PMID:9174344

Citation	Thoden, JB, Hegeman, AD, Wesenberg, G, Chapeau, MC, Frey, PA and Holden, HM (1997) Structural analysis of UDP-sugar binding to UDP-galactose 4-epimerase from Escherichia coli. Biochemistry 36:6294-304
Abstract	UDP-galactose 4-epimerase from Escherichia coli catalyzes the interconversion of UDP-galactose and UDP-glucose through the transient reduction of the tightly bound cofactor NAD+. The enzyme is unique among the NAD+-dependent enzymes in that it promotes stereospecific reduction of the cofactor but nonstereospecific hydride return during normal catalysis. In addition to hydride transfer, the reaction mechanism of epimerase involves two key features: the abstraction of a proton from the 4'-hydroxyl group of glucose or galactose by an active site base and the rotation of a 4-ketopyranose intermediate in the active site pocket. To address the second issue of movement within the active site, the X-ray structures of reduced epimerase complexed with UDP-mannose, UDP-4-deoxy-4-fluoro-alpha-D-galactose, or UDP-4-deoxy-4-fluoro-alpha-D-glucose have been determined and refined to 1.65, 1.8, and 1.65 A resolution, respectively. A comparison of these models to that of the previously determined epimerase/NADH/UDP-glucose abortive complex reveals that the active site accommodates the various sugars by simple rearrangements of water molecules rather than by large changes in side chain conformations. In fact, the polypeptide chains for all of the epimerase/NADH/UDP-sugar complexes studied thus far are remarkably similar and can be superimposed with root-mean-square deviations of not greater than 0.24 A. The only significant differences between the various enzyme/UDP-sugar models occur in two of the dihedral angles defining the conformation of the UDP-sugar ligands.
Links	PubMed Online version:10.1021/bi970025j
Keywords	Binding Sites; Crystallography, X-Ray; Escherichia coli/chemistry; Escherichia coli/enzymology; Macromolecular Substances; NAD/chemistry; NAD/metabolism; Substrate Specificity; UDPglucose 4-Epimerase/chemistry; UDPglucose 4-Epimerase/metabolism; Uridine Diphosphate/chemistry; Uridine Diphosphate/metabolism; Uridine Diphosphate Galactose/chemistry; Uridine Diphosphate Galactose/metabolism; Uridine Diphosphate Glucose/chemistry; Uridine Diphosphate Glucose/metabolism; Uridine Diphosphate Sugars/chemistry; Uridine Diphosphate Sugars/metabolism
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