The Regulation of Clotting Factors

Abstract

Blood clotting involves a multitude of proteins that act in concert in response to vascular injury to produce the procoagulant enzyme alpha-thrombin, which in turn is responsible for the generation of the fibrin plug. However, while generation of the fibrin plug is required for the arrest of excessive bleeding, unregulated clotting will result in the occlusion of the blood vessels and thrombosis. Thus, the regulation of the delicate balance between the procoagulant and anticoagulant mechanisms is of extreme importance for survival. While the majority of proteins involved in blood coagulation circulate as inactive zymogens that require proteolytic activation in order to function, approximately 1% of the circulating factor VII molecules are active. Factor VIIa, possess a serine protease active site, has poor catalytic activity, and is not inhibited by the circulating stoichiometric protease inhibitors. Following injury to the vasculature and subsequent exposure of the integral membrane glycoprotein, tissue factor (TF), the circulating factor VIIa molecules can bind to the exposed TF forming the extrinsic tenase complex (TF/factor VIIa) and initiate the blood coagulation process. Formation of the TF/factor VIIa complex increases the catalytic efficiency of the enzyme by four orders of magnitude when compared with factor VIIa alone. This cell-associated enzymatic complex initiates a series of enzymatic reactions, leading to the generation of alpha-thrombin and ultimately to the formation of the fibrin plug. The procoagulant enzymatic complexes (i.e., prothrombinase, intrinsic tenase, and extrinsic tenase) are similar in structure and composed of an enzyme, a cofactor, and the substrate associated on a cell surface in the presence of divalent metal ions. While the activity of the extrinsic tenase complex is limited by the availability (exposure) of its cell-associated cofactor (TF) it is remarkable that the activities of both the prothrombinase complex (factor Va/factor Xa) as well as the intrinsic tenase complex (factor VIIIa/factor IXa) are limited by the presence of the two soluble, nonenzymatic cofactors, factor Va and factor VIIIa. Factor Va and factor VIIIa, which are very similar in structure and function, are required for prothrombinase and intrinsic tenase activities, respectively, because both cofactors express a dual function in their respective complexes, acting as an enzyme receptor and catalytic effector on the cell surface. The cofactors derive from inactive plasma precursors by regulatory proteolytic events that involve alpha-thrombin. In general, bleeding tendencies are usually associated with defects in the activation of one of the zymogens or the cofactors of the procoagulant complexes. However, the activity of all of the complexes is also limited by the availability of an adequate membrane surface provided by endothelial cells, platelets, and monocytes. The cell surface provides a site for the recruitment of the appropriate proteins and allows for fast and efficient clot formation. In the absence o...

The Contribution of Amino Acid Region Asp695-Tyr698 of Factor V to Procofactor Activation and Factor Va Function

“…The prothrombinase complex, which is composed of the nonenzymatic cofactor factor Va, the enzyme, factor Xa, and the substrate, prothrombin, associated on a cell membrane-surface in the presence of Ca 2ϩ ions, is responsible for ␣-thrombin formation during blood coagulation (1,2). The prothrombinase complex catalyzes the activation of prothrombin ϳ300,000 times more efficiently than factor Xa alone (3).…”

Structural Requirements for Expression of Factor Va Activity

“…The prothrombinase complex responsible for the generation of ␣-thrombin in the hemostatic process is composed of factor Va and factor Xa associated on a phospholipid membrane in the presence of Ca 2ϩ (1,2). Although factor Xa alone can convert prothrombin to ␣-thrombin, the prothrombinase complex has a catalytic efficiency five orders of magnitude greater than factor Xa acting alone (3).…”

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Role of the Gla and First Epidermal Growth Factor-like Domains of Factor X in the Prothrombinase and Tissue Factor-Factor VIIa Complexes

“…It is clear that the catalytic specificities of blood coagulation proteases are supported by the carboxyl-terminal half of the enzymes, a trypsin-like serine protease domain (20). The prime role of the serine protease domain has also been demonstrated in the interaction of the blood coagulation proteases with their cofactors.…”