Structure of heparin and related molecules. (A) Heparin: the major disaccharide unit of heparin is 4-linked α-iduronic acid (IdoA) and 4-linked α-glucosamine (GlcNAc), in which IdoA is 2-O-sulfated and GlcNAc is 6-O- and de-acetyl-N-sulfated. Heparin can be modified by selective de-N-sulfation and/or de-O-sulfation. (B) K5 polysaccharide: this has the same backbone as the precursor to heparin and can be modified by addition of sulfate groups at indicated positions (arrows) and by epimerization of GlcA to IdoA (indicated in gray). This results in polysaccharides with various sulfation levels and patterns and structural features.