Figure 4.

Binding by a 6-O-sulfated oligosaccharide substrate to 3-OST-3. A: superposition of 3-OST-3 binding HS without 6-O-sulfo groups (protein gray, HS green; pdbcode 6XL8) with 3-OST-3 binding 6-O-sulfo containing HS in a productive binding mode consistent with catalysis (protein light blue, HS light purple; PDB 6XKG molecule B). Red arrow denotes acceptor 3OH position. B: superposition of the productive binding mode of HS with 6-O-sulfo group (protein light blue, HS light purple/transparent; PDB 6XKG 3-OST-3 molecule B) with 3-OST-3 binding the same 8-mer oligosaccharide in a nonproductive manner (protein and HS in tan; PDB 6XKG 3-OST-3 molecule A). The red asterisk is positioned at the 3OH that would typically be found bound at the active site near the PAP as in A. The structure of the octasaccharide substrate with 6-O-sulfation used in the crystal study is shown at the bottom of the figure. HS, heparan sulfate; PAP, 3′-phosphoadenosine 5′-phosphate; 3-OST, 3-O-sulfotransferase.