Heparin mechanism of action

Overview
Heparin is an indirect thrombin inhibitor. It's anticoagulation effects are due to activation of antithrombin III, that in turn inactivates thrombin (factor IIa) and factor Xa. The other minor mechanism of benefits are due to its inhibition of platelet aggregation.

Mechanism of action

 * Heparin is an indirect thrombin inhibitor. It has its effects through the following mechanisms-
 * It forms a complex with antithrombin / AT III that results in a conformational change in antithrombin, converting it from a slow to a rapid inactivator of thrombin. It also inactivates other factors like factor Xa, factors XIIa, XIa, and IXa.
 * Antithrombin has two active sites: reactive center, Arg393-Ser394 and heparin binding sites. The unfractionated heparins have long saccharide units that are lacking in in the low molecular weight heparins (LMW), and fondaparinux. These long chains of polysaccharides are required for heparins to bind to both AT and to a binding site on thrombin and formation of a tenary complex. Accordingly, LMW heparins and fondaparinux have less antithrombin activity than does unfractionated heparin.
 * Direct binding to platelets
 * Binding to heparin cofactor II (high concentrations).