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THE ASPERGILLUS FUMIGATUS SIALIDASE IS A KDNASE: STRUCTURAL AND MECHANISTIC INSIGHTS
Structural highlights
FunctionSIA_ASPFU Sialidase is able to release sialic acid from a wide variety of natural substrates including bovine salivary mucin, colominic acid, bovine fetuin, a serum glycoprotein containing both alpha-2-6 and alpha-2-3-linkages in a ratio of about 3:2, and glycoproteins and glycolipids from thermally denatured human lung epithelial cells. Does not show any trans-sialidase activity since it is able to remove terminal sialic acid residues but is unable to catalyze their transfer to the acceptor substrate. 2-keto-3-deoxynononic acid (KDN) is the preferred substrate and A.fumigatus can utilize KDN as a sole carbon source.[1] Publication Abstract from PubMedAspergillus fumigatus is a filamentous fungus that can cause severe respiratory disease in immunocompromised individuals. A putative sialidase from A. fumigatus was recently cloned and shown to be relatively poor in cleaving N-acetylneuraminic acid (Neu5Ac) in comparison to bacterial sialidases. Here we present the first crystal structure of a fungal sialidase. When the apo structure was compared to bacterial sialidase structures, the active site of the Aspergillus enzyme suggested that Neu5Ac would be a poor substrate due to a smaller pocket that normally accommodates the acetamido group of Neu5Ac in sialidases. A sialic acid with a hydroxyl in place of an acetamido group is 2-keto-3-deoxynononic acid (KDN). We show that KDN is the preferred substrate for the A. fumigatus sialidase and that A. fumigatus can utilise KDN as a sole carbon source. A 1.4Angstrom resolution crystal structure of the enzyme in complex with KDN reveals KDN in the active site in a boat conformation, and nearby a second binding site occupied by KDN in a chair conformation, suggesting that polyKDN may be a natural substrate. The enzyme is not inhibited by the sialidase transition state analogue 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (Neu5Ac2en) but is inhibited by the related KDN2en that we show bound to the enzyme in a 1.84Angstrom resolution crystal structure. Using a fluorinated KDN substrate, we present a 1.5Angstrom resolution structure of a covalently bound catalytic intermediate. The A. fumigatus sialidase is therefore a KDNase with a similar catalytic mechanism to Neu5Ac exosialidases, and this study represents the first structure of a KDNase. The Aspergillus fumigatus sialidase is a KDNase: structural and mechanistic insights.,Telford JC, Yeung JH, Xu G, Kiefel MJ, Watts AG, Hader S, Chan J, Bennet AJ, Moore MM, Taylor GL J Biol Chem. 2011 Jan 19. PMID:21247893[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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