Fig. 1.  Structure and function of heparin. Heparins are a heterogeneous mix of heparan sulfate (HS) glycosaminoglycans. Each HS strand is composed of repeating disaccharide units of N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcA) or iduronic acid (IdoA). GlcNAc can be sulfated at three distinct sites (-6S, -NS, and -3S) and IdoA at one (-2S). Unfractionated heparin is composed of HS chains that are >30 saccharides in length, whereas low-molecular weight heparin constituent HS chains are 22 saccharides or less (3). The charge distribution of heparin imparted by the presence of the precise pentasaccharide sequence shown allows for the binding of heparin to serine protease inhibitor antithrombin-III (AT3), conferring its primary anticoagulant effect. Innumerable other sulfation sequences are found in heparin preparations, which leads to binding and biologically relevant activity modulation of many other proteins.