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.

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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 Novel and Efficient Deprotection of the Allyl Group at the Anomeric Oxygen of Carbohydrates.

A convenient protocol for the β-stereoselective synthesis of 2-deoxy-C-arylglycosides has been developed. This reaction takes place in one step by using I2/Et3SiH to activate a glycosyl acetate to generate a glycosyl iodide intermediate in situ, which is captured by a naphthol; this is followed by a Fries-like O-to-C glycoside rearrangement to afford a β-C-aryl glycoside selectively. The approach is applicable to a wide range of naphthol moieties, and its utility was demonstrated in syntheses of 5-aza analogues of aquayamycin.

A Metal-Free β-Stereoselective Synthesis of 2-Deoxy-C-arylglycosides: Synthesis of 5-Aza Analogues of Aquayamycin

Sulfuric acid, immobilized on silica gel, is an efficient promoter for per-O-acetylation of carbohydrates with acetic anhydride.

H2SO4—SiO2: Highly Efficient and Reusable Catalyst for per‐O‐Acetylation of Carbohydrates under Solvent‐Free Conditions.

The invention discloses a method for preparing 5-aza-Marangucycline B. The method is characterized by comprising the following steps: adopting an intermediate of 2,6-dideoxy-peracetylated glucose prepared through carbon glycosylation, acyl protection, N-bromosuccinimide bromination, Diels-Alder and deacylation, sequentially performing Ferrier rearrangement, deacylation, allyl hydroxyl oxidation and Michael addition, and synthesizing the 5-aza-Marangucycline B with bioactive molecules. Compared with the prior art, the method disclosed by the invention has the advantages that the product has the bioactive molecules, the synthetic route is short and high-efficiency, reaction time of each step is short, the operation is simple and convenient, yield is high, reagents used in the reaction are cheap and readily available, the usage amount of organic solvents and catalysts is small, production cost is low, and industrial popularization is easily realized.

Jianbo Zhang

Papers

A Novel and Efficient Deprotection of the Allyl Group at the Anomeric Oxygen of Carbohydrates.

A Metal-Free β-Stereoselective Synthesis of 2-Deoxy-C-arylglycosides: Synthesis of 5-Aza Analogues of Aquayamycin

H2SO4—SiO2: Highly Efficient and Reusable Catalyst for per‐O‐Acetylation of Carbohydrates under Solvent‐Free Conditions.

Method for preparing 5-aza-Marangucycline B