Thrombin domains: structure, function and interaction with platelet receptors.

Thrombin plays a pivotal role in different biological phenomena, such as hemostasis, thrombosis, and cell differentiation. Indeed this protease catalyzes the conversion of fibrinogen into fibrin, the activation of coagulation factors V, VIII, XI, and XIII, but is also involved in the activation of many cell types and platelets. Thrombin bears some recognition domains and insertion loops, not found among other serine proteases of the coagulation system. In this review the properties of these thrombin domains, which regulate the specificity of the enzyme's interaction with substrates and inhibitors, are particularly emphasized. The example of thrombin interaction with the platelet membrane receptors, namely GpIb and PAR1, shows how the concerted action of the insertion loops and recognition domains is the key to solve the apparent enigma as to how thrombin can be at the same time a very efficient and specific enzyme for different substrates and inhibitors.