7-Fluorosialyl Glycosides Are Hydrolysis Resistant but Readily Assembled by Sialyltransferases Providing Easy Access to More Metabolically Stable Glycoproteins

Synthesis and unexpected binding of monofluorinated N,N'-diacetylchitobiose and LacdiNAc to wheat germ agglutinin

Fluorination of carbohydrate ligands of lectins is a useful approach to examine their binding profile, improve their metabolic stability and lipophilicity, and convert them into 19F NMR-active probes. However, monofluorination of monovalent carbohydrate ligands often leads to a decreased or completely lost affinity. By chemical glycosylation, we synthesized the full series of methyl β-glycosides of N,N'-diacetylchitobiose (GlcNAcβ(1-4)GlcNAcβ1-OMe) and LacdiNAc (GalNAcβ(1-4)GlcNAcβ1-OMe) systematically monofluorinated at all hydroxyl positions. A competitive enzyme-linked lectin assay revealed that the fluorination at the 6'-position of chitobioside resulted in an unprecedented increase in affinity to wheat germ agglutinin (WGA) by one order of magnitude. For the first time, we have characterized the binding profile of a previously underexplored WGA ligand LacdiNAc. Surprisingly, 4'-fluoro-LacdiNAc bound WGA even stronger than unmodified LacdiNAc. These observations were interpreted using molecular dynamic calculations along with STD and transferred NOESY NMR techniques, which gave evidence for the strengthening of CH/π interactions after deoxyfluorination of the side chain of the non-reducing GlcNAc. These results highlight the potential of fluorinated glycomimetics as high-affinity ligands of lectins and 19F NMR-active probes.

Exploring long-range fluorine-carbon J-coupling for conformational analysis of deoxyfluorinated disaccharides: A combined computational and NMR study

In this study, we investigated the potential of long-range fluorine-carbon J-coupling for determining the structures of deoxyfluorinated disaccharides. Three disaccharides, previously synthesized as potential galectin inhibitors, exhibited through-space fluorine-carbon J-couplings. In our independent conformational analysis of these disaccharide derivatives, we employed a combination of density functional theory (DFT) calculations and nuclear magnetic resonance (NMR) experiments. By comparing the calculated nuclear shieldings with the experimental carbon chemical shifts, we were able to identify the most probable conformers for each compound. A model comprising fluoromethane and methane molecules was used to study the relationship between molecular arrangements and intermolecular through-space J-coupling. Our study demonstrates the important effect of internuclear distance and molecular orientation on the magnitude of fluorine-carbon coupling. The experimental values for the fluorine-carbon through-space couplings (TSCs) of the disaccharides corresponded with values calculated for the most probable conformers identified by the conformational analysis. These results unlock the broader application of fluorine-carbon TSCs as powerful tools for conformational analysis of flexible molecules, offering valuable insights for future structural investigations.

Glycosyltransferases as versatile tools to study the biology of glycans

All cells are decorated with complex carbohydrate structures called glycans that serve as ligands for glycan-binding proteins (GBPs) to mediate a wide range of biological processes. Understanding the specific functions of glycans is key to advancing an understanding of human health and disease. However, the lack of convenient and accessible tools to study glycan-based interactions has been a defining challenge in glycobiology. Thus, the development of chemical and biochemical strategies to address these limitations has been a rapidly growing area of research. In this review, we describe the use of glycosyltransferases (GTs) as versatile tools to facilitate a greater understanding of the biological roles of glycans. We highlight key examples of how GTs have streamlined the preparation of well-defined complex glycan structures through chemoenzymatic synthesis, with an emphasis on synthetic strategies allowing for site- and branch-specific display of glyco-epitopes. We also describe how GTs have facilitated expansion of glyco-engineering strategies, on both glycoproteins and cell surfaces. Coupled with advancements in bioorthogonal chemistry, GTs have enabled selective glyco-epitope editing of glycoproteins and cells, selective glycan subclass labeling, and the introduction of novel biomolecule functionalities onto cells, including defined oligosaccharides, antibodies, and other proteins. Collectively, these approaches have contributed great insight into the fundamental biological roles of glycans and are enabling their application in drug development and cellular therapies, leaving the field poised for rapid expansion.

Cell-based production of Fc-GlcNAc and Fc-alpha-2,6 sialyl glycan enriched antibody with improved effector functions through glycosylation pathway engineering

Glycosylation of antibody plays an important role in Fc-mediated killing of tumor cells and virus-infected cells through effector functions such as antibody-dependent cellular cytotoxicity (ADCC), antibody dependent cell-mediated phagocytosis (ADCP) and vaccinal effect. Previous studies showed that therapeutical humanized antibodies with α2-6 sialyl complex type (SCT) glycan attached to Fc-Asn297 exhibited optimal binding to the Fc receptors on effector cells associated with ADCC, ADCP and vaccinal effect. However, the production of antibodies with homogeneous Fc-SCT needs multiple in vitro enzymatic and purification steps. In this study, we report two different approaches to shorten the processes to produce SCT-enriched antibodies. First, we expressed a bacterial endoglycosidase in GNT1-KO EXPI293 cells to trim all N -glycans to mono-GlcNAc glycoforms for in vitro transglycosylation to generate homogeneous SCT antibody. Second, we engineered the glycosylation pathway of HEK293 cells through knockout of the undesired glycosyltransferases and expression of the desired glycosyltransferases to produce SCT enriched antibodies with similar binding affinity to Fc receptors and ADCC activity to homogenous SCT antibody.

