by suitable
technological developments. However, the range of colours, hues, and depth
of shades so far obtained is too limited for enzymatic dyeing to be an attrac-
tive alternative to conventional chemical dyeing. Their technological per-
formance (reproducibility, colour fastness) is still a challenge. Further
studies are needed to explore a wider range of enzyme, phenols, and mod-

ifiers/additives combinations in order to widen the range of colours, to
enhance their performance, and to achieve industrially acceptable stan-
dards.
This overview of non-proteolytic enzymes of textile interest cannot be
considered complete without mentioning emerging enzymatic activities
that are likely to open new perspectives for the functionalisation of protein

fibres. These novel enzymes are sulfydryl oxidases (SOXs) and protein

disulfide isomerases (PDIs). In living organisms, these enzymes play a key

role in the post-translational modification and folding of newly synthesised
proteins leading to stabilisation and regulation of protein structure and
function. In particular, these enzymes are involved in the formation and

rearrangement of protein disulfide bonds (Appenzeller-Herzog and Ell-
gaard, 2008; Fass, 2008). Disulfides are covalent bonds produced by oxida-

tion of two free thiols of cysteine residues, both as intramolecular or
intermolecular bonds, to stabilise the protein structure, but they also have
the capacity to work as regulatory switches in redox signalling in cells. SOXs
have been identified in animal, plant, and fungal species, as well as in the

genomes of viruses. The eukaryotic SOXs characterised to date are classi-

fied into two families: Ero1 and Erv. All SOXs contain a fl avin adenine
dinucleotide (FAD) bound to the protein. SOXs promote cysteine pairing
by transfer of electrons from thiol groups to molecular oxygen. Common
to both eukaryotic families of SOXs is a CXXC motif (C is cysteine) adja-
cent to the FAD that participates in the two-electron transfer. The redox
activity of PDI enzymes is also governed by the CXXC active site. When

in the oxidised state, the disulfide can be transferred to the substrate to
catalyse its oxidation whereby the active site itself becomes reduced. When
in the reduced state, substrate disulfi des can be reduced and the active site

ends up in the oxidised state. These thiol–disulfide exchange reactions
proceed through the formation of a transient mixed disulfi de between
enzyme and substrate. Therefore, catalytic PDIs harboring a CXXC active
site sequence are specialised disulfide carrier proteins that act both as

disulfide-donor and -acceptor enzymes, i.e. catalysing both cysteine oxida-

tion and disulfide reduction in their substrates. Disulfide isomerisation is a

crucial reaction in biological systems not only for its key role in driving

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