A. Ribbon diagram of the crystal structure of human 3-OST-5 with bound PAP (5). Secondary structural elements are shown as α-helices (blue, labeled as A#), β-strands (green, labeled as B#), and random coils (yellow). The bound PAP molecule is drawn as ball-and-stick (cyan). This figure was created using Molscript and Raster3D. Panels B–D show substrate recognition by 3-OST enzymes via a gating structure. The bound PAP molecule is shown as ball-and-stick, though only the phosphate moieties (orange) are visible due to their buried location. Amino acid residues are shown on the surface in red, and are labeled in white text. Distances measured are shown as black dotted lines, and are labeled in white. The position of the substrate binding cleft is shown as a dashed green arrow. B. Key amino acid residues forming a ‘gate’ on the nonreducing end of the substrate binding cleft of m3-OST-1. C. Structurally corresponding gate residues in h3-OST-3. The bound tetrasaccharide substrate (ΔUA2S-GlcNS6S-IdoUA2S-GlcNS6S, 6) is drawn as ball-and-stick, with the glucosamine residue to be sulfated circled in yellow. D. Structurally corresponding gate residues in h3-OST-5. These molecular surfaces were generated using PyMOL.