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 thermodynamics in proteins. We substituted alanine at position 4 in S peptide by alpha-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 DeltaG degrees of binding of peptide to S pro, over the temperature range 5-30 degrees C, is virtually independent of temperature. At 25 degrees C, the DeltaDeltaG degrees, DeltaDeltaH degrees, DeltaDeltaS and DeltaDeltaCp of binding are 0.7 kcal/mol, 2.8 kcal/mol, 6 kcal/mol x K and -60 kcal/mol x K, respectively. The positive value of DeltaDeltaS is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of DeltaDeltaH: 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.
Ratnaparkhi
G S
GS
Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012, India.
Awasthi
S K
SK
Rani
P
P
Balaram
P
P
Varadarajan
R
R
D019943:Amino Acid Substitution; D000621:Aminoisobutyric Acids / Q000737:chemistry* / Q000494:pharmacology; D000818:Animals; D002417:Cattle; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D011487:Protein Conformation; D021122:Protein Subunits; D012260:Ribonucleases / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
D000621:Aminoisobutyric Acids; D010446:Peptide Fragments; D021122:Protein Subunits; D012260:Ribonucleases; C022542:ribonuclease S