Influenza, avian sialidase

An interesting feature of the influenza virus sialidase active site that offers the potential for developing inhibitors specific for N1 sialidases, including avian influenza A/H5N1 virus sialidase, has recently been revealed by X-ray crystallography. The... 

Chong AK, Pegg MS, Taylor NR, von Itzstein M (1992) Evidence for a sialosyl cation transition-state complex in the reaction of sialidase from influenza virus. Eur J Biochem 207 335-343 Cinatl J Jr, Michaelis M, Doerr HW (2007a) The threat of avian influenza A (H5N1). III. Antiviral therapy. Med Microbiol Immunol 196 203-212... 

Outbreaks of avian influenza starting near the beginning of the present century have posed the danger of a pandemic of disease comparable to that which swept the world in the late teens of the twentieth century. Dire results have been predicted should the virus mutate so as to be readily transmitted to, and by, humans. The research occasioned by this threat has led to several compounds that halt the replication of the influenza A (H5N1) virus. The final step in the replication of the virus comprises the extrusion of new virions from infected cells by way of buds on the cell membrane. The proteolytic enzyme sialidase, also known as neuramindase. 

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. 

Sialidases (Neuraminidases). The three sialidase families (GH 33, 34 and 83) all have the same catalytic machinery, an aspartate, which appears to act as a proton donor, and a probable nucleophilic tyrosine, rather than a carboxylate, activated in all likelihood by a glutamate. GH 33 contains all transialidases as well as simple hydrolytic enzymes, whereas GH 34 and GH 83 contain only enzymes from viruses which are mammalian or avian pathogens. GH 33 and GH 34 act with retention of the anomeric configuration and it is currently assumed that GH 83 is similar. Obtaining crystal structures with mechanistically informative ligands bound is complicated by the facility with which sialidases dehydrate A -acetylneuraminic acid to its 2,3-dehydro derivative, DANA the process is most facile with GH 33 enzymes. The influenza sialidase in GH 34 was more amenable and not only bound the minor anomer of NANA, but bound it in the conformation. Structures of GH 83 sialidases are available only with uninformative ligands such as p-NANA bound. 

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