 antithrombin (at) is a small protein molecule that inactivates several enzymes of the coagulation system. antithrombin is a glycoprotein produced by the liver and consists of 432 amino acids. it contains three disulfide bonds and a total of four possible glycosylation sites. Î±-antithrombin is the dominant form of antithrombin found in blood plasma and has an oligosaccharide occupying each of its four glycosylation sites. a single glycosylation site remains consistently un-occupied in the minor form of antithrombin, Î²-antithrombin. its activity is increased manyfold by the anticoagulant drug heparin, which enhances the binding of antithrombin to factor ii and factor x. == antithrombin nomenclature == antithrombin is also termed antithrombin iii (at iii). the designations antithrombin i through to antithrombin iv originate in early studies carried out in the 1950s by seegers, johnson and fell. antithrombin i (at i) refers to the absorption of thrombin onto fibrin after thrombin has activated fibrinogen. antithrombin ii (at ii) refers to a cofactor in plasma, which together with heparin interferes with the interaction of thrombin and fibrinogen. antithrombin iii (at iii) refers to a substance in plasma that inactivates thrombin. antithrombin iv (at iv) refers to an antithrombin that becomes activated during and shortly after blood coagulation. only at iii and possibly at i are medically significant. at iii is generally referred to solely as "antithrombin" and it is antithrombin iii that is discussed in this article. ==structure==  antithrombin has a half-life in blood plasma of around 3 days.{{cite journal | author = collen d, schetz j, de cock f, holmer e, verstraete m | title 
Coagulation_system Human_proteins Serine_protease_inhibitors