@article {2092,
	title = {Mechanistic Studies on N-Acetylmuramic Acid 6-Phosphate Hydrolase (MurQ): An Etherase Involved in Peptidoglycan Recycling},
	journal = {Biochemistry},
	volume = {47},
	number = {44},
	year = {2008},
	note = {ISI Document Delivery No.: 366TUTimes Cited: 4Cited Reference Count: 37Hadi, Timin Dahl, Ulrike Mayer, Christoph Tanner, Martin E.},
	month = {Nov},
	pages = {11547-11558},
	type = {Article},
	abstract = {Peptidoglycan recycling is a process in which bacteria import cell wall degradation products and incorporate them back into either peptidoglycan biosynthesis or basic metabolic pathways. The enzyme MurQ is an N-acetylmuramic acid 6-phosphate (MurNAc 6-phosphate) hydrolase (or etherase) that hydrolyzes the lactyl side chain from MurNAc 6-phosphate and generates GlcNAc 6-phosphate. This study supports a mechanism involving the syn elimination of lactate to give an alpha,beta-unsaturated aldehyde with (E)-stereochemistry, followed by the syn addition of water to give product. The observation of both a kinetic isotope effect slowing the reaction of [2-H-2]MurNAc 6-phosphate and the incorporation of solvent-derived deuterium into C2 of the product indicates that the C2-H bond is cleaved during catalysis. The observation that the solvent-derived 180 isotope is incorporated into the C3 position of the product, but not the C1 position, provides evidence of the cleavage of the C3-O bond and argues against imine formation. The finding that 3-chloro-3-deoxy-GlcNAc 6-phosphate serves as an alternate substrate is also consistent with an elimination-addition mechanism. Upon extended incubations of MurQ with GlcNAc 6-phosphate, the alpha,beta-unsaturated aldehydic intermediate accumulates in solution, and H-1 NMR analysis indicates it exists as the ring-closed form of the (E)-alkene. A structural model is developed for the Escherichia coli MurQ and is compared to that of the structural homologue glucosamine-6-phosphate synthase. Putative active site acid/base residues are probed by mutagenesis, and Glu83 and Glu114 are found to be crucial for catalysis. The Glu83Ala mutant is essentially inactive as an etherase yet is capable of exchanging the C2 proton of substrate with solvent-derived deuterium. This suggests that Glu83 may function as the acidic residue that protonates the departing lactate.},
	keywords = {CATALYSIS, CELL-WALL, COLI GLUCOSAMINE-6-PHOSPHATE SYNTHASE, DERIVATIVES, ELIMINATION, ENOYL-COA HYDRATASE, ENVIRONMENT, ESCHERICHIA-COLI, INHIBITION, PHOSPHATE},
	isbn = {0006-2960},
	url = {<Go to ISI>://000260507100019},
	author = {Hadi, T. and Dahl, U. and Mayer, C. and Tanner, M. E.}
}
@article {1279,
	title = {The enzymes of sialic acid biosynthesis},
	journal = {Bioorganic Chemistry},
	volume = {33},
	number = {3},
	year = {2005},
	note = {ISI Document Delivery No.: 931CJTimes Cited: 36Cited Reference Count: 59},
	month = {Jun},
	pages = {216-228},
	type = {Review},
	abstract = {The sialic acids are a family of nine carbon a-keto acids that play a wide variety of biological roles in nature. In mammals, they are found at the distal ends of cell surface glycoconjugates, and thus are major determinants of cellular recognition and adhesion events. In certain strains of pathogenic bacteria, they are found in capsular polysaccharides that mask the organism from the immune system by mimicking the exterior of a mammalian cell. This review outlines recent developments in the understanding of the two main enzymes responsible for the biosynthesis of the sialic acid, N-acetylneuraminic acid. The first, a hydrolyzing UDP-N-acetyl-glucosamine 2-epimerase, generates N-acetylmannosamine and UDP from UDP-N-acetylglucosamine. The second, sialic acid synthase, generates either N-acetylneuraminic acid (bacteria) or N-acetylneuraminic acid 9-phosphate (mammals) in a condensation reaction with phosphoenolpyruvate. An emphasis is placed on an understanding of the mechanistic and structural features of these enzymes. (c) 2005 Elsevier Inc. All rights reserved.},
	keywords = {2-EPIMERASE/N-ACETYLMANNOSAMINE KINASE, BIFUNCTIONAL KEY ENZYME, biosynthesis, ESCHERICHIA-COLI K1, FIRST 2, KDO8P SYNTHASE, MECHANISM, N-ACETYLGLUCOSAMINE 2-EPIMERASE, N-acetylmannosamine, N-ACETYLNEURAMINIC ACID, NEISSERIA-MENINGITIDIS, PHOSPHATE, phosphoenolpyruvate, RAT-LIVER, sialic acid synthase, STEPS, SYNTHASE GENE, UDP-GlcNAc 2-epimerase},
