@article {593,
	title = {Sialic acid biosynthesis: Stereochemistry and mechanism of the reaction catalyzed by the mammalian UDP-N-acetylglucosamine 2-epimerase},
	journal = {Journal of the American Chemical Society},
	volume = {125},
	number = {9},
	year = {2003},
	note = {ISI Document Delivery No.: 649YPTimes Cited: 19Cited Reference Count: 44},
	month = {Mar},
	pages = {2455-2461},
	type = {Article},
	abstract = {The bifunctional enzyme, UDP-N-acetylglucosamine 2-epimerase/ManNAc kinase, catalyzes the first two steps in the biosynthesis of the sialic acids in mammals. The epimerase domain converts UDP-GlcNAc into ManNAc and UDP. This paper demonstrates that a-ManNAc is the first formed anomer and therefore the reaction proceeds with a net retention of configuration at C-1. Studies in deuterated buffer show that solvent-derived deuterium is quantitatively incorporated into the C-2 position of the product during catalysis, but it is not incorporated into the remaining pool of substrate. This indicates that the inversion of stereochemistry is ultimately brought about by the removal and replacement of a proton at C-2 and is consistent with a two-base mechanism. Studies with O-18-labeled UDP-GlcNAc show that the anomeric oxygen of the glycosyl phosphate bond departs with the UDP product and therefore the net hydrolysis reaction involves C-O bond cleavage. Incubation of the putative intermediate, 2-acetamidoglucal, with the enzyme resulted in a slow hydration reaction to give the product, ManNAc. Additional kinetic isotope effect and positional isotope exchange (PIX) experiments address the nature of the rate-determining step of the reaction and show that C-H bond cleavage is not rate limiting. Overall, these results support a reaction mechanism involving an anti-elimination of UDP to give 2-acetamidoglucal, followed by a syn-addition of water.},
	keywords = {2-EPIMERASE/N-ACETYLMANNOSAMINE KINASE, BIFUNCTIONAL ENZYME CATALYZES, BINDING, CELL-SURFACE SIALYLATION, EPIMERIZATION, FIRST 2 STEPS, GLUTAMATE RACEMASE, POSITIONAL ISOTOPE EXCHANGE, RAT-LIVER},
	isbn = {0002-7863},
	url = {<Go to ISI>://000181235800030},
	author = {Chou, W. K. and Hinderlich, S. and Reutter, W. and Tanner, M. E.}
}
@article {4074,
	title = {Eliminations in the reactions catalyzed by UDP-N-acetylglucosamine 2-epimerase},
	journal = {Journal of the American Chemical Society},
	volume = {119},
	number = {43},
	year = {1997},
	note = {ISI Document Delivery No.: YD815Times Cited: 33Cited Reference Count: 65},
	month = {Oct},
	pages = {10269-10277},
	type = {Article},
	abstract = {Mechanistic studies have been carried out on the bacterial enzyme UDP-N-acetylglucosamine 2-epimerase, which catalyzes the interconversion of UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylmannosamine (UDP-ManNAc). This enzyme is interesting because it epimerizes a stereocenter that does not bear an acidic proton, and therefore it cannot utilize a simple deprotonation/reprotonation mechanism. A coupled enzyme assay employing UDP-ManNAc dehydrogenase has been developed. The epimerization in D2O is found to be accompanied by the incorporation of deuterium into the C-2 {\textquoteright}{\textquoteright} position of both epimers, supporting a mechanism that ultimately involves a proton transfer at this position. The epimerization of [2 {\textquoteright}{\textquoteright}-H-2]UDP-GlcNAc is slowed by a primary kinetic isotope effect indicating that C-H bond cleavage is occurring during a rate-determining step of the reaction. A positional isotope exchange (PM) experiment shows that an O-18 label in the sugar-UDP bridging position will scramble into nonbridging diphosphate positions during enzymatic epimerization. These observations are consistent with a mechanism that proceeds via cleavage of the anomeric C-O bond, with 2-acetamidoglucal and UDP as enzyme-bound intermediates. Additional evidence for this mechanism is found in the unusual observation that during extended incubations, the intermediates are gradually released from the enzyme and accumulate in solution. These intermediates are formed by an anti elimination of UDP from UDP-GlcNAc and a syn elimination of UDP from UDP-ManNAc. It is likely that El-like eliminations via oxocarbenium intermediates are involved in the reaction. Further experiments show that 3 {\textquoteright}{\textquoteright}-deoxy-UDP-GlcNAc is not a substrate for the enzyme and that the enzyme does not contain a tightly bound NAD(+) cofactor.},
	keywords = {BACILLUS-SUBTILIS, D-GLUCOSAMINE 2-EPIMERASE, enterobacterial common antigen, ESCHERICHIA-COLI, FATTY-ACID OXIDATION, GALACTOSE 4-EPIMERASE, POSITIONAL ISOTOPE EXCHANGE, RAT-LIVER, STAPHYLOCOCCUS-AUREUS-H, TEICHOIC-ACIDS, W23},
	isbn = {0002-7863},
	url = {<Go to ISI>://A1997YD81500003},
	author = {Morgan, P. M. and Sala, R. F. and Tanner, M. E.}
}
@article {3805,
	title = {Enzymatic formation and release of a stable glycal intermediate: The mechanism of the reaction catalyzed by UDP-N-acetylglucosamine 2-epimerase},
	journal = {Journal of the American Chemical Society},
	volume = {118},
	number = {12},
	year = {1996},
	note = {ISI Document Delivery No.: UC380Times Cited: 19Cited Reference Count: 25},
	month = {Mar},
	pages = {3033-3034},
	type = {Article},
	keywords = {ACID, BACTERIOPHAGE-N4 ADSORPTION, biosynthesis, enterobacterial common antigen, ESCHERICHIA-COLI, MUTANTS, POSITIONAL ISOTOPE EXCHANGE, SEQUENCE},
	isbn = {0002-7863},
	url = {<Go to ISI>://A1996UC38000035},
	author = {Sala, R. F. and Morgan, P. M. and Tanner, M. E.}
}
