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

The S protein-S peptide interaction is a model system to study binding ther modynamics in proteins. We substituted alanine at position 4 in S peptide b y a-aminoisobutyric acid (iiib) to investigate the effect of this substitut ion on the conformation of free S peptide and on its binding to S protein. The thermodynamic consequences of this replacement were studied using isoth ermal titration calorimetry, The structures of the free and complexed pepti des were studied using circular dichroic spectroscopy and X-ray crystallogr aphy, respectively. The alanine4Aib replacement stabilizes the free S pepti de helix and does not perturb the tertiary structure of RNase S, Surprising ly, and in contrast to the wild-type S peptide, the DeltaG degrees of bindi ng of peptide to S pro, over the temperature range 5-30 degreesC, is virtua lly independent of temperature, At 25 degreesC, the Delta DeltaG degrees, D elta DeltaH degrees, Delta DeltaS and Delta DeltaC(p) of binding are 0.7 kc al/mol, 2.8 kcal/mol, 6 kcal/mol.K and -60 kcal/mol.K, respectively. The po sitive value of MS is probably due to a decrease in the entropy of uncomple xed alanine4Aib relative to the wild-type peptide. The positive value of De lta DeltaH degrees is unexpected and is probably due to favorable interacti ons formed in uncomplexed alanine4Aib, This study addresses the thermodynam ic and structural consequences of a replacement of alanine by Aib both in t he unfolded and complexed states in proteins.