Recent Progress in Chemo-Enzymatic Methods for the Synthesis of N-Glycans

Chemoenzymatic synthesis of (1R,3R)-3-hydroxycyclopentanemethanol: An intermediate of carbocyclic-ddA

The synthesis of carbocyclic-ddA, a potent antiviral agent against hepatitis B, relies significantly on (1R,3R)-3-hydroxycyclopentanemethanol as a key intermediate. To effectively produce this intermediate, our study employed a chemoenzymatic approach. The selection of appropriate biocatalysts was based on substrate similarity, leading us to adopt the CrS enoate reductase derived from Thermus scotoductus SA-01. Additionally, we developed an enzymatic system for NADH regeneration, utilising formate dehydrogenase from Candida boidinii. This system facilitated the efficient catalysis of (S)-4-(hydroxymethyl)cyclopent-2-enone, resulting in the formation of (3R)-3-(hydroxymethyl) cyclopentanone. Furthermore, we successfully cloned, expressed, purified, and characterized the CrS enzyme in Escherichia coli. Optimal reaction conditions were determined, revealing that the highest activity occurred at 45 °C and pH 8.0. By employing 5 mM (S)-4-(hydroxymethyl)cyclopent-2-enone, 0.05 mM FMN, 0.2 mM NADH, 10 μM CrS, 40 μM formic acid dehydrogenase, and 40 mM sodium formate, complete conversion was achieved within 45 min at 35 °C and pH 7.0. Subsequently, (1R,3R)-3-hydroxycyclopentanemethanol was obtained through a simple three-step chemical conversion process. This study not only presents an effective method for synthesizing the crucial intermediate but also highlights the importance of biocatalysts and enzymatic systems in chemoenzymatic synthesis approaches.

Chemical and Chemoenzymatic Synthesis of Peptide and Protein Therapeutics Conjugated with Human N-Glycans

Glycosylation is one of the most ubiquitous post-translational modifications. It affects the structure and function of peptides/proteins and consequently has a significant impact on various biological events. However, the structural complexity and heterogeneity of glycopeptides/proteins caused by the diversity of glycan structures and glycosylation sites complicates the detailed elucidation of glycan function and hampers their clinical applications. To address these challenges, chemical and/or enzyme-assisted synthesis methods have been developed to realize glycopeptides/proteins with well-defined glycan morphologies. In particular, N-glycans are expected to be useful for improving the solubility, in vivo half-life and aggregation of bioactive peptides/proteins that have had limited clinical applications so far due to their short duration of action in the blood and unsuitable physicochemical properties. Chemical glycosylation performed in a post-synthetic procedure can be used to facilitate the development of glycopeptide/protein analogues or mimetics that are superior to the original molecules in terms of physicochemical and pharmacokinetic properties. N-glycans are used to modify targets because they are highly biodegradable and biocompatible and have structures that already exist in the human body. On the practical side, from a quality control perspective, close attention should be paid to their structural homogeneity when they are to be applied to pharmaceuticals.

N-Glycosylation as a Modulator of Protein Conformation and Assembly in Disease

Glycosylation, a prevalent post-translational modification, plays a pivotal role in regulating intricate cellular processes by covalently attaching glycans to macromolecules. Dysregulated glycosylation is linked to a spectrum of diseases, encompassing cancer, neurodegenerative disorders, congenital disorders, infections, and inflammation. This review delves into the intricate interplay between glycosylation and protein conformation, with a specific focus on the profound impact of N-glycans on the selection of distinct protein conformations characterized by distinct interactomes-namely, protein assemblies-under normal and pathological conditions across various diseases. We begin by examining the spike protein of the SARS virus, illustrating how N-glycans regulate the infectivity of pathogenic agents. Subsequently, we utilize the prion protein and the chaperone glucose-regulated protein 94 as examples, exploring instances where N-glycosylation transforms physiological protein structures into disease-associated forms. Unraveling these connections provides valuable insights into potential therapeutic avenues and a deeper comprehension of the molecular intricacies that underlie disease conditions. This exploration of glycosylation's influence on protein conformation effectively bridges the gap between the glycome and disease, offering a comprehensive perspective on the therapeutic implications of targeting conformational mutants and their pathologic assemblies in various diseases. The goal is to unravel the nuances of these post-translational modifications, shedding light on how they contribute to the intricate interplay between protein conformation, assembly, and disease.

