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Figure 6<br />

Stereo image of a ball-and-stick model of sialic acid bound to the active site of<br />

Tern/N9 neuraminidase. The hydrogen-bond interactions with conserved residues<br />

are shown as dotted lines. Nitrogen atoms are shaded black, oxygen atoms<br />

are shaded dark gray, and carbon atom are shaded light gray.<br />

Page 470<br />

a pocket of totally conserved (over all animal subtypes) residues [65]. In this way the enzyme active-site<br />

pocket is surrounded <strong>by</strong> highly variable surface residues that prevent immune recognition of the active<br />

site <strong>by</strong> antibody molecules [25]. The x-ray diffraction studies of neuraminidase—antibody complexes<br />

have shown that the footprint of an antibody in the complex is larger than the exposed surface of the<br />

conserved region of the active site [67–69]. These structural results indicate that antibodies are unable to<br />

exert mutational pressure on the conserved active site because they cannot bind there without engaging<br />

strain-variable residues as part of the binding surface. However, as antibodies bind to strain-variable<br />

elements of the structure, the virus can overcome host immune pressure <strong>by</strong> point mutations of the<br />

residues that do not have a catalytic or structural role [70,48] but are able to disrupt the<br />

antigen—antibody binding interface. The rapid emergence of these escape mutants explains the failure<br />

in producing a universal vaccine for influenza.<br />

E. The Enzyme Active Site<br />

The structures of N-acetyl neuraminic acid (sialic acid Neu5Ac) and the 2-deoxy-2,3-dehydro-N-acetyl<br />

neuraminic acid (Neu5Ac2en) inhibitor complexed with N2 neuraminidase [66] revealed the nature of<br />

the interactions of the<br />

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