2009_Book_FoodChemistry

1.4 Proteins 55
10 kJnm −2 . Therefore, the total hydrophobic contribution
to free energy due to folding is:
(1.92)
This relation is valid for a range of 6108 ≤ M
≤ 34,409, but appears to be also valid for larger
molecules since they often consist of several
loose associations of independent globular
portions called structural domains (Fig. 1.26).
Proteins with disulfide bonds fold at a significantly
slower rate than those without disulfide
bonds. Folding is not limited by the reaction
rate of disulfide formation. Therefore the folding
process of disulfide-containing proteins seems to
proceed in a different way. The reverse process,
protein unfolding, is very much slowed down
by the presence of disulfide bridges which
generally impart great stability to globular
proteins. This stability is particularly effective
against denaturation. An example is the
Bowman-Birk inhibitor from soybean (Fig. 1.27)
which inhibits the activity of trypsin and chymotrypsin.
Its tertiary structure is stabilized
by seven disulfide bridges. The reactive sites
Fig. 1.26. Globular protein with two-domain structure
(according to Schulz and Schirmer, 1979 )
of inhibition are Lys 16 -Ser 17 and Leu 43 -Ser 44 ,
i. e. both sites are located in relatively small
rings, each of which consists of nine amino
acid residues held in ring form by a disulfide
bridge. The thermal stability of this inhibitor is
high.
As examples of the folding of globular proteins,
Fig. 1.28 shows schematically the course of the
peptide chains in the β-chain of hemoglobin, in
triosephosphate isomerase and carboxypeptidase.
Other protein conformations are shown in the following
figures:
– Fig. 8.7 (cf. 8.8.4): Thaumatin and monellin
(two-dimensional)
– Fig. 8.8 (cf. 8.8.5): Thaumatin and monellin
(three-dimensional)
– Fig. 11.3 (cf. 11.2.3.1.4): Lysozyme
1.4.2.3.3 BSE
The origination of transmissible spongiform encephalopathies
(TESs) is explained by a change
in the protein conformation. (The name refers to
the spongy deformations which occur in the brain
in this disease. The resulting defects interrupt
the transmission of signals). One of the TESs
is bovine spongiform encephalopathy (BSE).
According to the current hypothesis, TESs are
caused by pathogenic prion proteins (PrPp),
which can be present in the animal meal used
as feed. PrPp are formed from normal prion
proteins (PrPn) found in all mammalian cells.
Fig. 1.27. Bowman-Birk inhibitor from soybean (according to Ikenaka et al., 1974)