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. 2001 Sep 18;98(20):11181–11186. doi: 10.1073/pnas.201401198

Figure 4.

Figure 4

Swine H9 and human H3 HA binding sites preferring α2,6 linkages are wider than avian H5 preferring α2,3. The H9 swine (gray), H3 human (green), and H5 avian (white) 226/228 loops are superimposed, showing that the H5 avian 220s loop (Gln-226/Gly-228) is closer to the opposing 130s loop than the H9 swine (Leu-226/Gly-228) or H3 human (Leu-226/Ser-228). Contact between Ala-138 and the lower methyl group of Leu-226 requires a more “open” site. The glycosidic oxygen of sialic acid (atom colors) is labeled with an asterisk. A water molecule (red sphere) mediates interactions between the amide group of Gly-228 and the 8- and 9-OHs of sialic acid in H9 swine and H5 avian HAs. The hydroxyl group of Ser-228 “replaces” the water molecule to form a hydrogen bond with 9-OH in the H3 human HA.