The thrombin high-affinity binding site on platelets is a negative regulator of thrombin-induced platelet activation. Structure-function studies using two mutant thrombins, Quick I and Quick II.

To elucidate the thrombin domains required for high-affinity binding and platelet activation, the platelet binding properties of thrombin and two mutant thrombins, thrombin Quick I and Quick II, were compared to their agonist effects in elevating intraplatelet Ca2+]. In Quick I, a mutation within the fibrinogen binding groove results in decreased clotting and platelet aggregating activities, whereas in Quick II, a mutation in the primary substrate binding pocket abolishes both activities. Dysthrombin binding was decreased compared to thrombin. The fibrinogen binding groove appeared more important than the primary substrate pocket for high-affinity binding since Quick I showed drastically reduced, and Quick II only slightly reduced, binding affinity (Kd approximately 200 and approximately 10 nM, respectively). The deduced interaction of thrombin with its high-affinity binding site indicated that the thrombin catalytic site is directed toward the platelet surface and therefore, when bound, is proteolytically inactive. Quick I (0.5-5 nM) elicited intraplatelet Ca2+] fluxes at concentrations where high-affinity binding was undetectable. Saturation of high-affinity binding sites with active-site-modified thrombin did not affect thrombin-induced (0.5 nM) or Quick I-induced (5 nM) responses. In contrast, addition of D-Phe-Pro-Arg chloromethyl ketone (FPRCK) subsequent to thrombin or Quick I stimulation of platelets abolished agonist-induced responses. Since Quick I was only 10-17% as effective as thrombin in increasing intraplatelet Ca2+], our data support a model in which thrombin acts enzymatically on a platelet membrane "substrate", through an interaction mediated in part by the fibrinogen binding groove of thrombin. This conclusion is consistent with the inhibition observed with high concentrations (greater than 100 nM) of Quick II and FPRCK-modified thrombin (FPR-thrombin) in platelets stimulated with low concentrations of thrombin (less than 0.5 nM) or Quick I (less than 2 nM), consistent with inhibition by substrate depletion. In contrast, concentrations of FPR-thrombin or Quick II (less than 100 nM), which saturated predominantly the high-affinity binding sites, enhanced the platelet responses induced by thrombin (less than 0.5 nM). Thus, occupation of the high-affinity sites with inactive thrombin increased the concentration of active thrombin available for substrate interaction. Quick I-induced responses were not enhanced, consistent with its inability to interact with the high-affinity site. Since thrombin bound to the high-affinity site is proteolytically inactive, we hypothesize that the thrombin high-affinity binding site on platelets functions to alter thrombin activity and platelet activation.