Recent advances in enzymatic and chemoenzymatic synthesis of N- and O-glycans

Glycosylation is one of the most common post-translational modifications of proteins, which plays essential roles in regulating the biological functions of proteins. Efficient and versatile methods for the synthesis of homogeneous and well-defined N- and O-glycans remain an urgent need for biological studies and biomedical applications. Despite their structural complexity, tremendous progress has been made in the synthesis of N- and O-glycans in recent years. This review discusses some recent advances in the enzymatic and chemoenzymatic synthesis of N- and O-glycans.

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.

Biomanufacturing of glycosylated antibodies: Challenges, solutions, and future prospects

Traditionally, recombinant protein production has been done in several expression hosts of bacteria, fungi, and majorly CHO (Chinese Hamster Ovary) cells; few have high production costs and are susceptible to harmful toxin contamination. Green algae have the potential to produce recombinant proteins in a more sustainable manner. Microalgal diversity leads to offer excellent opportunities to produce glycosylated antibodies. An antibody with humanized glycans plays a crucial role in cellular communication that works to regulate cells and molecules, to control disease, and to stimulate immunity. Therefore, it becomes necessary to understand the role of abiotic factors (light, temperature, pH, etc.) in the production of bioactive molecules and molecular mechanisms of product synthesis from microalgae which would lead to harnessing the potential of algal bio-refinery. However, the potential of microalgae as the source of bio-refinery has been less explored. In the present review, omics approaches for microalgal engineering, methods of humanized glycoproteins production focusing majorly on N-glycosylation pathways, light-based regulation of glycosylation machinery, and production of antibodies with humanized glycans in microalgae with a major emphasis on modulation of post-translation machinery of microalgae which might play a role in better understanding of microalgal potential as a source for antibody production along with future perspectives.

Multisite Partial Glycosylation Approach for Preparation of Biologically Relevant Oligomannosyl Branches Contribute to Lectin Affinity Analysis

High-mannose-type glycans play essential biological roles, e.g., immune response and glycoprotein quality control, and preparing a series of oligomannosyl branches of high-mannose-type glycans is critical for biological studies. However, obtaining sufficient amounts of the various oligomannosyl branches is challenging. In this study, we demonstrated a partial glycosylation strategy for the single-step synthesis of various biologically relevant oligomannosyl-branched structures. First, Manα1-6(Manα1-3)Man-type oligomannosyl branch was synthesized via double glycosylation from a 3,6-di-OH mannosyl acceptor and fluorinated mannosyl donor with perfect α-selectivity. Subsequent partial glycosylation by reducing the equivalent of the mannosyl donor enabled to obtain biologically relevant Manα1-2Manα1-6(Manα1-2Manα1-3)Man, Manα1-6(Manα1-2Manα1-3)Man, Manα1-2Manα1-6(Manα1-3)Man, and Manα1-6(Manα1-3)Man in one-pot. Each oligomannosyl branch could be easily purified by liquid chromatography. The resulting structural isomers were identified by 2D-HMBC NMR. A systematic lectin affinity assay using the prepared oligomannosyl branches showed different specificities for the Galanthus nivalis lectin between structural isomers of the oligomannosyl branches with the same number of mannose residues..

