The structure of thrombin: a janus-headed proteinase.

Through a series of successive, cascade-like proteinase activation and amplification steps, any vascular injury triggers a rapid burst of alpha-thrombin, a trypsin-like serine proteinase. Thrombin, the main executioner of the coagulation cascade, has procoagulant as well as anticoagulant and antifibrinolytic properties. It exhibits quite diverse physiological functions, but also gives rise to several thrombotic disorders, such as thromboembolism, myocardial infarction, and stroke, thus making it an attractive target for antithrombotic agents. Thrombin interacts specifically with several protein substrates, receptors, cofactors, inhibitors, carbohydrates, and modulators. It cleaves fibrinogen, factors XI (FXI) and FXIII, cofactors V and VIII, and the thrombin receptors; uses thrombomodulin to activate protein C and thrombin-activatable-fibrinolysis inhibitor; is inhibited by heparin cofactor II and antithrombin III with the help of acidic carbohydrates; and its activity/specificity is modulated by sodium ions. A large number of crystal structures of alpha-thrombin in complexes with synthetic polypeptides and protein inhibitors, substrate fragments, cofactors, and carbohydrates have displayed extended recognition sites on the thrombin surface, reflecting the versatility and multifunctional specificity of this remarkable proteinase. These structures essentially show that the thrombin surface can be subdivided into several functional regions, which recognize different chemical moieties. By using different combinations of these surface elements, thrombin can interact with a variety of molecules with high specificity, accounting for its multifunctional properties.