The role of complex formation and epidermal growth factor-like domains in the regulation of blood coagulation by the thrombomodulin-protein C system.

The explosive nature of the coagulation cascade led many scientists to investigate how it is regulated. Proteinase inhibitors such as antithrombin III inhibit active proteases of the coagulation cascade. Cofactors such as factor Va and factor VIIIa are proteolytically inactivated by activated protein C. Protein C is activated by the thrombin-thrombomodulin complex on the endothelial cell surface. Thus, the independent actions of the proteinase inhibitor system and the thrombomodulin-protein C system complement each other to maintain regulation of blood coagulation. The thrombin binding site of thrombomodulin was identified to be the fifth and sixth repeats of the epidermal growth factor-like domain. The same binding template contains sufficient information to block the functions of thrombin as a procoagulant. However, additional repeats are required for the activation of protein C. Thrombomodulin is the first example which illustrates that the epidermal growth factor-like domain functions as a binding template for thrombin and as a switch to turn off the procoagulant activity of thrombin as well as to trigger the protein C anticoagulant pathway. Epidermal growth factor-like structures are found in many of the coagulation factors. Complex formation is a repeated theme not only in the blood coagulation cascade but also in the thrombomodulin-protein C anticoagulant pathway.