A First Expression, Purification and Characterization of Endo-β-1,3-Glucanase from Penicillium expansum

β-1,3-glucanase plays an important role in the biodegradation, reconstruction, and development of β-1,3-glucan. An endo-β-1,3-glucanase which was encoded by PeBgl1 was expressed, purified and characterized from Penicillium expansum for the first time. The PeBgl1 gene was amplified and transformed into the competent cells of E. coli Rosetta strain with the help of the pET-30a cloning vector. The recombinant protein PeBgl1 was expressed successfully at the induction conditions of 0.8 mmol/L IPTG at 16 °C for 16 h and then was purified by nickel ion affinity chromatography. The optimum reaction temperature of PeBgl1 was 55 °C and it had maximal activity at pH 6.0 according to the enzymatic analysis. Na2HPO4-NaH2PO4 buffer (pH 6.0) and NaCl have inhibitory and enhancing effects on the enzyme activities, respectively. SDS, TritonX-100 and some metal ions (Mg2+, Ca2+, Ba2+, Cu2+, and Zn2+) have an inhibitory effect on the enzyme activity. The results showed that PeBgl1 protein has good enzyme activity at 50-60 °C and at pH 5.0-9.0, and it is not a metal dependent enzyme, which makes it robust for storage and transportation, ultimately holding great promise in green biotechnology and biorefining.

Cell-free N-glycosylation of peptides using synthetic lipid-linked hybrid and complex N-glycans

Cell-free, chemoenzymatic platforms are emerging technologies towards generating glycoconjugates with defined and homogeneous glycoforms. Recombinant oligosaccharyltransferases can be applied to glycosylate "empty," i.e., aglycosyalted, peptides and proteins. While bacterial oligosaccharlytransferases have been extensively investigated, only recently a recombinant eukaryotic single-subunit oligosaccharyltransferase has been successfully used to in vitro N-glycosylate peptides. However, its applicability towards synthesizing full-length glycoproteins and utilizing glycans beyond mannose-type glycans for the transfer have not be determined. Here, we show for the first time the synthesis of hybrid- and complex-type glycans using synthetic lipid carriers as substrates for in vitro N-glycosylation reactions. For this purpose, transmembrane-deleted human β-1,2 N-acetylglucosamintransferase I and II (MGAT1ΔTM and MGAT2ΔTM) and β-1,4-galactosyltransferase (GalTΔTM) have been expressed in Escherichia coli and used to extend an existing multi-enzyme cascade. Both hybrid and agalactosylated complex structures were transferred to the N-glycosylation consensus sequence of peptides (10 amino acids: G-S-D-A-N-Y-T-Y-T-Q) by the recombinant oligosaccharyltransferase STT3A from Trypanosoma brucei.

Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.

Generation and reaction of alanyl radicals in open flasks

The generation and Giese-type reaction of alanyl radicals under metal-free reaction conditions is described. The procedure is operationally simple, occurring at ambient temperature in an open reaction vessel, and requiring short reaction times (≤5 min). The reaction occurs without epimerization and provides ready access to non-proteinogenic amino acids and peptides. Importantly, the process is tolerant of light absorbing groups including commonly used fluorescent tags.

New insight into strategies used to develop long-acting G-CSF biologics for neutropenia therapy

Over the last 20 years, granulocyte colony-stimulating factors (G-CSFs) have become the major therapeutic option for the treatment of patients with neutropenia. Most of the current G-CSFs require daily injections, which are inconvenient and expensive for patients. Increased understanding of G-CSFs' structure, expression, and mechanism of clearance has been very instrumental in the development of new generations of long-acting G-CSFs with improved efficacy. Several approaches to reducing G-CSF clearance via conjugation techniques have been investigated. PEGylation, glycosylation, polysialylation, or conjugation with immunoglobulins or albumins have successfully increased G-CSFs' half-lives. Pegfilgrastim (Neulasta) has been successfully approved and marketed for the treatment of patients with neutropenia. The rapidly expanding market for G-CSFs has increased demand for G-CSF biosimilars. Therefore, the importance of this review is to highlight the principle, elimination's route, half-life, clearance, safety, benefits, and limitations of different strategies and techniques used to increase the half-life of biotherapeutic G-CSFs. Understanding these strategies will allow for a new treatment with more competitive manufacturing and lower unit costs compared with that of Neulasta.

