What is the mechanism of action of heparin and LMWH?

Heparin acts at a number of molecular targets, but its anticoagulant effect is a consequence of binding to antithrombin III (pentasaccharide sequence of heparin), with the subsequent rapid inactivation of coagulation factors. Heparin serves as a true catalyst (cofacteur), allowing antithrombin III to rapidly combine with and inhibit circulating thrombin and Factor Xa. LMWHs complex with antithrombin III and inactivate Factor Xa but do not bind as avidly to thrombin. **AT III: Antithrombin III is an α-globulin. It inhibits serine proteases, including several of the clotting factors most importantly, thrombin (Factor IIa) and Factor Xa