Evidence for Allosteric Linkage between Exosites 1 and 2 of Thrombin*

Investigations to date have demonstrated that ligand binding to exosites 1 or 2 on thrombin produces conformational changes at the active site. In this study, we directly compared the effect of ligand binding to exosites 1 and 2 on the structure and function of the active site of thrombin and investigated functional linkage between the two exosites. Binding studies were performed in solution with fluorescein-Phe-Pro-Arg-CH2Cl (FPR)-thrombin. Hirudin-(54-65) and sF2, a synthetic peptide corresponding to residues 63-116 of prothrombin fragment 2, were used as ligands for exosites 1 and 2 of thrombin, respectively. The two ligands produce diametric changes in the fluorescence of fluorescein-FPR-thrombin and also have opposing effects on the rate of thrombin hydrolysis of a number of chromogenic substrates. These results indicate that sF2 and hirudin-(54-65) differentially affect the conformation of the active site. Experiments then were performed to investigate whether both ligands can bind to thrombin simultaneously. When thrombin-bound fluorescein-sF2 is titrated with hirudin-(54-65), complete displacement of fluorescein-sF2 is observed. Likewise, when thrombin-bound fluorescein-hirudin-(54-65) is titrated with sF2, complete displacement occurs. Additional support for reciprocal binding was obtained in fluorescence experiments where both probes were labeled and in experiments monitoring ligand binding to agarose-immobilized thrombin. This mutually exclusive binding of either ligand can be explained by reciprocal, allosteric modulation of ligand affinity between the two exosites. Thus, not only do the two exosites differentially influence the active site, they also affect the binding properties of the opposing exosite.