Intrinsic Prothrombin Activation Pathway
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Intrinsic Prothrombin Activation Pathway

The process by which the body prevents blood loss is referred to as coagulation. Thrombin/TFIIA (Activated Factor-II) is a multifunctional serine proteinase, which serves as an essential component of the process of Blood Coagulation - the hemostatic process of greatest interest. When a blood vessel is injured, bleeding is stopped by clotting (Coagulation) factors that form a thrombus (clot) of Fibrin threads which trap platelet aggregates and other blood cells. Clotting is a mechanism used by the body to stop bleeding. Our body needs to be able to clot blood as this is the normal way bleeding is stopped to begin the healing following an injury. The first step in clotting is adhesion of platelets, which are fragments of blood cells that circulate in the blood, to the cut edges of a damaged blood vessel. In this way, a platelet plug is formed and external bleeding stops. Next, small molecules, called clotting factors, cause Fibrin strands to stick together and seal the inside of the wound. Eventually, the cut blood vessel heals and the blood clot dissolves after a few days. The protease Thrombin commands an indispensable role in the Coagulation Cascade and is essential for Fibrin formation and platelet activation. It resides in the cell in an inactive form, called Prothrombin. Basing on the activation of Prothrombin, Blood Coagulation Cascade operates in 3 essential phases: 1) Formation of a Prothrombin Activator complex; 2) Activation of Prothrombin; and 3) Clot formation as a result of Fibrinogen cleavage by activated Thrombin (Ref.1, 2 & 3).

The process of generation of Thrombin can be divided into three phases: the Intrinsic and Extrinsic pathways which provide alternative routes for the generation of the clotting factor: FXa (Activated Factor-X), and the final Common pathway which results in Thrombin formation. Both the Intrinsic and Extrinsic pathways lead to the formation of Prothrombin Activation Complex, each giving rise to a different form of the Prothrombin Activator. The Intrinsic mechanism of Prothrombin activator formation begins with trauma to the blood or exposure of blood to Collagen in a traumatized vessel wall. The Intrinsic pathway consists of the clotting factors: FVIII (Factor-VIII), FIX, FX, FXI, FXII, FV and Prothrombin (FII), which is converted to Thrombin (FIIa). Also required are the proteins PK (Prekallikrein) and HK (High-molecular-weight Kininogen), as well as Calcium ions and PF3 (Platelet Factor-3) secreted from platelets. Each of these pathway constituents contribute to the conversion of FX (Factor-X) (inactive) to FXa (a signifies active). Initiation of the Intrinsic pathway occurs when Prekallikrein, HK, FXI and FXII are exposed to a negatively charged surface. This is termed the contact phase. Exposure of Collagen to a vessel surface is the primary stimulus for the contact phase (Ref.1, 3 & 4).

The assemblage of contact phase components results in conversion of Prekallikrein to Kallikrein, which in turn activates FXII to FXIIa. FXIIa can then hydrolyze more Prekallikrein to Kallikrein, establishing a reciprocal activation cascade. FXIIa also activates FXI to FXIa. In the presence of Ca2+, FXIa activates FIX to FIXa. FIX is a proenzyme that contains vitamin K-dependent Gamma-Carboxyglutamate residues, whose serine protease activity is activated following Ca2+ binding to these Gamma-Carboxyglutamate residues. FIXa cleaves FX leading to its activation to FXa. The activation of FXa is facilitated and stabilized by the assemblage of the Tenase Complex (Ca2+ and Factors: FVIIIa, FIXa and FX) on the surface of activated platelets. The role of FVIII in this process is to act as a receptor, in the form of FVIIIa, for factors IXa and X. The activation of FVIII to FVIIIa (the actual receptor) occurs in the presence of minute quantities of Thrombin. As the concentration of Thrombin increases, FVIIIa is ultimately cleaved by Thrombin and inactivated. This dual action of Thrombin, upon FVIII, acts to limit the extent of Tenase Complex formation and thus the extent of the coagulation cascade. FVIII, is proteolytically activated by Thrombin to FVIIIa in a second autocatalytic process leading to Prothrombin activation.  The activation of the Tenase Complex ultimately leads to the FX to FXa. FXa in turn binds FVa, a membrane-associated cofactor for FXa-mediated cleavage of Prothrombin to active Thrombin. Finally, a complex, known as the Prothrombin Activator Complex is formed, consisting of FXa, FVa, FVIIIa, and Phopspholipid from platelets. This complex, in turn mediates the cleavage of Prothrombin to Thrombin. This conversion requires a phospholipid surface and Calcium ion. Thrombin converts Fibrinogen (Factor-I) to Fibrin (Factor-Ia). The Fibrin undergoes polymerization and the Fibrin polymer is further stabilized by FXIIIa (Activated Factor-XIII), which sews up the clot (much like forming an intricate network of cross-stitched strands of Fibrin). The conversion of FXIII to FXIIIa is aided by Thrombin. Fibrin forms a mesh that, in concert with the platelets, and some blood cells (RBCs (Red Blood Cells and WBCs (White Blood Cells)) plugs the break in the vessel wall, termed as the Blood Clot. Eventually, the cut blood vessel heals and the blood clot dissolves after a few days (Ref.2, 4, 5 & 6).

Blood clot formation is essential for the maintenance of Hemostasis.  The absence of any of the clotting factors predisposes animals to severe, often life-threatening, bleeding disorders. In a similar way, overactivity of this system can produce unwanted blood clots, resulting in blockages to critical blood vessels, as occurs in such diseases as heart attack and stroke. In addition to the clotting fators and proteins, several other proteins operate to slow down the clotting process. Antithrombin serves to block the actions of multiple clotting factors, involved in the formation of the Prothrombin Activator Complex. The newly formed Thrombin activates the formation of a complex of Protein-C, Protein-S and TM (Thrombomodulin) that can inactivate FVIII and FV. As enhanced coagulation and thrombosis are linked to a variety of cardiovascular and metabolic diseases, as well as to cancer, inhibition is essential in order to keep a check on excessive clotting, and hence, the unwanted clots (Ref.7 & 8).