From Levy and Connors:
Heparin polymers bind to antithrombin and thereby accelerate the interaction between antithrombin and thrombin or antithrombin and factor Xa, either of which results in the inhibition of prothrombotic activity. A specific pentasaccharide sequence of unfractionated heparin and low-molecular-weight heparin binds to antithrombin and can inhibit factor Xa, but longer, unfractionated heparin polymers containing 18 or more polysaccharide units are required to inhibit thrombin. A specific pentasaccharide sequence of unfractionated heparin and low-molecular-weight heparin binds to antithrombin and can inhibit factor Xa, but longer, unfractionated heparin polymers containing 18 or more polysaccharide units are required to inhibit thrombin.
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