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Protein Engineering, Vol. 13, No. 10, 697-702, October 2000
© 2000 Oxford University Press

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

Girish S. Ratnaparkhi1,2, Satish Kumar Awasthi1,3, P. Rani1, P. Balaram1,4,4 and R. Varadarajan51,4,4

1 Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012 and 4 Chemical Biology Unit, Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur, Bangalore, 560 004, India

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 {Delta}G° of binding of peptide to S pro, over the temperature range 5–30°C, is virtually independent of temperature. At 25°C, the {Delta}{Delta}G°, {Delta}{Delta}H°, {Delta}{Delta}S and {Delta}{Delta}Cp 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 {Delta}{Delta}S is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of {Delta}{Delta}H° 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.


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 J. Biol. Chem.Home page
G. S. Ratnaparkhi and R. Varadarajan
Osmolytes Stabilize Ribonuclease S by Stabilizing Its Fragments S Protein and S Peptide to Compact Folding-competent States
J. Biol. Chem., July 27, 2001; 276(31): 28789 - 28798.
[Abstract] [Full Text] [PDF]


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