10.12.2012 Views

netLibrary - eBook Summary Structure-based Drug Design by ...

netLibrary - eBook Summary Structure-based Drug Design by ...

netLibrary - eBook Summary Structure-based Drug Design by ...

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Document<br />

Page 471<br />

molecules in the active-site pocket (Figure 6). Sialic acid binds in the active site in the α-anomer and, in<br />

a distorted half-chair conformation, through the same face as used in its interaction with hemagglutinin<br />

[71]. The carboxylate group of the sugar interacts with three guanidinium groups of argine residues 118,<br />

292, and 371 and has an equatorial conformation with respect to the sugar ring (this group, axial toward<br />

the floor in the undistorted structure, is probably held equatorial <strong>by</strong> interactions with these arginine<br />

residues). The NH group of the 5-N-acetyl side chain interacts with the floor of the active-site cavity via<br />

a bound water molecule. The oxygen of the 5-N-acetyl side chain is hydrogen bonded to the N ε of Arg<br />

152, while the methyl group lies in a hydrophobic pocket near Ile222 and Trp178. The last two hydroxyl<br />

groups of the 6-glycerol side chain are hydrogen bonded to carboxylate oxygens of Glu276 and the 4hydroxyl<br />

is directed to a carboxylate oxygen of Glu119. The glycosidic oxygen O2 interacts with a<br />

carboxylate oxygen of Asp151. Similar binding of sialic acid in the active site was observed in type B<br />

virus [50].<br />

Comparison of active sites of N2, N9, and type B neuraminidase [72] show there are no significant<br />

differences between active-site orientations, except for some minor displacements of Arg224 and<br />

Glu276, where the major interactions with the 6-glycerol group of sialic acid occur. However there are<br />

differences in the water structure in the active sites of the different subtypes. The Gly405 residue (in N2<br />

and N9) is replaced <strong>by</strong> a tryptophan in type B, which displaces four water molecules that lie in a solvent<br />

pocket bounded <strong>by</strong> arginines at residues 371 and 118. The Val240 residue (in N2 and N9) is replaced <strong>by</strong><br />

a methionine in type B, which displaces two water molecules that form a channel under Arg 224,<br />

decreasing the flexibility of the active site of type B in this region. These waters are not displaced in the<br />

sialic acid/neuraminidase complex in N9 and N2 and would alter the hydrogen-bonding pattern of the<br />

complexes when compared to type B.<br />

Comparison of the active sites of influenza neuraminidases and bacterial sialidases [51,52] indicates that<br />

there is considerable conservation of the catalytic site at the carboxylate-binding end. The residues<br />

Asp151, Arg118, Glu277, Arg292, Val or Ile349, Arg371, Tyr406, and Glu425 are conserved over all<br />

known viral and bacterial strains. The arginyl residues 118, 292, and 371 position the 2-carboxylate<br />

group and the Val(or Ile)349, Glu425, and Glu277 are important in positioning the triarginyl cluster. The<br />

residues Asp151 and Tyr406 are presumably important in bond cleavage, but the precise mechanism is<br />

still unclear. These eight residues (Figure 7) are thus most likely to be conserved in all neuraminidases.<br />

Differences between viral, bacterial, and mammalian neuraminidase structures may correspond with the<br />

different role these enzymes have in vivo. These differences are likely to be in the interactions of the 6glycerol,<br />

5-N-acetyl, and 4-OH groups of silaic acid. In the influenza virus, the turnover rate must be<br />

balanced against the requirement to maintain<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_471.html [4/9/2004 12:22:26 AM]

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!