Serine protease inhibitor (serpin) that neutralizes Thrombin by forming a 1:1 stoichiometric complex between the two proteins by way of antithrombin’s reactive site (arginine) and Thrombin’s active center (serine) site. Binding of heparin to antithrombin accelerates this interaction nearly 1000 fold. Antithrombin also inactivates hemostatic enzymes of the intrinsic pathway of coagulation (factors IXa, Xa, XIa, and XIIa) by a similar mechanism.
