Antithrombin–heparin complexes

Antithrombin is a serine protease inhibitor (serpin) that inhibits many plasma proteases. In particular, antithrombin functions as one of the major natural anticoagulants by irreversibly inhibiting enzymes formed during activation of the coagulation cascade in vivo [1]. Antithrombin can form serpin-protease inhibitor complexes with activated factor (F) XII (FXIIa), FXIa, FIXa, FXa, and thrombin [1-3]. Within this group of coagulant proteases, antithrombin has the fastest rate of reaction with thrombin, its preferred reactant [4].