Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2-like mechanism involving an enzymatic base catalyst. Leaving group departure in GT-A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT-B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two-step double-displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short-lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.

Glycosyltransferases: structures, functions, and mechanisms.

Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery. These endeavors are, however, complicated by a lack of general methods for the routine preparation of this important class of compounds. Recent development such as one-pot multi-step protecting group manipulations, the use of unified monosaccharide building blocks, the introduction of stereoselective glycosylation protocols, and convergent strategies for oligosaccharide assembly, are beginning to address these problems. Furthermore, oligosaccharide synthesis can be facilitated by chemo-enzymatic methods, which employ a range of glycosyl transferases to modify a synthetic oligosaccharide precursor. Glycosynthases, which are mutant glycosidases, that can readily form glycosidic linkages are addressing a lack of a wide range glycosyltransferases. The power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.

/pdf/opportunities-and-challenges-in-synthetic-oligosaccharide-4802bp03rp.pdf

Opportunities and challenges in synthetic oligosaccharide and glycoconjugate research

Sialic acids are a subset of nonulosonic acids, which are nine-carbon α-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis, as well as large-scale E. coli cell-based production, have provided a large library of sialoside standards and derivatives in amounts sufficient for structure−activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts not only will lead to a better understanding of the biological and pathological importance of sialic acids and their diversi...

Advances in the Biology and Chemistry of Sialic Acids

The discovery of previously unknown functions associated with carbohydrates and the study of their structure-function relations are of current interest in carbohydrate chemistry and biology. Progress in this area is, however, hampered by the lack of convenient and effective tools for the synthesis and analysis of oligosaccharides and glycoconjugates. Development of automated synthesis of such materials is necessary to facilitate research in this field. This review describes recent advances in carbohydrate synthesis, with particular focus on developments that have potential application to the automated synthesis of oligosaccharides, glycopeptides, and glycoproteins.

https://science.sciencemag.org/content/sci/291/5512/2344.full.pdf

Toward automated synthesis of oligosaccharides and glycoproteins.

The review compiles artificial cascades involving enzymes with a focus on the last 10 years. A cascade is defined as the combination of at least two reaction steps in a single reaction vessel without isolation of the intermediates, whereby at least one step is catalyzed by an enzyme. Additionally, cascades performed in vivo and in vitro are discussed separately, whereby in vivo cascades are defined here as cascades relying on cofactor recycling by the metabolism or on a metabolite from the living organism. The review introduces a systematic classification of the cascades according to the number of enzymes in the linear sequence and differentiates between cascades involving exclusively enzymes and combinations of enzymes with non-natural catalysts or chemical steps. Since the number of examples involving two enzymes is predominant, the two enzyme cascades are further subdivided according to the number, order, and type of redox steps. Furthermore, this classification differentiates between cascades where al...

Artificial Biocatalytic Linear Cascades for Preparation of Organic Molecules

A multifunctional sialyltransferase has been cloned from Pasteurella multocida strain P-1059 and expressed in E. coli as a truncated C-terminal His6-tagged recombinant protein (tPm0188Ph). Biochemical studies indicate that the obtained protein is (1) an alpha2,3-sialyltransferase (main function), (2) an alpha2,6-sialyltransferase, (3) an alpha2,3-sialidase, and (4) an alpha2,3-trans-sialidase. The recombinant tPm0188Ph is a powerful tool in the synthesis of structurally diverse sialoside libraries due to its relaxed substrate specificity, high solubility, high expression level, and multifunctionality.

A multifunctional Pasteurella multocida sialyltransferase: a powerful tool for the synthesis of sialoside libraries.

Xi Chen, Jianwen Fang, Jianbo Zhang, Ziye Liu, Jun Shao, Przemyslaw Kowal, Peter Andreana, and Peng George Wang* Department of Chemistry, Wayne State Uni Versity Detroit, Michigan 48202 Recei Ved October 27, 2000 Application of carbohydrates in modern medicine is limited by the high cost of synthesis of most biologically important glycoconjugates. It is generally recognized that glycosyltransferasecatalyzed glycosylation is one of the most practical approaches. 1

Sugar nucleotide regeneration beads (superbeads): a versatile tool for the practical synthesis of oligosaccharides.

