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Critical Reviews™ in Eukaryotic Gene Expression

Impact factor: 4.111

ISSN Print: 1045-4403
ISSN Online: 2162-6502

Critical Reviews™ in Eukaryotic Gene Expression

DOI: 10.1615/CritRevEukarGeneExpr.v7.i3.40
pages 241-280

The Regulation of Clotting Factors

Michael Kalafatis
Department of Biochemistry, Given Building, Health Science Complex, University of Vermont, College of Medicine, Burlington, Vermont 05405-0068
Jack O. Egan
Department of Biochemistry, University of Vermont, College of Medicine, Burlington, Vermont 05405
Cornells van't Veer
Department of Biochemistry, Given Building, Health Science Complex, University of Vermont, College of Medicine, Burlington, Vermont 05405-0068
Kevin M. Cawthern
Department of Biochemistry, Given Building, Health Science Complex, University of Vermont, College of Medicine, Burlington, Vermont 05405-0068
Kenneth G. Mann
Department of Biochemistry, Given Building, Health Science Complex, University of Vermont, College of Medicine, Burlington, Vermont 05405-0068

ABSTRACT

Blood clotting involves a multitude of proteins that act in concert in response to vascular injury to produce the procoagulant enzyme α-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 VIla 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 α-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/factorXa) 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 α-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 of an appropriate membrane surface, the procoagulant complexes have limited catalytic efficiency. Thus, timely exposure of the adequate membrane surface is an additional step in the regulation of α-thrombin formation, α-Thrombin participates in its own down-regulation by binding to the endothelial cell receptor thrombomodulin, initiating the protein C pathway, which in turn leads to the formation of activated protein C (APC). APC is required for efficient neutralization of factor Va cofactor activity, which results in the inactivation of the prothrombin-activating complex. This inactivation can only occur in the presence of the appropriate membrane surface. Thus, while following α-thrombin activation, factor VIIIa is rapidly and spontaneously inactivated by dissociation of the A2 domain from the rest of the cofactor, APC is required for down-regulation of α-thrombin formation by prothrombinase. APC down-regulates the prothrombinase complex by cleaving specific peptide bonds on the heavy chain of factor Va, which results in the dissociation of the A2 domain of factor Va from the rest of the molecule. Dissociation of the A2 domain of both cofactors impairs their ability to interact with the other protein components of prothrombinase and intrinsic tenase and results in the arrest of α-thrombin formation. The other regulatory proteins in plasma that can prevent continuous activation of prothrombin are the stoichiometric protease inhibitors tissue factor pathway inhibitor (TFPI), which inhibits the factor VIIa/TF complex activity in association with factor Xa and antithrombin III (AT-III), which covalently inhibits α-thrombin, factor IXa, and factor Xa by trapping the active sites of the enzymes. The combination of the protein C system with TFPI and AT-III is very efficient in preventing explosive thrombin generation. Any irregularities in the mechanism of inactivation of factor Va by APC, or in the mechanism of the direct inhibition of the serine protease by the stoichiometric inhibitors, or the unnecessary exposure of a procoagulant membrane surface to the blood flow, is associated with thrombotic episodes due to sustained prothrombin activation. The severity of the thrombotic episodes is directly correlated to the importance of the molecular basis of the defect associated with any protein involved in the down-regulation of α-thrombin formation.