Interactions between σ1 and sialic acid.

<p>(<b>A</b>) Simulated annealing omit difference density map contoured at 3σ and displayed with a radius of 2.2 Å around the bound α-2,3-sialyllactose. The sugar moieties are labeled Sia (sialic acid), Gal (galactose), and Glc (glucose) here and in subsequent figures. (<b>B</b>) Detailed interactions between σ1 and the terminal sialic acid of α-2,3-sialyllactose. Residues in the binding region are drawn in ball and stick representation, while the rest of the protein is shown as a ribbon drawing. The σ1 residues forming hydrogen bonds or salt bridges with the ligand are shown in green, and residues forming van der Waals contacts are shown in cyan. The side chain of Asn189 (colored dark blue) is contributed by a neighboring σ1 monomer (see also <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1002166#ppat-1002166-g001" target="_blank">Figure 1A</a>). Sialic acid is shown in ball-and-stick representation, with carbons colored orange, oxygens colored red, and nitrogens colored blue. Bridging waters are shown as orange spheres. Hydrogen bonds and salt bridges are represented with broken lines. (<b>C</b>) Superposition of all three bound ligands into a single binding site. The superposition was performed using σ1 residues only. While the orientation of the terminal sialic acid is nearly identical, the lactose moieties are facing in different orientations as a result of their participation in different crystal contacts.