@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 {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.}
}