Site Specific Preparation of N-Glycosylated Peptides: Thioamide-Directed Activation of Aspartate

A site-specific method for the preparation of N-glycosylated peptides is described. Incorporation of a peptide backbone thioamide linkage adjacent to an Asp residue facilitates a Ag^I -promoted, site-specific conversion to N-glycosylated Asn residues in peptides.

Recent applications of ionic liquid-based tags in glycoscience

The functionalization of glycosides with ionic compounds such as ionic liquids provides enhanced polarity for the labelled glycans thanks to the presence of a permanent positive charge. The chemical derivatisation of glycans with ionic liquids constitutes an emerging strategy to boost the detection sensitivity in MS applications. This allows the straightforward monitoring and detection of the presence of labelled glycans in complex matrices and in those cases where very limited amounts of material were available such as in biological samples and chemoenzymatic reactions. The use of ionic liquid based derivatisation agents can be further exploited for the labelling of live cells via metabolic oligosaccharide engineering for the detection of cancer biomarkers and for the tuning of live cells-surface properties with implications in cancer prognosis and progression. In this mini-review we summarise the latest development of the ionic liquid based derivatisation agents in glycoscience focussing on their use for sensitive MS applications.

Glycan microarrays from construction to applications

Through their specific interactions with proteins, cellular glycans play key roles in a wide range of physiological and pathological processes. One of the main goals of research in the areas of glycobiology and glycomedicine is to understand glycan-protein interactions at the molecular level. Over the past two decades, glycan microarrays have become powerful tools for the rapid evaluation of interactions between glycans and proteins. In this review, we briefly describe methods used for the preparation of glycan probes and the construction of glycan microarrays. Next, we highlight applications of glycan microarrays to rapid profiling of glycan-binding patterns of plant, animal and pathogenic lectins, as well as other proteins. Finally, we discuss other important uses of glycan microarrays, including the rapid analysis of substrate specificities of carbohydrate-active enzymes, the quantitative determination of glycan-protein interactions, discovering high-affinity or selective ligands for lectins, and identifying functional glycans within cells. We anticipate that this review will encourage researchers to employ glycan microarrays in diverse glycan-related studies.

Unusual β1-4-galactosidase activity of an α1-6-mannosidase from Xanthomonas manihotis in the processing of branched hybrid and complex glycans

Mannosidases are a diverse group of glycoside hydrolases that play crucial roles in mannose trimming of oligomannose glycans, glycoconjugates, and glycoproteins involved in numerous cellular processes, such as glycan biosynthesis and metabolism, structure regulation, cellular recognition, and cell–pathogen interactions. Exomannosidases and endomannosidases cleave specific glycosidic bonds of mannoside linkages in glycans and can be used in enzyme-based methods for sequencing of isomeric glycan structures. α1-6-mannosidase from Xanthomonas manihotis is known as a highly specific exoglycosidase that removes unbranched α1-6 linked mannose residues from oligosaccharides. However, we discovered that this α1-6-mannosidase also possesses an unexpected β1-4-galactosidase activity in the processing of branched hybrid and complex glycans through our use of enzymatic reactions, high performance anion-exchange chromatography, and liquid chromatography mass spectrometric sequencing. Our docking simulation of the α1-6-mannosidase with glycan substrates reveals potential interacting residues in a relatively shallow pocket slightly differing from its homologous enzymes in the glycoside hydrolase 125 family, which may be responsible for the observed higher promiscuity in substrate binding and subsequent terminal glycan hydrolysis. This observation of novel β1-4-galactosidase activity of the α1-6-mannosidase provides unique insights into its bifunctional activity on the substrate structure-dependent processing of terminal α1-6-mannose of unbranched glycans and terminal β1-4-galactose of hybrid and complex glycans. The finding thus suggests the dual glycosidase specificity of this α1-6-mannosidase and the need for careful consideration when used for the structural elucidation of glycan isomers.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Mass Spectrometry-Based Shotgun Glycomics Using Labeled Glycan Libraries

