Allosteric changes of thrombin catalytic site induced by interaction of bothrojaracin with anion-binding exosites I and II.

Abstract Bothrojaracin, a 27-kDa C-type lectin from Bothrops jararaca venom, is a selective and potent thrombin inhibitor ( K d = 0.6 nM) which interacts with the two thrombin anion-binding exosites (I and II) but not with its catalytic site. In the present study, we analyzed the allosteric effects produced in the catalytic site by bothrojaracin binding to thrombin exosites. Opposite effects were observed with α-thrombin, which possesses both exosites I and II, and with γ-thrombin, which lacks exosite I. On the one hand, bothrojaracin altered both kinetic parameters K m and k cat of α-thrombin for small synthetic substrates, resulting in an increased efficiency of α-thrombin catalytic activity. This effect was similar to that produced by hirugen, a peptide based on the C-terminal hirudin sequence (residues 54–65) which interacts exclusively with exosite I. On the other hand, bothrojaracin decreased the amidolytic activity of γ-thrombin toward chromogenic substrates, although this effect was observed with higher concentrations of bothrojaracin than those used with α-thrombin. In agreement with these observaions, bothrojaracin produced opposite effects on the fluorescence intensity of α- and γ-thrombin derivatives labeled at the active site with fluorescein-Phe-Pro-Arg-chloromethylketone. These observations support the conclusion that bothrojaracin binding to thrombin produces two different structural changes in its active site, depending on whether it interacts exclusively with exosite II, as seen with γ-thrombin, or with exosite I (or both I and II) as observed with α-thrombin. The ability of bothrojaracin to evoke distinct modifications in the thrombin catalytic site environment when interacting with exosites I and II make this molecule an interesting tool for the study of allosteric changes in the thrombin molecule.