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

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 a-aminoisobutyric acid (iiib) 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 DeltaG degrees of binding of peptide to S pro, over the temperature range 5-30 degreesC, is virtually independent of temperature, At 25 degreesC, the Delta DeltaG degrees, Delta DeltaH degrees, Delta DeltaS and Delta DeltaC(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 MS is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of Delta DeltaH degrees 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.