583Theme Issue Article Natural inhibitors of thrombin

The serine protease thrombin is the effector enzyme of blood coagu-lation. It has many activities critical for the formation of stable clots, including cleavage of fibrinogen to fibrin, activation of platelets and conversion of procofactors to active cofactors. Thrombin carries-out its multiple functions by utilising three special features: a deep active site cleft and two anion binding exosites (exosite I and II). Similarly, thrombin inhibitors have evolved to exploit the unique features of thrombin to achieve rapid and specific inactivation of thrombin. Ex-ogenous thrombin inhibitors come from several different protein families and are generally found in the saliva of haematophagous ani-mals (blood suckers) as part of an anticoagulant cocktail that allows them to feed. Crystal structures of several of these inhibitors reveal how peptides and proteins can be targeted to thrombin in different and interesting ways. Thrombin activity must also be regulated by en-dogenous inhibitors so that thrombi do not occlude blood flow and cause thrombosis. A single protein family, the serpins, provides all four of the endogenous thrombin inhibitors found in man. The crystal struc-tures of these serpins bound to thrombin have been solved, revealing a similar exosite-dependence on complex formation. In addition to forming the recognition complex, serpins destroy the structure of thrombin, allowing them to be released from cofactors and substrates for clearance. This review examines how the special features of throm-bin have been exploited by evolution to achieve inhibition of the ulti-mate coagulation protease