Thrombin, the protease cleavage product of prothrombin by prothrombinase, is a two chain protein (Mr 37,000) consisting of an amino-terminal A chain (Mr 6,000) disulfide-bonded to a carboxyl-terminal B chain (Mr 31,000). Thrombin has many physiological roles including cleavage of Aα and Bβ peptides of fibrinogen to form fibrin; activation of factors V and VIII, protein C; and platelet activation via cleavage of PAR-1 and PAR-4.