Antithrombin III (ATIII)

Antithrombin III is a plasma serine protease inhibitor (serpin) that plays a central role in regulating blood coagulation by inactivating thrombin (factor IIa) and factor Xa, among other serine proteases. Its inhibitory activity is greatly enhanced in the presence of glycosaminoglycans such as heparin, which induces a conformational change in the antithrombin molecule, increasing its affinity for the coagulation enzymes. The ATIII-thrombin (or ATIII-Xa) complex is then rapidly cleared from the circulation. Antithrombin III deficiency, either congenital or acquired, is a risk factor for thromboembolic disease, while excessive ATIII activation can lead to bleeding. Drugs like heparin act by potentiating the anticoagulant action of ATIII, forming the mechanistic basis for their clinical use in thrombosis prevention and treatment.

Other names

AntithrombinAntithrombin III (ATIII)Serpin C1AT-III

Drugs such as heparin and fondaparinux *potentiate antithrombin III activity* by inducing a conformational change, dramatically enhancing the rate at which ATIII inhibits factors IIa and Xa. Sulfated polysaccharides similarly increase ATIII’s ability to inactivate IIa/Xa.