Tumor-Associated Extracellular Microvesicles with Fluorine-Modified Carbohydrate Antigens Trigger a Stronger Antitumor Immune Response

Abnormal glycosylation is a hallmark of tumor development, and tumor-associated carbohydrate antigens are potential immune targets for tumor therapy. Tumor-associated extracellular microvesicles are subcellular vesicles released from cell membranes that have immunogenicity similar to that of precursor cells. However, unmodified tumor-derived microvesicles have weaknesses, such as low immunogenicity, poor biostability, and short half-life in vivo. For the first time, we herein generated extracellular microvesicles containing modified tumor-associated carbohydrate antigens by constructing a cell line with highly expressed antigen-processing enzymes utilizing fluorine-modified monosaccharide substrates via a metabolic oligosaccharide engineering strategy. The microvesicles were applied to tumor immunity, achieving enhanced immunoprophylaxis and immunotherapy effects. Furthermore, the mechanisms of antitumor immunity were explored. Our findings may provide new insights into the adhibition of suitably modified extracellular microvesicles and the development of more effective carbohydrate-based anticancer vaccines.

Glycoprotein In Vitro N-Glycan Processing Using Enzymes Expressed in E. coli

Protein N-glycosylation is a common post-translational modification that plays significant roles on the structure, property, and function of glycoproteins. Due to N-glycan heterogeneity of naturally occurring glycoproteins, the functions of specific N-glycans on a particular glycoprotein are not always clear. Glycoprotein in vitro N-glycan engineering using purified recombinant enzymes is an attractive strategy to produce glycoproteins with homogeneous N-glycoforms to elucidate the specific functions of N-glycans and develop better glycoprotein therapeutics. Toward this goal, we have successfully expressed in E. coli glycoside hydrolases and glycosyltransferases from bacterial and human origins and developed a robust enzymatic platform for in vitro processing glycoprotein N-glycans from high-mannose-type to α2-6- or α2-3-disialylated biantennary complex type. The recombinant enzymes are highly efficient in step-wise or one-pot reactions. The platform can find broad applications in N-glycan engineering of therapeutic glycoproteins.

Biocatalytic Approaches to Building Blocks for Enzymatic and Chemical Glycan Synthesis

While the field of biocatalysis has bloomed over the past 20-30 years, advances in the understanding and improvement of carbohydrate-active enzymes, in particular, the sugar nucleotides involved in glycan building block biosynthesis, have progressed relatively more slowly. This perspective highlights the need for further insight into substrate promiscuity and the use of biocatalysis fundamentals (rational design, directed evolution, immobilization) to expand substrate scopes toward such carbohydrate building block syntheses and/or to improve enzyme stability, kinetics, or turnover. Further, it explores the growing premise of using biocatalysis to provide simple, cost-effective access to stereochemically defined carbohydrate materials, which can undergo late-stage chemical functionalization or automated glycan synthesis/polymerization.

Pseudo-glycoconjugates with a C-glycoside linkage

Work by the author and colleagues has been focused on the development of pseudo-glycans (pseudo-glycoconjugates), in which the O-glycosidic linkage of the natural-type glycan structure is replaced by a C-glycosidic linkage. These analogs are not degraded by cellular glycoside hydrolases and are thus expected to be useful molecular tools that may maintain the original biological activity for a long period in the cell. However, their biological potential is not yet well understood because only a few pseudo glycans have so far been synthesized. This article aims to provide a bird's-eye view of our recent studies on the creation of C-glycoside analogs of ganglioside GM3 based on the CHF-sialoside linkage, and summarizes the chemical insights acquired during our stereoselective synthesis of the C-sialoside bond, ultimately leading to pseudo-GM3. Conformational analysis of the synthesized CHF-sialoside disaccharides confirmed that the anticipated conformational control by F-atom introduction was successful, and furthermore, enhanced the biological activity. In order to improve access to C-glycoside analogs based on pseudo-GM3, it is still important to streamline the synthesis process. With this in mind, we designed and developed a direct C-glycosylation method using atom-transfer radical coupling, and employed it in syntheses of pseudo-isomaltose and pseudo-KRN7000.

