Modulation of fibrin cofactor activity in plasminogen activation

Fibrin is a cofactor for the formation of plasmin from plasminogen as catalyzed by tissue plasminogen activator. Initial cleavages of fibrin by plasmin upregulates the cofactor activity of fibrin by exposing carboxyl terminal lysine residues. This effect is eliminated by a carboxypeptidase B-like enzyme generated from the precursor, thrombin activatable fibrinolysis inhibitor (TAFI) that is generated by thrombin during the formation of fibrin. Thus, TAFI and its activation to TAFIa create a link between the coagulation and fibrinolytic cascade, such that activation of the former suppresses the latter. Complete solubilization of fibrin results in a family of very large fibrin degradation products. These also have very substantial tissue plasminogen activator cofactor activity that is very highly downregulated by TAFIa.

Thrombin, thrombomodulin and TAFI in the molecular link between coagulation and fibrinolysis

The thrombin thrombomodulin dependent activation of the plasma protein TAFI (Thrombin Activatable Fibrinolysis Inhibitor) and Subsequent Inhibition of Fibrinolysis by the TAFIa is described. Work to date indicates that TAFIa is a carboxypeptidase B enzyme that suppress fibrinolysis most likely by down regulating the cofactor functions of partially degraded fibrin. The existence of TAFI provides the explanation for the apparent profibrinolytic effect of activated protein C. and implies the existence of an explicit molecular connection between the blood coagulation of fibrinolytic cascades that is expressed through the thrombin thrombomodulin dependent activation of TAFI. Thus, thrombin generation can, in principle, result in the suppression of fibrinolysis.