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Page 474<br />

of the oxycarbonium ion into Neu5Ac2en, which is produced irreversibly at low levels from sialic acid<br />

<strong>by</strong> the enzyme [78]. An S N1-type mechanism has been suggested [88] that is facilitated <strong>by</strong> an activated<br />

water molecule, which can be expelled upon inhibitor binding. The catalytic mechanism could possibly<br />

proceed without an acid group: the electrostatic potential of the enzyme could lower the barrier<br />

preventing the breaking of the ketosidic bond and the solvent could protonate the aglycon after release<br />

[89]. Clearly details of the enzyme mechanism have yet to be elucidated definitively. Structural<br />

considerations indicate that only Tyr406 (and possibly Glu277) and the triarginyl cluster are essential in<br />

the enzyme mechanism and that Asp151 is implicated.<br />

B. Inhibitor <strong>Design</strong> Principles<br />

All the nearest-neighbor interactions between sialic acid or Neu5Ac2en and the protein are with totally<br />

conserved amino acids. Thus an inhibitor designed to bind only to the conserved active-site residues of<br />

neuraminidase would inhibit neuraminidase activity across all strains of influenza. This would enable<br />

the development of an antiviral drug that would affect the spread of viral replication potentially in three<br />

ways, i.e., transport through the protective mucosal layer, desialyation of freshly synthesized viral<br />

glycoproteins, and elution of progeny virions from infected cells.<br />

The development of potent inhibitors was <strong>based</strong> on the structural information of the N2 neuraminidase<br />

conserved active site and its complex with sialic acid and Neu5Ac2en [66]. There are no reports of de<br />

novo molecules designed to fit into the cavity, and the most useful approach was to consider molecules<br />

that were structurally related to Neu5Ac2en. This involved the design of molecules that would bind<br />

isosterically to Neu5Ac2en but which were modified to increase the number of favorable interaction<br />

with the protein. The method of Goodford [90] enabled the calculation of favorable binding sites for a<br />

variety of chemical probes. The validity of this method was indicated <strong>by</strong> its ability to identify the<br />

positions of the carboxylate binding site of sialic acid as an energy minima for a carboxylate probe<br />

(Figure 8a), and the successful prediction of known bound-water sites in the active sites <strong>by</strong> a water<br />

probe. Utilizing this methodology, predictions of energetically favorable substitutions to Neu5Ac2en<br />

were examined [91]. A replacement of the hydroxyl at the 4-position of the pyranose ring of Neu5Ac2en<br />

<strong>by</strong> an amino group was identified <strong>by</strong> this procedure as an energetically favorable substitution. A<br />

protonated primary amine probe identified a favorable binding site of -16 kcal mol -1 at this location and<br />

in a pocket in the active site near two conserved glutamate residues Glu119 and Glu227 (Figure 8b).<br />

This suggested that the substitution of the 4-hydroxyl group <strong>by</strong> an amino group would increase the<br />

overall binding interactions <strong>by</strong> forming a salt link with Glu119. Furthermore the substitution of the 4hydroxyl<br />

group with a much bulkier guanidinyl group would lead to even tighter binding as a result<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_474.html [4/9/2004 12:23:05 AM]

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