Biochemistry of Platelet Function
The prothrombinase complex is a membrane-bound, Ca2+-dependent complex comprised of two proteins: factor Xa, a serine protease, and factor Va, its essential cofactor. Prothrombinase catalyzes the generation of the multifunctional enzyme thrombin, thus serving a critical role in hemostasis. Factor Va is derived from the procofactor, factor V, and while approximately 80% of factor V is found in plasma, it is the remaining 20% found in the α-granules of platelets that constitutes the physiologically-relevant source of the molecule. The activation of platelets at sites of vascular injury results in release of the α-granule’s contents, providing a highly-concentrated bolus of the platelet-derived cofactor at a membrane surface where it can immediately support clot formation. As a member of Dr. Paula Tracy’s research team, I am interested in the role that platelets play in hemostasis, with a focus on understanding how the unique structural properties of platelet-derived factor Va make it such a robust cofactor for platelet-bound prothrombinase. To this end, a variety of physical and biochemical techniques are being used to characterize the post-translational modifications found on both plasma-derived and platelet-derived factor Va, including N-linked and O-linked glycosylation, phosphorylation, sulfation, proteolysis, and lipid anchor attachment, with the goal of correlating these structural attributes with cofactor function. Of similar interest are the origins of these unique modifications. Platelet-derived factor Va is formed within megakaryocytes, the progenitor cells of platelets, subsequent to the endocytosis of factor V from plasma. Such alteration and repackaging of an endocytosed molecule in preparation for subsequent use represents a novel paradigm in cellular trafficking, and identifying the locations and mechanisms involved in this process will provide new insights into the biological processes of cells. Ultimately, our detailed understanding of platelet-bound prothrombinase will enable increasingly well informed decisions to be made in the context of clinical interventions aimed at maintaining hemostasis and ameliorating its pathological extension, thrombosis.