Salt bridges or ion-pairs are a special form of particularly strong hydrogen
bonds made up of the interaction between two charged residues.
An example of a salt bridge is found
at this PPS link.
The contribution of salt bridges to protein stability is a somewhat contentious
issue in the literature. On the one hand is the observation that thermophilic
and hyper-thermophilic analogues of mesophilic proteins tend to have increased
numbers of salt-bridges (Tanner
et al., 1996; Perutz
& Raidt, 1975; Perutz,
1978; Dekker et al., 1991).
On the other hand are mutational studies showing that the contribution of
salt bridges to stability is small. Perhaps at higher temperatures salt
bridges make more of a contribution to stability.
Horovitz et al
. (1990) measured the stability of a surface salt bridge triad between
Asp8, Asp12 and Arg110 on the surface of barnase by construction of a thermodynamic
cycle of all possible combinations of 1, 2, or 3 alanine mutants. The free
energy contribution to the stability of the protein is only 1.25 kcal/mol
for the Asp12/Arg110 pair and 0.98 kcal/mol for the Asp8/Arg110 pair. Removal
of Arg 110 has no effect on stability!
Click here for gif of barnase salt
bridge and link to pdb.
Other studies have had similar results (Akke & Forsen, 1990). One reason that these contributions are not as large as might be expected from the strength of such an ion pair is that, in order form a salt bridge, strong hydrogen bonds with water have to be broken. In fact, it is possible that most of the energy of stabilization comes from the increase in solvent entropy upon formation of the ion pair.
In contrast to surface salt bridges, removal of one partner from a buried
salt bridge leads to destabilization of 3-4 kcal/mol. However, it has been
found that replacing a buried salt-bridge triad by well packed hydrophobic
residues (found by random mutagenesis at the three charged residue positions)
leads to an increase in stability of
4.5 kcal/mol in the arc repressor (Waldburger
et al., 1995). Thus, hydrophobic interactions contribute more
to stability than a salt bridge triad. This is illustrated below and is
presumably due to the cost of desolvating the charged groups on going from
the unfolded to the folded state. The mutant is R31M, E36Y, R40L.
Click here for gifs of the arc repressor
wild-type and mutant and pdb links.
Interestingly, it was also shown that the wild-type arc repressor folds between 10 and 1250 more slowly than the mutant (Waldburger et al., 1996). This is proposed to be due to the high energy barrier to burying charged residues. The transition state would have a particularly high energy if one charged residue had to be buried before the other. Even if the salt bridge had formed prior to the transition state, the geometry of the salt bridge might well be sub-optimal until that part of the protein attained its native conformation. Hydrophobic interactions have less of a steric requirement.
Other Factors Aromatic-Aromatic InteractionsBeginning