Nonpolar interactions of thrombin and its inhibitors at the fibrinogen recognition exosite: Thermodynamic analysis

Nonpolar interactions play a major role in the association of the fibrinogen recognition exosite of thrombin with the C-terminal fragment (55-65), Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-Leu-Gln, of hirudin, which is a naturally occurring thrombin inhibitor. The thermodynamic details (free energy, enthalpy, entropy, and heat capacity) of the molecular recognition are studied by using five analogs of a synthetic bivalent thrombin inhibitor (P552), tert-butylbenzensulfonyl-Arg-(D-pipecoli acid)-(12-aminododecanoic acid)-(gamma-aminobutyric acid)-hirudin(55-65). The residue of phe(H56), Ile(H59), Pro(H60), Tyr(H63), or Leu(H64) in hirudin(55-65) segment is substituted by Gly in each analog in order to elucidate the contributions of these nonpolar side chains. The results show that the interactions of these nonpolar side chains with thrombin are enthalpy-driven, except for the contribution of the phe(H56) Side chain which is entropy-driven. Interestingly, molecular modeling predicts a large conformational change due to the Gly substitution of Phe(H56). I, analyzing the correlation among the thermodynamic and structural properties of the nonpolar interaction, a good correlation is observed between the binding free energy and the hydrophobicity of the molecular surface; i.e., tighter binding is observed as more nonpolar atoms are buried and more polar atoms are exposed upon molecular association.