What are the anticoagulant effects of heparin?
Heparin binds to antithrombin (AT) and catalyzes the inactivation of thrombin and factors IIa, Xa, IXa, XIa and XIIa. This is the major mechanism for heparin's anticoagulant effect, and is produced by only one-third of heparin molecules (those containing the unique AT-binding pentasaccharide). Heparin also binds to heparin cofactor II (HCII) and catalyzes inactivation of factor IIa, but this requires high concentrations of heparin and is independent of the pentasaccharide. Lastly, heparin binds to factor IXa and inhibits factor X activation, which requires very high concentrations of heparin and is AT- and HCII-independent.