elongated structures of mostly not very hydrophobic peptide chains; and (c)
disordered, which structure somewhat resembles a random coil. The
structures are often stabilized by posttranslational modifications, which
may include glycosylation of some residues, and formation of intramole-
cular cross-links in the form of 22 S 22 S 22 bridges between Cys residues.
Many large protein molecules form separate globular domains of 100–200
residues. Quaternary structure involves association of protein molecules into
larger entities of specific order.
Conformational Stability. The compact conformation of
globular proteins is due to a great number of weak intramolecular bonds.
If the molecule unfolds, which leads to a greatly increased conformational
entropy, this is generally via a cooperative transition, which implies that
intermediate conformational states do not or hardly occur. The stability,
defined as the free energy difference between the folded and unfolded states,
is fairly small. It is, however, the sum of two very large terms, one
promoting and the other opposing unfolding. This means that small changes
in conditions can already lead to unfolding. The bonds involved are for the
greater part H-bonds, but these can only be strong in an apolar
environment, implying that the presence of hydrophobic residues is
essential in obtaining a folded, i.e., globular, conformation. Unfolding
generally occurs at high or very low temperature, at extreme pH,
at very high pressure, and upon adsorption onto hydrophobic surfaces
(solid, oil, or air). Several solutes may cause unfolding due to altering
the solvent quality, such as salts that are ‘‘high’’ in the Hofmeister series.
Other solutes have specific effects, such as the breakage of 22 S 22 S 22
bridges.
Denaturationof a globular protein may be equated to the unfolding of
its peptide chain; it can also be related to the effects it has, such as loss of
biological (e.g., enzyme) activity, or aggregation. If these changes are to be
permanent, refolding of the peptide chain into its native conformation
should be prevented. Several reactions, which especially occur at high
temperature or high pH, can cause changes in configuration that do prevent
refolding. The kinetics of denaturation, particularly of heat denaturation, is
of great practical importance. In general, the kinetics is intricate, though in
many cases the denaturation rate is controlled by the unfolding reaction.
Then, the reaction is first-order and has a very steep temperature
dependence. The latter is due to the very large activation enthalpy
(numerous bonds have to be broken simultaneously). The very large
entropy change (e.g., the increase in conformational entropy) causes the
reaction nevertheless to proceed at a reasonable rate at moderate