Heparin structure.

Heparins used in therapy are largely constituted by sequences of the trisulfated disaccharide L-iduronic acid-2-sulfate----D-glucosamine-N,6-disulfate. These regular sequences are interrupted by undersulfated (occasionally, oversulfated) sequences containing D-glucuronic acid and N-acetylated D-glucosamine. Different heparin sequences are binding domains for heparin cofactors and plasma proteins. The active site for antithrombin is a specific pentasaccharide sequence containing 3-O-sulfated D-glucosamine. Heparin cofactor-II binds, less specifically, mostly to the regular sequences. The conformational flexibility of iduronic acid residues contributes to the binding versatility and to the 'biological reactivity' of heparin.