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Figure 7<br />

Stereo image of the active site of Tern/N9 showing the active-site residues<br />

surrounding 4-guanidino-Neu5Ac2en (black). Those residues that are conserved<br />

in both influenza and bacterial neuraminidase are shaded gray, and those that are<br />

conserved only in influenza virus neuraminidase are not shaded.<br />

sufficient sialic acid at the cell surface to enable attachment via the hemagglutinin. This balance may<br />

require some configuration of residues in the active site not directly responsible for catalysis but only<br />

involved in the binding and release of sialic acid.<br />

IV. Neuraminidase Inhibitor <strong>Design</strong><br />

Page 472<br />

Earlier screening programs [73] failed to identify potent inhibitors of viral neuraminidase. The first<br />

inhibitor synthesized [74] with a K i value in the micromolar range was Neu5Ac2en. This was <strong>based</strong> on<br />

the proposed transition state of the reaction, where the anomeric carbon (C2) bound to the ketosidic<br />

oxygen has a trigonal state. Several analogues of Neu5Ac2en were synthesized soon after, and the most<br />

potent of these, a trifluoracetyl derivative, had a K i of only 0.8 μM [75]. While this compound showed<br />

that in cell culture it retarded virus shedding [34,76], it failed as an effective antiviral agent in animals<br />

[77].<br />

The x-ray structure of Neu5Ac2en/neuraminidase complexes have been determined for N2 [66], type B<br />

[78], and N9 [79]. The Neu5Ac2en molecule binds in the active site of neuraminidase with the<br />

carboxylate oxygen atoms placed in the same location as the carboxylate of sialic acid. The 5-N-acetyl, 4-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_472.html [4/9/2004 12:22:48 AM]

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