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Protein stabilization by salt bridges: concepts, experimental approaches and clarification of some misunderstandings
Author(s) -
Bosshard Hans Rudolf,
Marti Daniel N.,
Jelesarov Ilian
Publication year - 2004
Publication title -
journal of molecular recognition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.401
H-Index - 79
eISSN - 1099-1352
pISSN - 0952-3499
DOI - 10.1002/jmr.657
Subject(s) - salt bridge , chemical physics , static electricity , chemistry , charge (physics) , electrostatics , biomolecule , salt (chemistry) , electric potential energy , dipole , coupling (piping) , folding (dsp implementation) , protein folding , crystallography , energy (signal processing) , mutant , materials science , physics , biochemistry , organic chemistry , quantum mechanics , electrical engineering , metallurgy , gene , engineering
Salt bridges in proteins are bonds between oppositely charged residues that are sufficiently close to each other to experience electrostatic attraction. They contribute to protein structure and to the specificity of interaction of proteins with other biomolecules, but in doing so they need not necessarily increase a protein's free energy of unfolding. The net electrostatic free energy of a salt bridge can be partitioned into three components: charge–charge interactions, interactions of charges with permanent dipoles, and desolvation of charges. Energetically favorable Coulombic charge–charge interaction is opposed by often unfavorable desolvation of interacting charges. As a consequence, salt bridges may destabilize the structure of the folded protein. There are two ways to estimate the free energy contribution of salt bridges by experiment: the p K a approach and the mutation approach. In the p K a approach, the contribution of charges to the free energy of unfolding of a protein is obtained from the change of p K a of ionizable groups caused by altered electrostatic interactions upon folding of the protein. The p K a approach provides the relative free energy gained or lost when ionizable groups are being charged. In the mutation approach, the coupling free energy between interacting charges is obtained from a double mutant cycle. The coupling free energy is an indirect and approximate measure of the free energy of charge–charge interaction. Neither the p K a approach nor the mutation approach can provide the net free energy of a salt bridge. Currently, this is obtained only by computational methods which, however, are often prone to large uncertainties due to simplifying assumptions and insufficient structural information on which calculations are based. This state of affairs makes the precise thermodynamic quantification of salt bridge energies very difficult. This review is focused on concepts and on the assessment of experimental methods and does not cover the vast literature. Copyright © 2004 John Wiley & Sons, Ltd.