O o d h e a d p u b L i s h I n g L i m I t e d

1.5
How might wool quality be modified by
affecting fibre molecular structure?
1.5.1
The protein structure of wool and its relation
to physical properties
The physical properties of wool fibres such as elasticity and strength and the
distinctive thermal attributes of wool textiles are a consequence of their
cellular structure, their specific proteins and the organisation of the proteins
within the cells of the cortex of the fibre. The detailed structures of the wool
fibre cortex and cuticle and the proteins that compose them have been studied
at the chemical and electron microscopic levels for decades (see Powell and
Rogers, 1997). Primarily the cortical cells are filled with 8 nm diameter
keratin filaments (referred to as keratin IF or KIF) that are members of the
large intermediate filament family found in animal cells. In the wool cortex
the KIFs are separated by a matrix that appears under the electron microscope
as dense material between individual IFs. The matrix is a complex of non-
fibrous protein families and referred to as keratin-associated proteins or
KAPs. They fall into two main types, one with a composition characterised
by a high content of amino acids glycine and tyrosine (‘high glycine/tyrosine
proteins’) and the other of cystine (‘high-sulphur proteins’).
In fine wool fibres the cells of the orthocortex and paracortex that were
originally distinguished by their differing degrees of access to dyestuffs,
display a distinct organisation of the intermediate filaments, the KIFs, and
matrix. In the paracortical cells the KIFs are strongly aligned with the fibre
axis and in cross-section they show a quasi-regular packing, whereas in the
orthocortical cells the KIFs are inclined to the fibre axis and packed into
fibre aggregates called macrofibrils (
Fig. 1.3).
 In cross-section view the
macrofibrils have the appearance of ‘whorls’ because of the tilt and individual
KIFs become indistinguishable out from the centre.
The protein structure of a KIF is complex in that it requires two distinct
protein types, Type I and Type II, in order for it to form. The two types are
only distantly related in amino acid sequence but nevertheless very similar
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Improvement of wool production through genetic manipulation
11
in their secondary structure. The stability of the cortex is dependent upon
chemical bonds that are present between the KIFs and also between the KIFs
and matrix molecules. They include hydrogen bonds of the 
α-helix and
disulphide bonds that are present within the KIFs and connect them to the
matrix molecules. Very little is known about these intermolecular bonds in
terms of what particular amino acids in the protein sequences are involved.
Presumably, overall there is a high degree of specificity in the intermolecular
bonding. A diagram illustrating these aspects is given in 
Fig. 1.4.
The typical behaviour of a wool fibre when stretched in the presence of
water molecules (the stress/strain curve) results from the breakage of the
hydrogen (non-covalent) bonds as the 
α-helical proteins of the keratin IFs
unwind (the yield region of the stress/strain curve). The yield region ends
when the disulphide (covalent) bonds resist further extension until the breakage
of the fibre (see Feughelman, 1997, for a detailed discussion of these physical
properties).
The contents of the cells of the cuticle consist of amorphous proteins in
three layers that are distinguishable by electron microscopy (
Fig. 1.5).
  The
1.3
 Transmission electron micrograph of a cross-section of a fine
Merino wool fibre showing the ortho- (O) and paracortical (P) cortical
cell types with differing organisation of the keratin intermediate
filaments. The cells are separated by a boundary consisting of the
cell membrane complex (m) present between all of the cortical cells.
P
m
O
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Advances in wool technology
12
layer beneath the epicuticle on the surface of each cuticle cell is the exocuticle
that has an outer or A-layer. An innermost layer in each cell is the endocuticle.
The proteins of the cuticle have been only partially characterised in the
exocuticle that has two proteins identified as KAP-type proteins and related
to the sulphur-rich KAPs of the matrix in the fibre cortex. Proteins of the
endocuticle have not been identified but are usually regarded as cytoplasmic
proteins that remain after cell differentiation. The exocuticle KAP proteins
are crosslinked by disulphide bonds and produce a stiff protective covering
Glycine/tyrosine matrix molecule
Cysteine-rich matrix molecule
Intermediate filament
IF protein molecule
linked to matrix
IF
IF
Matrix
IF protein molecule
linked to matrix
1.4
 A diagram illustrating the linkages by disulphide bonds (dotted
lines) present between globular-type matrix molecules and between
the matrix molecules and the rod-like 
α-helical protein molecules in
neighbouring intermediate filaments.
1.5
 Diagrammatic representation of the layers constituting the
protein layers in a cuticle cell.
External surface
Epicuticle
A-layer
Exocuticle – KAP5 & KAP10
Endocuticle – unknown
composition
Lipid and protein
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Improvement of wool production through genetic manipulation
13
for the fibre. The influence of the overlapping flattened cells of the cuticle
(Fig. 1.6)
 on the elastic properties of wool fibres is minimal but they play a
major role on fibre migration in concert with the elasticity of the fibre cortex
in the shrinkage of wool textiles. The effect of the cuticle is significantly
diminished by chemical treatments that flattens the cells and reduces the
ratchet-like profile.
For example, the chlorination step of the commercial Hercosett process