@article {4750,
	title = {The structure of UDP-N-acetylglucosamine 2-epimerase reveals homology to phosphoglycosyl transferases},
	journal = {Biochemistry},
	volume = {39},
	number = {49},
	year = {2000},
	note = {ISI Document Delivery No.: 383GTTimes Cited: 58Cited Reference Count: 48},
	month = {Dec},
	pages = {14993-15001},
	type = {Article},
	abstract = {Bacterial UDP-N-acetylglucosamine 2-epimerase catalyzes the reversible epimerization at C-2 of UDP-N-acetylglucosamine (UDP-GlcNAc) and thereby provides bacteria with UDP-N-acetylmannosamine (UDP-ManNAc), the activated donor of ManNAc residues. ManNAc is critical for several processes in bacteria, including formation of the antiphagocytic capsular polysaccharide of pathogens such as Streptococcus pneumoniae types 19F and 19A. We have determined the X-ray structure (2.5 Angstrom) of UDP-GlcNAc 2-epimerase with bound UDP and identified a previously unsuspected structural homology with the enzymes glycogen phosphorylase and T4 phage beta -glucosyltransferase. The relationship to these phosphoglycosyl transferases is very intriguing in terms of possible similarities in the catalytic mechanisms. Specifically, this observation is consistent with the proposal that the UDP-GlcNAc 2-epimerase-catalyzed elimination and re-addition of UDP to the glycal intermediate may proceed through a transition state with significant oxocarbenium ion-like character. The homodimeric epimerase is composed of two similar alpha/beta/alpha sandwich domains with the active site-located in the deep cleft at the domain interface. Comparison of the multiple copies in the asymmetric unit has revealed that the epimerase can undergo a 10 degrees interdomain rotation that is implicated in the regulatory mechanism. A structure-based sequence alignment has identified several basic residues in the active site that may be involved in the proton transfer at C-2 or stabilization of the proposed oxocarbenium ion-like transition state. This insight into the structure of the bacterial epimerase is applicable to the homologous N-terminal domain of the bifunctional mammalian UDP-GlcNAc "hydrolyzing" 2-epimerase/ManNAc kinase that catalyzes the rate-determining step in the sialic acid biosynthetic pathway.},
	keywords = {2-EPIMERASE/N-ACETYLMANNOSAMINE KINASE, ACETYLNEURAMINIC ACID BIOSYNTHESIS, BETA-GLUCOSYLTRANSFERASE, BIFUNCTIONAL ENZYME CATALYZES, CELL-SURFACE SIALYLATION, DNA MODIFYING, enterobacterial common antigen, ENZYME, FIRST 2 STEPS, GLYCOGEN-PHOSPHORYLASE, RAT-LIVER},
	isbn = {0006-2960},
	url = {<Go to ISI>://000165875300001},
	author = {Campbell, R. E. and Mosimann, S. C. and Tanner, M. E. and Strynadka, N. C. J.}
}
@article {7321,
	title = {BINDING-ENERGY AND CATALYSIS - FLUORINATED AND DEOXYGENATED GLYCOSIDES AS MECHANISTIC PROBES OF ESCHERICHIA-COLI (LACZ) BETA-GALACTOSIDASE},
	journal = {Biochemical Journal},
	volume = {286},
	year = {1992},
	note = {ISI Document Delivery No.: JN815Times Cited: 86Cited Reference Count: 40Part 3},
	month = {Sep},
	pages = {721-727},
	type = {Article},
	abstract = {Kinetic parameters for the hydrolysis of a series of deoxy and deoxyfluoro analogues of 2{\textquoteright},4{\textquoteright}-dinitrophenyl beta-D-galactopyranoside by Escherichia coli (lacZ) beta-galactosidase have been determined and rates found to be two to nine orders of magnitude lower than that for the parent compound. These large rate reductions result primarily from the loss of transition-state binding interactions due to the replacement of sugar hydroxy groups, and such interactions are estimated to contribute at least 16.7 kJ (4 kcal).mol-1 to binding at the 3, 4 and 6 positions and more than 33.5 kJ (8 kcal).mol-1 at the 2 position. The existence of a linear free-energy relationship between log(k(cat.)/K(m)) for these compounds and the logarithm of the first-order rate constant for their spontaneous hydrolysis demonstrates that electronic effects are also important and provides direct evidence for oxocarbonium ion character in the enzymic transition state. A covalent intermediate which turns over only extremely slowly (t1/2 = 45 h) accumulates during hydrolysis of the 2-deoxyfluorogalactoside, and kinetic parameters for its formation have been determined. This intermediate is nonetheless catalytically competent, since it re-activates much more rapidly in the presence of the transglycosylation acceptors methanol or glucose, thereby providing support for the notion of a covalent intermediate during hydrolysis of the parent substrates.},
	keywords = {ALPHA-D-GLUCOPYRANOSYL, D-GLUCOPYRANOSYL PHOSPHATES, ENZYME, GLYCOGEN-PHOSPHORYLASE, HYDROLYSIS, inhibitors, OLIGOSACCHARIDE, SITE, SPECIFICITY, SUBSTRATE},
	isbn = {0264-6021},
	url = {<Go to ISI>://A1992JN81500010},
	author = {McCarter, J. D. and Adam,Michael J. and Withers, S. G.}
}
