What is the function of antithrombin and what is its interaction with thrombin and heparin?

Antithrombin is the most important natural inhibitor of the coagulation cascade, accounting for approximately 80% of the thrombin inhibitory activity in plasma. By inhibiting the coagulation proteases, especially thrombin, FXa, and FIXa, AT prevents uncontrolled coagulation and thrombosis. Inhibition of antithrombin involves the formation of a stable 1:1 complex between the active domain of the serine protease such as thrombin, and the reactive site of antithrombin, which proteases initially recognize as a substrate. During the cleavage of the reactive site bond in antithrombin, a conformational change occurs in the inhibitor that traps the protease.

Slow protease-antithrombin interactions are enhanced dramatically in the presence of certain sulfated polysaccharides like heparan sulfate. Heparin is a commercial preparation of heparan sulfate, and binds antithrombin and thrombin, thereby catalyzing the thrombin-AT reaction. Binding to antithrombin induces a conformational change in AT that facilitates its reaction with thrombin. Thrombin binds to heparin in a non-specific manner and slides along the chain until it encounters the bound AT. The affinity of heparin to the thrombin-AT (TAT) complex is much lower than to free AT. Heparin will therefore dissociate from the TAT complex, which is rapidly removed from the blood circulation by the liver.