Structure of Sialidase

Two calcium ion binding sites have been identified in the protein, one on the four-fold symmetry axis near the outermost surface of the tetramer, and one approximately 8 A from the active site. Calcium has been shown to increase the catalytic rate of the enzyme, although it is not absolutely necessary for activity [21,22]. Four carbohydrate attachment sites have also 

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Importantly, the crystal structure of 34 complexed with N9 sialidase (Fig. 8) indicated differences in the orientation of the guanidino group in subsite S2, and in its interaction with the active site residues, compared to that of zanamivir (Babu et al. 2000). These differences have implications for cross-reactivity of 34 with zanamivir-resistant influenza viruses that have Glul 19 mutations in the sialidase S2 subsite (see Sect. 5.1). 

Chavas LMG, Tringah C, Fusi P, Venerando B, Tettamanti G, Kato R, Monti E, Wakatsuki S (2005) Crystal structure of the human cytosolic sialidase Neu2. Evidence for the dynamic nature of substrate recognition. J Biol Chem 280 469-475... 

Exact analysis of sialic acid is required in biologieal experiments where the biological role of sialic acid is frequently studied with the aid of sialidases, and the amount of sialic acids released is determined. This is also important for periodate oxidation studies on biological systems, where modification of sialic acids by periodate is only assumed, but chemical analysis of this effect by isolation and analysis of the modified sialic acids is seldom performed. These uncertainties in determinations of sialic acid can be overcome by the purification procedures already described. Furthermore, it must be stressed that unequivocal determination of the structure of a sialic acid, especially... 

Crystals of pronase-released heads of the N2 human strains of A/Tokyo/3/67 [44] and A/RI/5+/57 were used for an x-ray structure determination. The x-ray 3-dimensional molecular structure of neuraminidase heads was determined [45] for these two N2 subtypes by a novel technique of molecular electron density averaging from two different crystal systems, using a combination of multiple isomorphous replacement and noncrystallographic symmetry averaging. The structure of A/Tokyo/3/67 N2 has been refined [46] to 2.2 A as has the structures of two avian N9 subtypes [47-49]. Three influenza type structures [50] have also been determined and found to have an identical fold with 60 residues (including 16 conserved cysteine residues) being invariant. Bacterial sialidases from salmonella [51] and cholera [52] have homologous structures to influenza neuraminidase, but few of the residues are structurally invariant. 

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... 

A convenient chemoenzymatic access to sialic acid mimetics as important inhibitors of influenza sialidases has been established by Nelson et al. (Fig. 35b) [191, 192]. Application of a pyruvate-dependent sialic acid aldolase improved by directed evolution disclosed a new route to the core structure of important pharmaceuticals, such as zanamivir (Relenza ). 

The lipase-catalysed hydrolysis of methyl 2-fluoro-2-arylpropionates was proposed to proceed via a mechanism whereby, after ester hydrolysis, the enzyme facilitates the elimination of fluoride ion with the formation of a carbocation stabilized by the adjacent C02 group.230 Determination of the crystal structure of human sialidase Neu2, an enzyme that catalyses the hydrolysis of sialic acids, reveals a tyrosine residue that is positioned in the active site to stabilize the carbocation proposed as an intermediate in the hydrolysis.231 ll-Fluoro-all-frans-retinol is found to undergo isomerization to its 11 -cis form in the presence of visual cycle enzymes, in contrast to a previous study where no isomerization was reported.232 The result of the prior study was taken as evidence for a carbonium ion pathway in the isomerization. Although the authors of the present study do not rule out such a mechanism, they suggest that the isomerization mechanism remains unknown. Data obtained in a study of the oxidation of... 

FIGURE 17.6 X-ray structures of influenza virus sialidases. (a) Structure of influenza B virus sialidase tetramer (PDB 1 a4g) viewed from the top [62]. (b) Structure of influenza A virus sialidase N9 monomer (PDB lmwe) with a-Neu5Ac (shown as spheres) bound in the active site [63]. 

X-ray crystal structures of a-Neu5Ac and 11 in complex with influenza virus sialidases in the 1980s and early 1990s led to the opportunity for structure-based design and development of influenza virus sialidase inhibitors [66, 85],... 

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