THERMODYNAMIC AND STRUCTURAL-ANALYSIS OF PHOSPHOTYROSINE POLYPEPTIDE BINDING TO GRB2-SH2

A thermodynamic analysis using isothermal titration calorimetry (ITC) has been performed to examine the binding interaction between the SH2 (Src homology 2) domain of growth factor receptor binding protein 2 (G rb2-SH2) and one of its phosphotyrosine (pY) polypeptide ligands, Inte raction of the She-derived phosphotyrosine hexapeptide Ac-SpYVNVQ-NH2 with Grb2-SH2 was both enthalpically and entropically favorable (Delta H = -7.55 kcal mol(-1), -T Delta S = -1.46 kcal mol(-1), Delta G = -9 .01 kcal mol(-1), T = 20 degrees C), ITC experiments using five alanin e-substituted peptides were performed to examine the role of each side chain in binding, The results were consistent with homology models of the Grb2-SH2-Shc hexapeptide complex which identified several possibl e hydrogen bonds between Grb2-SH2 and the phosphotyrosine and conserve d asparagine(+2) side chains of the She hexapeptide, These studies als o demonstrated that the hydrophobic valine(+1) side chain contributes significantly to the favorable entropic component of binding, The ther modynamic and structural data are consistent with a Grb2-SH2 recogniti on motif of pY-hydrophobic-N-X (where X is any amino acid residue). Th e measured heat capacity of binding (Delta C-p = -146 cal mol(-1) K-1) was very similar to computed values using semiempirical estimates (De lta C-p = -106 to -193 cal mol(-1) K-1) derived from apolar and polar accessible surface area values calculated from several homology models of the Grb2-SH2-Shc hexapeptide complex, The homology model which mos t closely reproduced the measured Delta C-p value is also the model wh ich had the lowest RMS deviation from the subsequently determined crys tal structure, Calculations based on the thermodynamic data and these semiempirical estimates indicated that the binding event involves buri al of nearly comparable apolar (677 Angstrom(2)) and polar (609 Angstr om(2)) surface areas.