Regeneration of sugar nucleotides is a critical step in the biosynthetic pathway for the formation of oligosaccharides. To alleviate the difficulties in the production of sugar nucleotides, we have developed a method to produce uridine diphosphate galactose (UDP-galactose). The combined biosynthetic pathway, which involves seven enzymes, is composed of three parts: i) the main pathway to form UDP-galactose from galactose, with the enzymes galactokinase, galactose-1-phosphate uridyltransferase, UDP-glucose pyrophosphorylase, and inorganic pyrophosphatase, ii) the uridine triphosphate supply pathway catalyzed by uridine monophosphate (UMP) kinase and nucleotide diphosphate kinase, and iii) the adenosine triphosphate (ATP) regeneration pathway catalyzed by polyphosphate kinase with polyphosphate added as an energy resource. All of the enzymes were expressed individually and immobilized through their hexahistidine tags onto nickel agarose beads ("super beads"). The reaction requires a stoichiometric amount of UMP and galactose, and catalytic amounts of ATP and glucose 1-phosphate, all inexpensive starting materials. After continuous circulation of the reaction mixture through the super-bead column for 48 h, 50 % of the UMP was converted into UDP-galactose. The results show that de novo production of UDP-galactose on the super-bead column is more efficient than in solution because of the stability of the immobilized enzymes.

/pdf/combined-biosynthetic-pathway-for-de-novo-production-of-udp-34ruic8a81.pdf

Combined biosynthetic pathway for de novo production of UDP-galactose: Catalysis with multiple enzymes immobilized on agarose beads

Despite the increasing availability of various glycosyltransferases, the high cost and inadequate availability of sugar nucleotides required by these enzymes have severely limited their applications.3 Since sugar nucleotides only serve as cofactors in the overall glycosylation in biological system, the best way is mimicking nature to recycle them in situ. 4 Among the existing uridine 5′-diphosphoglucose (UDP-Glc) and uridine 5 ′-diphosphogalactose (UDP-Gal) recycling schemes, the most elegant one is based on a sucrose synthase (SusA, EC 2.4.1.13). 5 The synthesis and cleavage of sucrose catalyzed by sucrose synthase (sucrose+ UDP S UDP-Glc + fructose) is the only readily reversible transglycosylation reaction involving a sugar nucleotide 6

Transferring a biosynthetic cycle into a productive Escherichia coli strain: large-scale synthesis of galactosides.

A metabolic pathway engineered Escherichia coli strain (superbug) containing one plasmid harboring an artificial gene cluster encoding all the five enzymes in the biosynthetic pathway of Galalpha l,3Lac through galactose metabolism has been developed The plasmid contains a lambda promoter, a c1857 repressor gene, an ampicillin resistance gene, and a T7 terminator Each gene was preceded by a Shine - Dalgarno sequence for ribosome binding In a reaction catalyzed by the recombinant E coli strain, Galalpha 1,3Lac trisaccharide accumulated at concentrations of 142 mM (72 gL(-1)) in a reaction mixture containing galactose, glucose, lactose, and a catalytic amount of uridine 5'-diphosphoglucose This work demonstrates that large-scale synthesis of complex oligosaccharides can be achieved economically and efficiently through a single, biosynthetic pathway engineered microorganism

http://chemgroups.ucdavis.edu/~chen/publication%20files/No-19.pdf

Jianbo Zhang

Papers

A multifunctional Pasteurella multocida sialyltransferase: a powerful tool for the synthesis of sialoside libraries.

Sugar nucleotide regeneration beads (superbeads): a versatile tool for the practical synthesis of oligosaccharides.

Combined biosynthetic pathway for de novo production of UDP-galactose: Catalysis with multiple enzymes immobilized on agarose beads

Transferring a biosynthetic cycle into a productive Escherichia coli strain: large-scale synthesis of galactosides.

Reassembled biosynthetic pathway for large-scale carbohydrate synthesis: α-Gal epitope producing "superbug"