THE REACTION OF THROMBIN WITH PLATELET-DERIVED NEXIN REQUIRES A SECONDARY RECOGNITION SITE IN ADDITION TO THE CATALYTIC SITE

A protease nexin released by activated platelets forms stable complexes with α-thrombin. Active-site-blocked thrombin does not form the stable complex, but it inhibits formation of the stable complex by active α-thrombin. γ-Thrombin, which has a damaged substrate recognition site (the anion-binding exosite), did not form the complex and did not inhibit formation of the stable complex by α-thrombin. Complex formation was inhibited by the C-terminal dodecapeptide of hirudin, which has been shown to bind to the anion-binding exosite. A monoclonal antibody that blocks reactions of thrombin that involve the anion-binding exosite also inhibited formation of a stable complex of α-thrombin and the platelet-derived protease nexin. It is concluded that the anion-binding exosite of thrombin, a site that confers a high degree of specificity for substrates with a complementary site, binds to the platelet nexin prior to reaction of the catalytic site with the serpin. © 1991.