Online

Ayombil, Francis

Biochemistry

2016

Ph.D.

At sites of vascular injury, the critical blood clotting enzyme thrombin is generated from prothrombin via Prothrombinase, a macromolecular, Ca2+-dependent enzymatic complex consisting of the serine protease factor Xa and the non-enzymatic cofactor factor Va, assembled on the membranes of activated platelets. Platelets regulate thrombin formation by providing specific binding sites for the components of Prothrombinase and by releasing a platelet-derived factor V/Va molecule that is more procoagulant than its plasma counterpart and partially resistant to proteolytic inactivation. This dissertation identifies and characterizes the subpopulation of platelet-derived factor V/Va that is responsible for the observed protease resistance, and the mechanism by which Prothrombinase bound to platelets differs from a model system using vesicles composed of 75% phosphatidylcholine (PC) and 25% phosphatidylserine (PS), PCPS vesicles. Previous studies have demonstrated that activated platelets release a dissociable pool of factor V/Va and a non-dissociable, membrane-bound pool, which is covalently attached to the platelet membrane through a glycosylphosphatidyl inositol (GPI) anchor. Data described herein demonstrate unequivocally that the pool of platelet-derived factor V/Va that is resistant to proteolytic inactivation by activated protein C is provided exclusively by the non-dissociable GPI-anchored pool. Further, although this factor Va pool is susceptible to proteolysis by plasmin, the fragments formed are associated with sustained and increased cofactor activity. These observations indicate that tethering of factor Va to the membrane surface via a GPI anchor imparts resistance to proteolytic inactivation and sustained thrombin generation at sites of vascular injury. For several years it has been known that Prothrombinase assembled on PCPS vesicles does not mimic that bound to platelets. While both enzymes cleave prothrombin at Arg271 and Arg320 to form thrombin, prothrombin activation proceeds via the prethrombin-2 pathway (initial cleavage at Arg271) on the platelet surface, in contrast to the meizothrombin pathway (initial cleavage at Arg320) on PCPS vesicles. Using thrombin active site inhibitors, we demonstrate that the preference for either pathway is dictated by the conformation in which prothrombin is bound by the membrane-bound enzyme. The prethrombin-2 pathway of prothrombin activation catalyzed by platelet-bound Prothrombinase is a direct consequence of configuring prothrombin in a proteinase-like state resulting in the exposure of a pseudo-active site that can be stabilized by active site thrombin inhibitors. Conversely, prothrombin is preferentially configured in the zymogen-like state on PCPS vesicles where the meizothrombin pathway is preferred. Additional support for the differential assembly of Prothrombinase on the platelet surface is provided by observations made using prethrombin-1, an intermediate formed by cleavage of prothrombin at Arg155 by the formed thrombin. Prethrombin-1 is converted into fragment-2 and thrombin by platelet-bound Prothrombinase at a substantially higher rate than vesicle-bound Prothrombinase. The decreased rate of prethrombin-1 activation in the model system is due, in part, to inhibition of the vesicle-bound enzyme by the fragment-2 generated in the reaction. Taken together, these data not only provide important molecular insights into the mechanisms by which Prothrombinase bound to activated platelets at sites of vascular injury regulates the procoagulant response to effectively support robust thrombin generation, but also provides potential mechanistic sites that could be targeted therapeutically.