Structural and thermodynamic consequences of introducing α-aminoisobutyric acid in the S peptide of ribonuclease S

Abstract

The S protein-S peptide interaction is a model system to study binding thermodynamics in proteins. We substituted alanine at position 4 in S peptide by α-aminoisobutyric acid (Aib) to investigate the effect of this substitution on the conformation of free S peptide and on its binding to S protein. The thermodynamic consequences of this replacement were studied using isothermal titration calorimetry. The structures of the free and complexed peptides were studied using circular dichroic spectroscopy and X-ray crystallography, respectively. The alanine4Aib replacement stabilizes the free S peptide helix and does not perturb the tertiary structure of RNase S. Surprisingly, and in contrast to the wild-type S peptide, the ΔG° of binding of peptide to S pro, over the temperature range 5-30°C, is virtually independent of temperature. At 25°C, the ΔΔG°, ΔΔH°, ΔΔS and ΔΔC_p of binding are 0.7 kcal/mol, 2.8 kcal/mol, 6 kcal/mol.K and -60 kcal/mol.K, respectively. The positive value of ΔΔS is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of ΔΔH° is unexpected and is probably due to favorable interactions formed in uncomplexed alanine4Aib. This study addresses the thermodynamic and structural consequences of a replacement of alanine by Aib both in the unfolded and complexed states in proteins.