Carbohydrate-active enzymes (CAZymes) in the gut microbiome

The 10¹³-10¹⁴ microorganisms present in the human gut (collectively known as the human gut microbiota) dedicate substantial percentages of their genomes to the degradation and uptake of carbohydrates, indicating the importance of this class of molecules. Carbohydrates function not only as a carbon source for these bacteria but also as a means of attachment to the host, and a barrier to infection of the host. In this Review, we focus on the diversity of carbohydrate-active enzymes (CAZymes), how gut microorganisms use them for carbohydrate degradation, the different chemical mechanisms of these CAZymes and the roles that these microorganisms and their CAZymes have in human health and disease. We also highlight examples of how enzymes from this treasure trove have been used in manipulation of the microbiota for improved health and treatment of disease, in remodelling the glycans on biopharmaceuticals and in the potential production of universal O-type donor blood.

Neighboring Group Participation of Benzoyl Protecting Groups in C3‐ and C6‐Fluorinated Glucose

Fluorination is a potent method to modulate chemical properties of glycans. Here, we study how C3‐ and C6‐fluorination of glucosyl building blocks influence the structure of the intermediate of the glycosylation reaction, the glycosyl cation. Using a combination of gas‐phase infrared spectroscopy and first‐principles theory, glycosyl cations generated from fluorinated and non‐fluorinated monosaccharides are structurally characterized. The results indicate that neighboring group participation of the C2‐benzoyl protecting group is the dominant structural motif for all building blocks, correlating with the β‐selectivity observed in glycosylation reactions. The infrared signatures indicate that participation of the benzoyl group in enhanced by resonance effects. Participation of remote acyl groups such as Fmoc or benzyl on the other hand is unfavored. The introduction of the less bulky fluorine leads to a change in the conformation of the ring pucker, whereas the structure of the active dioxolenium site remains unchanged.

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.

Impacting Bacterial Sialidase Activity by Incorporating Bioorthogonal Chemical Reporters onto Mammalian Cell-Surface Sialosides

Bioorthogonal chemical reporters, in synergy with click chemistry, have emerged as a key technology for tagging complex glycans in living cells. This strategy relies on the fact that bioorthogonal chemical reporters are highly reactive species while being biologically noninvasive. Here, we report that chemical reporters and especially sydnones may have, on the contrary, enormous impact on biomolecule processing enzymes. More specifically, we show that editing cell-surface sialic acid-containing glycans (sialosides) with bioorthogonal chemical reporters can significantly affect the activity of bacterial sialidases, enzymes expressed by bacteria during pathogenesis for cleaving sialic acid sugars from mammalian cell-surface glycans. Upon screening various chemical reporters, as well as their position on the sialic acid residue, we identified that pathogenic bacterial sialidases were unable to cleave sialosides displaying a sydnone at the 5-position of sialic acids in vitro as well as in living cells. This study highlights the importance of investigating more systematically the metabolic fate of glycoconjugates modified with bioorthogonal reporters.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

The Metabolic Chemical Reporter Ac46AzGal Could Incorporate Intracellular Protein Modification in the Form of UDP-6AzGlc Mediated by OGT and Enzymes in the Leloir Pathway

Galactose is a naturally occurring monosaccharide used to build complex glycans that has not been targeted for labeling as a metabolic reporter. Here, we characterize the cellular modification of proteins by using Ac₄6AzGal in a dose- and time-dependent manner. It is noted that a vast majority of this labeling of Ac₄6AzGal occurs intracellularly in a range of mammalian cells. We also provided evidence that this labeling is dependent on not only the enzymes of OGT responsible for O-GlcNAcylation but also the enzymes of GALT and GALE in the Leloir pathway. Notably, we discover that Ac₄6AzGal is not the direct substrate of OGT, and the labeling results may attribute to UDP-6AzGlc after epimerization of UDP-6AzGal via GALE. Together, these discoveries support the conclusion that Ac₄6AzGal as an analogue of galactose could metabolically label intracellular O-glycosylation modification, raising the possibility of characterization with impaired functions of the galactose metabolism in the Leloir pathway under certain conditions, such as galactosemias.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Chemoenzymatic Total Synthesis of GM3 Gangliosides Containing Different Sialic Acid Forms and Various Fatty Acyl Chains

Gangliosides are sialic acid-containing glycosphingolipids that have been found in the cell membranes of all vertebrates. Their important biological functions are contributed by both the glycan and the ceramide lipid components. GM3 is a major ganglioside and a precursor for many other more complex gangliosides. To obtain structurally diverse GM3 gangliosides containing various sialic acid forms and different fatty acyl chains in low cost, an improved process was developed to chemically synthesize lactosyl sphingosine from an inexpensive l-serine derivative. It was then used to obtain GM3 sphingosines from diverse modified sialic acid precursors by an efficient one-pot multienzyme sialylation system containing Pasteurella multocida sialyltransferase 3 (PmST3) with in situ generation of sugar nucleotides. A highly effective chemical acylation and facile C18-cartridge purification process was then used to install fatty acyl chains of varying lengths and different modifications. The chemoenzymatic method represents a powerful total synthetic strategy to access a library of structurally defined GM3 gangliosides to explore their functions.