	isbn = {0045-2068},
	url = {<Go to ISI>://000229463100007},
	author = {Tanner, M. E.}
}
@article {520,
	title = {Understanding nature{\textquoteright}s strategies for enzyme-catalyzed racemization and epimerization},
	journal = {Accounts of Chemical Research},
	volume = {35},
	number = {4},
	year = {2002},
	note = {ISI Document Delivery No.: 544TBTimes Cited: 65Cited Reference Count: 36},
	month = {Apr},
	pages = {237-246},
	type = {Review},
	abstract = {Epimerases and racemases are enzymes that catalyze the inversion of stereochemistry in biological molecules. In this article, three distinct examples are used to illustrate the wide range of chemical strategies employed during catalysis, and the diverse set of ancestors from which these enzymes have evolved. Glutamate racemase is an example of an enzyme that operates at an "activated" stereocenter (bearing a relatively acidic proton) and employs a nonstereospecific deprotonation/reprotonation mechanism. UDP-N-Acetylglucosamine 2-epimerase acts at an "unactivated" stereocenter and uses a mechanism involving a nonstereospecific elimination/addition of UDP. L-Ribulose phosphate 4-epimerase also acts at an unactivated stereocenter and uses a nonstereospecific retroaldol/aldol mechanism.},
	keywords = {ALDOLASE, CLASS-II, ESCHERICHIA-COLI, GLUTAMATE RACEMASE, IDENTIFICATION, L-FUCULOSE-1-PHOSPHATE, L-RIBULOSE-5-PHOSPHATE 4-EPIMERASE, MECHANISM, N-ACETYLGLUCOSAMINE 2-EPIMERASE, PHOSPHATE, RESIDUES},
	isbn = {0001-4842},
	url = {<Go to ISI>://000175175800005},
	author = {Tanner, M. E.}
}
@article {5183,
	title = {Catalysis and binding in L-ribulose-5-phosphate 4-epimerase: A comparison with L-fuculose-1-phosphate aldolase},
	journal = {Biochemistry},
	volume = {40},
	number = {49},
	year = {2001},
	note = {ISI Document Delivery No.: 500CJTimes Cited: 15Cited Reference Count: 15},
	month = {Dec},
	pages = {14772-14780},
	type = {Article},
	abstract = {L-Ribulose-5-phosphate (L-Ru5P) 4-epimerase and L-fuculose-1-phosphate (L-Fuc1P) aldolase are evolutionarily related enzymes that display 26\% sequence identity and a very high degree of structural similarity. They both employ a divalent cation in the formation and stabilization of an enolate during catalysis, and both are able to deprotonate the C-4 hydroxyl group of a phosphoketose substrate. Despite these many similarities, subtle distinctions must be present which allow the enzymes to catalyze two seemingly different reactions and to accommodate substrates differing greatly in the position of the phosphate (C-5 vs C-1). Asp76 of the epimerase corresponds to the key catalytic acid/base residue Glu73 of the aldolase. The D76N mutant of the epimerase retained considerable activity, indicating it is not a key catalytic residue in this enzyme. In addition, the D76E mutant did not show enhanced levels of background aldolase activity. Mutations of residues in the putative phosphate-binding pocket of the epimerase (N28A and K42M) showed dramatically higher values of K-M for L-Ru5P. This indicates that both enzymes utilize the same phosphate recognition pocket, and since the phosphates are positioned at opposite ends of the respective substrates, the two enzymes must bind their substrates in a reversed or "flipped" orientation. The epimerase mutant D120N displays a 3000-fold decrease in the value of k(cat), suggesting that Asp 120{\textquoteright} provides a key catalytic acid/base residue in this enzyme. Analysis of the D120N mutant by X-ray crystallography shows that its structure is indistinguishable from that of the wild-type enzyme and that the decrease in activity was not simply due to a structural perturbation of the active site. Previous work [Lee, L.V., Poyner, R.R., Vu, M.V., and Cleland, W.W. (2000) Biochemistry 39, 4821-4830] has indicated that Tyr229{\textquoteright} likely provides the other catalytic acid/base residue. Both of these residues are supplied by an adjacent subunit. Modeling Of L-Ru5P into the active site of the epimerase structure suggests that Tyr229{\textquoteright} is responsible for deprotonating L-Ru5P and Asp 120{\textquoteright} is responsible for deprotonating its epimer, D-Xu5P.},
	keywords = {CLASS-II, CLEAVAGE, ESCHERICHIA-COLI, MECHANISM, MUTAGENESIS, PHOSPHATE},
	isbn = {0006-2960},
	url = {<Go to ISI>://000172608100006},
	author = {Samuel, J. and Luo, Y. and Morgan, P. M. and Strynadka, N. C. J. and Tanner, M. E.}
}
