Sialidases catalytic site

Sialidase is a homotetramer with C4 symmetry composed of identical disulfide-linked subunits (Fig. 17.6a [62]). Each monomer is a glycosylated polypeptide with six p-sheets assuming the appearance of six blades of a propeller [6, 64] with a right-handed twist. The catalytic site is observed to be at the center of the sixfold pseudosymmetry axis, which passes through the center of each monomer and relates the six p-sheets to each other (Fig. 17.6b [63]) [64, 65], The active site contains a large number of conserved amino acid residues, which are involved in binding to sialic acid in the substrate sialoglycoconjugate [64],... 

FIGURE 17.16 Comparison of group 1 and group 2 sialidase active sites, (a) Oseltamivir carboxylate binds into the open catalytic site of group 1 sialidase Nl. (b) Binding between oseltamivir carboxylate and group 2 sialidase N9 with the closed 150-loop. 

Another important feature, especially from the viewpoint of rational drug design, zanamivir (12) shows a remarkable degree of selectivity towards influenza virus sialidase. In experiments aimed at determining the specificity of (12) towards influenza sialidase it was shown that zanamivir [as well as the 4-amino derivative (11)] did not show any increase in inhibition of other viral, bacterial, or mammalian sialidases when compared with Neu5Ac2en (6) [73, 87]. The results of some of these studies are presented in the clinical summary section of this article. This selectivity has been explained on the basis of the active site architecture of the different sialidases, in that it is only influenza virus sialidase which can accomodate the relatively bulky and basic C-4 substituent in its catalytic site [73]. 

Figure 2. The structure of the sialidase of influenza A virus. The model has been derived from the crystallographic data obtained on A/tem/Australia/G70C/75 sialadase (Baker et al., 1987). View of the monomer is down the fourfold symmetry axis. The catalytic site (white) is indicated. The receptor binding site (black) observed on some sialidase molecules is also shown. For details see text.

The X-ray crystal structure of virus sialidase was first solved in early 1980s.24,25 While the active site could be identified, resolution was insufficient to determine the orientation of the bound ligand (Neu5Ac) within the catalytic center. Subsequent efforts in protein crystallography allowed further refinement... 

M. Ribeirao, V. L. Pereira-Chioccola, D. Eichinger, M. M. Rodrigues, and S. Schenkman, Temperature differences for frans-glycosylation and hydrolysis reaction reveal an acceptor binding site in the catalytic mechanism of Trypanosoma cruzi trans-sialidase, Glycobiology, 1 (1997) 1237-1246. 

Fig. 8. Alternative catalytic nucleophiles, (a) Tyrosine functions as the catalytic nucleophile in Clan GH-E sialidases and trans-silalidases, with the assistance of a proximal carboxylate amino acid side chain, (b) Anchimeric assistance in hexosaminidases involving reaction of the A -acetyl group of the substrate with the anomeric carbon to form an oxazoline intermediate in the enzyme active site, (c) Sugar thiazolines are noncleavable S-linked inhibitors of hexosaminidases that use an oxazoline intermediate. Enz5mie selectivity can be further tuned by varying i . ...

The six-bladed P-propeller-like domain is now recognized as the canonical siali-dase structure, found in each of the viral, bacterial, trypanosomal, and human exo-sialidases crystallized [63-66], despite wide differences in sequence similarities [67]. Key catalytic residues in the active site are also well conserved (reviewed in [63]). 

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