Clotting factors, surface-activated

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

The initial steps of the intrinsic pathway are somewhat more complicated. This system requires the presence of clotting factors VIII, IX, XI and XII, all of which, except for factor VIII, are endo-acting proteases. As in the case of the extrinsic pathway, the intrinsic pathway is triggered upon exposure of the clotting factors to proteins present on the surface of body tissue exposed by vascular injury. These protein binding/activation sites probably include collagen. 

Figure 17.1 Summary of the four cascades that result from trauma or bleeding and the reactions they catalyse. These are all activated by the blood clotting factor, Xlla (also known as the Hageman factor). Details of each cascade are presented in Figures 17.2, 17.4 and 17.6. Factor XII is activated by collagen and negatively charged surfaces to form the active form, Xlla.

The clotting factors are protein molecules. Activation mostly means proteolysis (cleavage of protein fragments) and, with the exception of fibrin, conversion into protein-hydrolyzing enzymes (proteases). Some activated factors require the presence of phospholipids (PL) and Ca + for their proteolytic activity. Conceivably, Ca + ions cause the adhesion of factor to a phospholipid surface, as depicted in C. Phospholipids are contained in platelet factor 3 (PF3), which is released from ag-Lullmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of license. 

It is also important to note that the coagulation mechanism in vivo does not occur in solution, but is localized to activated cell surfaces expressing anionic phospholipids such as phosphatidylserine, and is mediated by Ca2+ bridging between the anionic phospholipids and 7-carboxyglutamic acid residues of the clotting factors. This is the basis for using calcium chelators such as ethylenediamine tetraacetic acid (EDTA) or citrate to prevent blood from clotting in a test tube. 

Thrombin cannot be allowed to circulate freely in the blood, but must be produced rapidly when clotting is initiated. Thrombin is derived from its inactive precursor, prothrombin, in a cascade of reactions in which a sequence of inactive factors is activated each factor activates the next by proteolytic cleavage. The whole process is initiated by exposure of blood to an abnormal surface, such as collagen, which initially triggers the aggregation of platelets. 

Figure 10.37. Blood-Clotting Cascade. A fibrin clot is formed by the interplay of the intrinsic, extrinsic, and final common pathways. The intrinsic pathway begins with the activation of factor XII (Hageman factor) by contact with abnormal surfaces produced by injury. The extrinsic pathway is triggered by trauma, which activates factor VII and releases a lipoprotein, called tissue factor, from blood vessels. Inactive forms of clotting factors are shown in red their activated counterparts (indicated by the subscript "a") are in yellow. Stimulatory proteins that are not themselves enzymes are shovm in blue. A striking feature of this process is that the activated form of one clotting factor catalyzes the activation of the next factor.

Factor VII is the most sensitive of the vitamin K-dependent clotting factors. The mode of action is tissue factor-dependent activation of factors Xa and IXa on the surfaces of activated platelets (1). Factor Xa leads to thrombin generation and hemostasis, by converting fibrinogen to fibrin. This process is limited to the site of injury, since exposure of tissue factor from the subendothelial matrix has a role in the action of recombinant factor Vila, thereby reducing the risk of thromboembohc events (2). 

Fig. 11.4 Mechanism of clotting factor localization to an activated platelet surface. Left After synthesis in the liver, certain blood clotting proteins are posttranslationally modified in the endoplasmic reticulum by a vitamin K-dependent Vit K carboxylase. This enzyme forms carboxyglutamate residues (top center) that chelate calcium ions. Right In the bloodstream, clotting factor-bound calcium ions attach to negatively charged phosphatidylserine that appears on the surface of activated platelets. Certain therapeutic drugs or acquired deficiencies inhibit this process - see text (Original figure submitted by Dr Paul DeAngelis, Department of Biochemistry, University of Oklahoma HSC, Oklahoma City, OK, USA)...

The activity of surface-bound factor Xa is controlled by a third serpin, Z-dependent protease inhibitor (ZPI) which circulates in blood along with protein Z. The latter has a gla domain and it attaches to the activated platelet surface alongside the activated clotting factors. Protein Z attaches ZPI in which it alters the conformation of a surface loop and causes a residue, not arginine, to protrude and inactivate the bound factor Xa. Protein Z dissociates from the ZPI-factor Xa complex, but ZPI itself gets cut by factor Xa into two fragments that soon dissociate from the complex. Protein Z replaces a glycosaminoglycan in the previous mechanism, but ZPI inhibition of factor Xa is transient. ZPI may work best where an... 

Tissue factor pathway inhibitor (TFPI) is a Kunitz type inhibitor composed of three Kunitz domains. The N-terminal Kunitz domain binds to and inhibits the VHa-TF complex that activates factor X (Fig. 11.6a), whereas the downstream Kunitz domain binds directly to factor Xa and inhibits it strongly and permanently. No function has yet been demonstrated for the third Kunitz domain. The C-terminal region of TFPI is basic and remains attached to endothelial cell surfaces where it inhibits inadvertent factor Xa activation. Studies in mice indicate that knocking out the gene for TFPI stops fetal development. Indeed, clotting diseases such as a stroke and heart attacks are associated with mutations of AITH, HCII and ZPI, but not of TFPI. Like TF (Sect. 11.3.1), diminished TFPI activity may be incompatible with life. 

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