FIG. 3.

Positions of residues 226 and 228 in the RBS of influenza virus HA in relation to the Neu5Ac2,3Gal moiety. Only the relevant portion of the human virus X31 HA complexed with 3′ sialylactose (1HGG structure, Brookhaven Protein Databank [19]) and the Neu5Acα2-3Gal moiety of 3′ sialyl lactose (heavy atoms, stick presentation) are shown for clarity. Atoms of sialic acid (NAN) and galactose (GAL) that are thought to participate in hydrogen bonding with HA (19) are shown as small white balls; atoms of the protein that participate in these hydrogen bonds are shown as white on black background. Substitution 226S→G results in a loss of a hydrogen bond between the hydroxyl group of serine and the O-9 hydroxyl of sialic acid, whereas mutation 226L→Q results in new hydrogen bonds between the amide side chain of glutamine and the C-8 hydroxyl and carboxylic groups of Neu5Ac (30). This figure was generated with a WebLab viewer (Molecular Simulations, Inc., San Diego, Calif.).