Chemical synthesis of UDP-Glc-2,3-diNAcA, a key intermediate in cell surface polysaccharide biosynthesis in the human respiratory pathogens B. pertussis and P. aeruginosa

D-Hexopyranosides with Vicinal Nitrogen-Containing Functionalities

Various substituted D-hexopypyranosides units with nitrogen-containing functionalities are present in many important natural compounds and pharmaceutical substances. Since their complex structural diversity contributes to a broad spectrum of biological functions and activities, these derivatives are frequently studied. This review covers syntheses of D-hexopyranosides with vicinal nitrogen-containing functionalities since the 1960s, when the first articles emerged. The syntheses are arranged according to the positions of substitutions, to form a relative configuration of vicinal functionalities, and synthetic methodologies.

Chemical Synthesis of a Keto Sugar Nucleotide

Keto sugar nucleotides (KSNs) are common and versatile precursors to various deoxy sugar nucleotides, which are substrates for the corresponding glycosyltransferases involved in the biosynthesis of glycoproteins, glycolipids, and natural products. However, there has been no KSN synthesized chemically due to the inherent instability. Herein, the first chemical synthesis of the archetypal KSN TDP-4-keto-6-deoxy-d-glucose (1) is achieved by an efficient and optimized route, providing feasible access to other KSNs and analogues, thereby opening a new avenue for new applications.

Cofactor-Driven Cascade Reactions Enable the Efficient Preparation of Sugar Nucleotides

Glycosylation is catalyzed by glycosyltransferases using sugar nucleotides or occasionally lipid-linked phosphosugars as donors. However, only very few common sugar nucleotides that occur in humans can be obtained readily, while the majority of sugar nucleotides that exist in bacteria, plants, archaea, or viruses cannot be synthesized in sufficient quantities by either enzymatic or chemical synthesis. The limited availability of such rare sugar nucleotides is one of the major obstacles that has greatly hampered progress in glycoscience. Herein we describe a general cofactor-driven cascade conversion strategy for the efficient synthesis of sugar nucleotides. The described strategy allows the large-scale preparation of rare sugar nucleotides from common sugars in high yields and without the need for tedious purification processes.

Detachable nanoladders: A new method for signal identification and their application in the detection of ochratoxin A (OTA)

A highly sensitive fluorescence turn-off biosensor for the detection of ochratoxin A (OTA) was developed based on graphene oxide (GO) and an aptamer-induced detachable nanoladders. In this assay, two types of ssDNA (H1 and H2) were involved in the assembly of the DNA nanoladders, in which H1 was labeled with fluorophore, and H2 was the OTA binding aptamer. Self-assembly of the DNA nanoladders with the addition of GO weakened its adsorption and the fluorescence intensity remained strong. In the presence of OTA, the aptamer was specifically recognized and an aptamer-OTA complex was formed, leading to the detached of DNA nanoladders. With the addition of GO, the released H1 was adsorbed on the GO surface, thus efficiently quenching the fluorescence signal (turning off). The detection limit of OTA in this assay was 4.59 nM. To improve the sensitivity of this strategy, we creatively developed an alternative strategy to replace the sturdy nanoladders with frail nanoladders. More precisely, the sequences of H1 had mismatched bases, which, when hybridized with H2 could be used to create long non-perfect complementary nanoladders. For the mismatched bases-based frail nanoladders, it was easier for OTA to bind its aptamer sequence, thus enabling a more thorough and faster detachment of the nanoladders, along with a greater degree of fluorescence quenching. The detection limit for OTA was estimated to be 0.1 nM. The biosensors we developed were sensitive, specific, enzyme-free, cost-effective and can be applied in red wine samples spiked with known concentration of OTA.

Chemoenzymatic Synthesis and Applications of Prokaryote-Specific UDP-Sugars

This method describes the chemoenzymatic synthesis of several nucleotide sugars, which are essential substrates in the biosynthesis of prokaryotic N- and O-linked glycoproteins. Protein glycosylation is now known to be widespread in prokaryotes and proceeds via sequential action of several enzymes, utilizing both common and modified prokaryote-specific sugar nucleotides. The latter, which include UDP-hexoses such as UDP-diNAc-bacillosamine (UDP-diNAcBac), UDP-diNAcAlt, and UDP-2,3-diNAcManA, are also important components of other bacterial and archaeal glycoconjugates. The ready availability of these "high-value" intermediates will enable courses of study into inhibitor screening, glycoconjugate biosynthesis pathway discovery, and unnatural carbohydrate incorporation toward metabolic engineering.

