Contribution of Cation−π Interactions to Protein Stability

• Published in: Biochemistry (Easton) Vol. 45; no. 50; pp. 15000 - 15010
• Main Authors: Prajapati, Ravindra S, Sirajuddin, Minhajuddin, Durani, Venuka, Sreeramulu, Sridhar, Varadarajan, Raghavan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 19-12-2006
• Subjects: Amino Acid Substitution; Cations - chemistry; Escherichia coli - chemistry; Escherichia coli - genetics; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Lysine - chemistry; Lysine - genetics; Models, Molecular; Periplasmic Proteins - chemistry; Periplasmic Proteins - genetics; Protein Folding; Protein Structure, Secondary; Static Electricity
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi061275f

Calculations predict that cation−π interactions make an important contribution to protein stability. While there have been some attempts to experimentally measure strengths of cation−π interactions using peptide model systems, much less experimental data are available for globular proteins. We have attempted to determine the magnitude of cation−π interactions of Lys with aromatic amino acids in four different proteins (LIVBP, MBP, RBP, and Trx). In each case, Lys was replaced with Gln and Met. In a separate series of experiments, the aromatic amino acid in each cation−π pair was replaced by Leu. Stabilities of wild-type (WT) and mutant proteins were characterized by both thermal and chemical denaturation. Gln and aromatic → Leu mutants were consistently less stable than corresponding Met mutants, reflecting the nonisosteric nature of these substitutions. The strength of the cation−π interaction was assessed by the value of the change in the free energy of unfolding [ΔΔG° = ΔG°(Met) − ΔG°(WT)]. This ranged from +1.1 to −1.9 kcal/mol (average value −0.4 kcal/mol) at 298 K and +0.7 to −2.6 kcal/mol (average value −1.1 kcal/mol) at the T m of each WT. It therefore appears that the strength of cation−π interactions increases with temperature. In addition, the experimentally measured values are appreciably smaller in magnitude than calculated values with an average difference |ΔG°expt − ΔG°calc|av of 2.9 kcal/mol. At room temperature, the data indicate that cation−π interactions are at best weakly stabilizing and in some cases are clearly destabilizing. However, at elevated temperatures, close to typical T m's, cation−π interactions are generally stabilizing.