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Page 476<br />

nM, respectively. These compounds were later shown <strong>by</strong> x-ray studies of 4-guanidino and 4-amino-<br />

Neu4Ac2en complexed with A/Tokyo/3/67 neuraminidase to bind close to that predicted <strong>by</strong> the design<br />

studies. However details of the interactions of the guanidinyl group of the 4-guanidino-Neu4Ac2en with<br />

the glutamic acid groups (Glu119 and Glu277) in the floor of the active site were slightly different. This<br />

was confirmed on a higher resolution x-ray study (Figure 9) of a 4-guanidino-Neu4Ac2en complexed<br />

with Tern N9 neuraminidase [72]. One of the primary guanidinyl nitrogens of 4-guanidino-Neu4Ac2en<br />

is hydrogen bonded to the main-chain oxygen at residue 178, a carboxylate oxygen of Glu227, and a<br />

water molecule. The other primary guanidinyl nitrogen interacts with the main-chain oxygen of residues<br />

178 and 151. The secondary guanidinyl nitrogen interacts with the carboxylate of Glu119 and Asp151.<br />

The interactions with Glu119 are electrostatic and van der Waals in character and lack hydrogenbonding<br />

geometry (postulated in the design study) as the carboxylate group of Glu119 stacks parallel to<br />

the guanidinyl group. Furthermore, theoretical energy-minimized structures of the complex using<br />

AMBER [92] converged to the x-ray structure only if the protein nonhydrogen atoms were kept rigid in<br />

the x-ray structure [72]. Otherwise this resulted in active site residues showing large distortions in their<br />

conformation. This is an example of the difficulty in correctly modeling even modest changes in the<br />

interactions of an inhibitor/active site complex.<br />

The 4-guanidino analog shows potent inhibition of neuraminidase activity in all known wild strains of<br />

influenza. Furthermore it is very specific to in-<br />

Figure 9<br />

Stereo image of the Tern/N9 4-guanidino-Neu5Ac2en complex showing the<br />

hydrogen-bond interactions (dotted lines) of the inhibitor with conserved residues<br />

in the active site of the enzyme. Nitrogen, oxygen, and carbon atoms are<br />

shaded black, dark gray, and light gray, respectively.<br />

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