title: High resolution x-ray structure of tyvelose epimerase from Salmonella typhi.
abstract: Tyvelose epimerase catalyzes the last step in the biosynthesis of tyvelose by converting CDP-d-paratose to CDP-d-tyvelose. This unusual 3,6-dideoxyhexose occurs in the O-antigens of some types of Gram-negative bacteria. Here we describe the cloning, protein purification, and high-resolution x-ray crystallographic analysis of tyvelose epimerase from Salmonella typhi complexed with CDP. The enzyme from S. typhi is a homotetramer with each subunit containing 339 amino acid residues and a tightly bound NAD+ cofactor. The quaternary structure of the enzyme displays 222 symmetry and can be aptly described as a dimer of dimers. Each subunit folds into two distinct lobes: the N-terminal motif responsible for NAD+ binding and the C-terminal region that harbors the binding site for CDP. The analysis described here demonstrates that tyvelose epimerase belongs to the short-chain dehydrogenase/reductase superfamily of enzymes. Indeed, its active site is reminiscent to that observed for UDP-galactose 4-epimerase, an enzyme that plays a key role in galactose metabolism. Unlike UDP-galactose 4-epimerase where the conversion of configuration occurs about C-4 of the UDP-glucose or UDP-galactose substrates, in the reaction catalyzed by tyvelose epimerase, the inversion of stereochemistry occurs at C-2. On the basis of the observed binding mode for CDP, it is possible to predict the manner in which the substrate, CDP-paratose, and the product, CDP-tyvelose, might be accommodated within the active site of tyvelose epimerase.
authors.affiliation: Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Koropatkin
authors.forename: Hung-Wen
authors.initials: HW
authors.lastname: Liu
authors.forename: Hazel M
authors.initials: HM
authors.lastname: Holden
mesh_terms: D001665:Binding Sites; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D006601:Hexoses / Q000378:metabolism*; D008968:Molecular Conformation; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D019998:Racemases and Epimerases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D012485:Salmonella typhi / Q000201:enzymology*; D014534:UDPglucose 4-Epimerase / Q000737:chemistry
chemical_list: D006601:Hexoses; C009616:tyvelose; D019998:Racemases and Epimerases; D014534:UDPglucose 4-Epimerase