Structural features of disaccharides in heparan sulfate within the sites modified by 3-OST-1 and 3-OST-3_A or 3-OST-3_B. Substrate recognition by each enzyme probably involves binding to more than two residues in the polymer, but the groups colored in green may contribute to recognition by each enzyme. The sulfate group added by each enzyme is shown in red. The disaccharide found in 3-OST-1–modified heparan sulfate is part of the pentasaccharide to which antithrombin binds. The disaccharide found in 3-OST-3–modified heparan sulfate is part of the binding site for gD, the size and overall structure of which is not yet known.