- Salt bridges. This concerns only a few bonds; they may on average
contribute by an amount of about20 kJ?mol^1 toDH.
Some other terms are positive, hencepromote the unfolded state: - Conformational entropy. In the unfolded state each peptide unit
would contribute about 4 degrees of freedomðOÞ, but many side groups
would also have more conformational freedom. Assuming the increase inO
to be 6 to 8 per residue, this would lead to aDS¼RlnO^100 between 1500
and 1700 J?mol^1 ?K^1. Altogether,DU?NGwould be about 500 kJ?mol^1. - Hydrationof polar side groups and peptide bonds. This must
provide a large term inDG, including enthalpic and entropic contributions.
It cannot be separated from the interactions mentioned in items 1 and 2. - Bending and stretching of covalent bonds. Although such
deviations from the state of lowest energy may be minor, it is inconceivable
that all the bonds in a tightly folded protein molecule would remain
completely undistorted. Since about a thousand bonds are present, the
contribution toDHmay be significant. - Mutualrepulsion of charged groupson the surface. This is an
enthalpic term, that may be appreciable if the pH is far away from the
isoelectric point.
Despite the uncertainties in the magnitude of the various terms, it is
obvious that the thermodynamic stability of a globular protein results from
the difference between two large terms. For a 100 residue protein, these
terms are about 10^3 kJ?mol^1 or more, whereas their difference would only
be about 25 kJ?mol^1. Observed values ofDU?NGfor globular proteins
under physiological conditions range for the most part between20 and
65 kJ?mol^1 , far less than the strength of one covalent bond (about
500 kJ?mol^1 ). This implies that even small changes in conditions may
lead to unfolding. At high temperature, the increase in conformational
entropy of the protein in the unfolded state will generally be overriding: the
mentionedDSof about 1:6kJ?mol^1 ?K^1 (item 5, above) leads to a change
in DðTSÞ, hence in DG, by about 50 kJ?mol^1 for a 30K increase in
temperature. At very low temperature, hydrophobic interactions become
very small or may even become repulsive (see Figure 3.4). At extreme pH
values, electrostatic repulsion (item 8, above) may be sufficient to cause
unfolding. Changes in solvent quality may significantly affect solvation free
energy.
The stability is thus due to a very subtle balance. Small changes in the
primary structure may leave the conformation virtually the same, but it is
also possible that replacement of only one residue by another one leads to
an unstable molecule. Globular proteins evolved to be stable under
physiological conditions, and stability under other conditions may not be
needed. Although more stable conformations would in principle be possible,
singke
(singke)
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