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Clot formation and fibrinolysis is a balance of plasmin activation/inhibition and thrombin-thrombomodulin activity that regulates fibrin polymer formation and degradation. Active thrombin is produced by the cleavage of prothrombin in the intrinsic thrombin activation pathway or the extrinsic thrombin activation pathway. Cleavage of fibrinogen by thrombin releases the fibrin monomers that auto-polymerize within seconds into fibrin threads or fibers. The coagulation cascade has many feedback loops. One example is the binding of thrombin to the fibrin polymers resulting in a reduction in soluble thrombin. The fibers form a more stable clot as a result of the covalent bonds formed by activated factor XIII enzyme (also known as Fibrin-stabilizing factor). These fibers form a mesh that traps platelets, blood cells and plasma to form a clot. The removal of the clot is caused by plasmin cleavage of the fibrin monomers into soluble fibrin degradation products. Plasmin is formed by the cleavage of plasminogen between Arg561 and Val562. Plasmin is a two-chain trypsin-like serine protease. Plasminogen activator inhibitor 1 (PAI1) and plasminogen activator inhibitor 2 (PAI2) inhibit cleavage of plasminogen by tissue-type plasminogen activator (tPA) or urokinase plasminogen activator (uPA). The presence of fibrin fibers and fibrin degradation products [(DD)E1 and (DD)E2] exert a two-fold stimulation of tPA and uPA. Plasmin activity is also inhibited by alpha2-antiplasmin. Thrombin activatable fibrinolysis inhibitor (TAFI) is a carboxypeptidase B-like proenzyme activated by the thrombin-thrombomodulin dimer. TAFI cleaves (DD)E2 to separate DD and E fragments which do not enhance the activation of tPA or uPA and results in a reduced feedback signal. Overabundance or increased activity of the plasminogen activator inhibitors or reduced presence or function of tPA or uPA can result in atherosclerotic disease and venous thrombosis due to an increase in fibrin deposition or formation of a thrombus. Thrombosis can also result from plasminogen deficiency caused by a lack of protein or lack of functional protein. Reduced or depleted levels of alpha2-antiplamin can result in severe bleeding disorders. (This definition may be outdated - see the DesignNote.)
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