Figure 3

Location of added N-linked glycans on the HA head domains of H3, H1 and H5 hemagglutinins. The structures of the H1, H3, and H5 HA monomer head regions are shown as white ribbons with the Ca atoms of the corresponding glycosylation sites listed in Table 2 highlighted as colored balls. PDB1RVZ coordinates of the 1934 H1N1 HA complexed with LSTc trisaccharide [31], PDB1HGG coordinates of the X31 H3N2 HA complexed with (a2-3)sialyllactose [38], and PDB12FK0 coordinates of the 2004 Vietnamese H5N1 HA [39] were displayed using Accelrys DS Visualizer 1.7. The trisaccharide (a2-3)sialyllactose was modeled into the H5N1 structure by superpositioning the H5N1 structure onto that of H3N2 structure complexed with (a2-3) sialyllactose to illustrate the primary sugar binding site on HA. Increased sensitivity to inhibition by SP-D appears to correlate with acquisition of certain added glycans on the HA head region (see Table 2). The Ca atoms of these sensitive glycan positions are depicted as magenta balls while the other glycan Ca atoms are shown as blue balls. Although the glycan at 122 in H3 (purple ball) was identified as potentially important based on hemagglutination inhibition, it is significantly further removed from the sialic acid binding site (and therefore less likely to be important) than the glycans at 133 and 144. The trisaccharide sugars are depicted as red sticks.