Discrimination of epimeric glycans and glycopeptides using IM-MS and its potential for carbohydrate sequencing

Mass spectrometry is the primary analytical technique used to characterize the complex oligosaccharides that decorate cell surfaces. Monosaccharide building blocks are often simple epimers, which when combined produce diastereomeric glycoconjugates indistinguishable by mass spectrometry. Structure elucidation frequently relies on assumptions that biosynthetic pathways are highly conserved. Here, we show that biosynthetic enzymes can display unexpected promiscuity, with human glycosyltransferase pp-α-GanT2 able to utilize both uridine diphosphate N-acetylglucosamine and uridine diphosphate N-acetylgalactosamine, leading to the synthesis of epimeric glycopeptides in vitro. Ion-mobility mass spectrometry (IM-MS) was used to separate these structures and, significantly, enabled characterization of the attached glycan based on the drift times of the monosaccharide product ions generated following collision-induced dissociation. Finally, ion-mobility mass spectrometry following fragmentation was used to determine the nature of both the reducing and non-reducing glycans of a series of epimeric disaccharides and the branched pentasaccharide Man3 glycan, demonstrating that this technique may prove useful for the sequencing of complex oligosaccharides.

Synthesis of a tetrasaccharide related to the repeating unit of the O-antigen from Escherichia coli K-12

Synthesis of a tetrasaccharide related to the repeating unit of the O-antigen from Escherichia coli K-12 is reported in the form of its octyl glycoside. Syntheses of the 1,2-cis glycosidic linkages have been accomplished by using NIS in conjunction with H(2)SO(4)-silica, and it was found to be stereoselective and productive. The synthesized tetrasaccharide will be utilized as the substrate for galactofuranosyltransferase, WbbI.

Characterization of WbpB, WbpE, and WbpD and reconstitution of a pathway for the biosynthesis of UDP-2,3-diacetamido-2,3-dideoxy-D-mannuronic acid in Pseudomonas aeruginosa

The lipopolysaccharide of Pseudomonas aeruginosa PAO1 contains an unusual sugar, 2,3-diacetamido-2,3-dideoxy-d-mannuronic acid (d-ManNAc3NAcA). wbpB, wbpE, and wbpD are thought to encode oxidase, transaminase, and N-acetyltransferase enzymes. To characterize their functions, recombinant proteins were overexpressed and purified from heterologous hosts. Activities of His(6)-WbpB and His(6)-WbpE were detected only when both proteins were combined in the same reaction. Using a direct MALDI-TOF mass spectrometry approach, we identified ions that corresponded to the predicted products of WbpB (UDP-3-keto-d-GlcNAcA) and WbpE (UDP-d-GlcNAc3NA) in the coupled enzyme-substrate reaction. Additionally, in reactions involving WbpB, WbpE, and WbpD, an ion consistent with the expected product of WbpD (UDP-d-GlcNAc3NAcA) was identified. Preparative quantities of UDP-d-GlcNAc3NA and UDP-d-GlcNAc3NAcA were enzymatically synthesized. These compounds were purified by high-performance liquid chromatography, and their structures were elucidated by NMR spectroscopy. This is the first report of the functional characterization of these proteins, and the enzymatic synthesis of UDP-d-GlcNAc3NA and UDP-d-GlcNAc3NAcA.

Sugar nucleotide recognition by Klebsiella pneumoniae UDP-D-galactopyranose mutase: fluorinated substrates, kinetics and equilibria

A series of selectively fluorinated and other substituted UDP-D-galactose derivatives have been evaluated as substrates for Klebsiella pneumoniae UDP-D-galactopyranose mutase. This enzyme, which catalyses the interconversion of the pyranose and furanose forms of galactose as its UDP adduct, is a prospective drug target for a variety of microbial infections. We show that none of the 2''-, 3''- or 6''-hydroxyl groups of UDP-D-galactopyranose are essential for substrate binding and turnover. However, steric factors appear to play an important role in limiting the range of substitutions that can be accommodated at C-2'' and C-6'' of the sugar nucleotide substrate. Attempts to invert the C-2'' stereochemistry from equatorial to axial, changing D-galacto- to D-talo-configuration, in an attempt to exploit the higher percentage of furanose at equilibrium in the talo-series, met with no turnover of substrate.