Mass spectrometry-based shotgun glycomics (MS-SG) is a rapid, sensitive, label-, and immobilization-free approach for the discovery of natural ligands of glycan-binding proteins (GBPs). To perform MS-SG, natural libraries of glycans derived from glycoconjugates in cells or tissues are screened against a target GBP using catch-and-release electrospray ionization mass spectrometry (CaR-ESI-MS). Because glycan concentrations are challenging to determine, ligand affinities cannot be directly measured. In principle, relative affinities can be ranked by combining CaR-ESI-MS data with relative concentrations established by hydrophilic interaction liquid chromatography (HILIC) performed on the fluorophore-labeled glycan library. To validate this approach, as well as the feasibility of performing CaR-ESI-MS directly on labeled glycans, libraries of labeled N-glycans extracted from the human monocytic U937 cells or intestinal tissues were labeled with 2-aminobenzamide (2-AB), 2-aminobenzoic acid (2-AA), or procainamide (proA). The libraries were screened against plant and human GBPs with known specificities for α2-3- and α2-6-linked sialosides and quantified by HILIC. Dramatic differences, in some cases, were found for affinity rankings obtained with libraries labeled with different fluorophores, as well as those produced using the combined unlabeled/labeled library approach. The origin of these differences could be explained by differential glycan labeling efficiencies, the impact of specific labels on glycan affinities for the GBPs, and the relative efficiency of release of ligands from GBPs in CaR-ESI-MS. Overall, the results of this study suggest that the 2-AB(CaR-ESI-MS)/2-AB(HILIC) combination provides the most reliable description of the binding specificities of GBPs for N-glycans and is recommended for MS-SG applications.

Insights into Selectin Inhibitor Design from Endogenous Isomeric Ligands of SLea and SLex

Selectins interact with cell-surface glycans to promote the initial tethering and rolling of leukocytes, and these interactions are targets for designs of inhibitors to neutralize diseases related to excessive inflammatory responses in many cardiovascular and immune dysfunctions, as well as tumor markers in different cancers. The isomeric endogenous tetrasaccharides, sialyl Lewis X (sLe^x) and sialyl Lewis A (sLe^a), are minimal sugar structures required for selectin binding. Understanding their subtle structural variances and significant advanced binding strengths of sLe^a over sLe^x could benefit the rational designs for selectin inhibitors. Modeling based on the E-selectin-sLe^x crystal structure in the present study demonstrated that the N-acetyl group of GlcNAc in sLe^x could form steric hindrances in the E-selectin-sLe^x complex, but the hydroxy methylene group of GlcNAc in sLe^a at the same position allows for stronger binding interactions. The subsequent designed inhibitor with a synthetic accessible linker molecule that has no exo-cyclic moieties replacing GlcNAc displayed comparable dynamic and energetic binding features to sLe^a. The present study deciphered the clues from endogenous isomeric sLe^a and sLe^x and provided insights into designing selectin inhibitors with simplified synthesis.

General Tolerance of Galactosyltransferases toward UDP-galactosamine Expands Their Synthetic Capability

Accessing large numbers of structurally diverse glycans and derivatives is essential to functional glycomics. We showed a general tolerance of galactosyltransferases toward uridine-diphosphate-galactosamine (UDP-GalN), which is not a commonly used sugar nucleotide donor. The property was harnessed to develop a two-step chemoenzymatic strategy for facile synthesis of novel and divergent N-acetylgalactosamine (GalNAc)-glycosides and derivatives in preparative scales. The discovery and the application of the new property of existing glycosyltransferases expand their catalytic capabilities in generating novel carbohydrate linkages, thus prompting the synthesis of diverse glycans and glycoconjugates for biological studies.

Chemical Synthesis of Sialyl N-Glycans and Analysis of Their Recognition by Neuraminidase

The chemical synthesis of a fully sialylated tetraantennary N-glycan has been achieved for the first time by using the diacetyl strategy, in which NHAc is protected as NAc₂ to improve reactivity by preventing intermolecular hydrogen bonds. Another key was the glycosylation to the branched mannose in an ether solvent, which promoted the desired glycosylation by stabilizing the oxocarbenium ion intermediate. Furthermore, high α-selectivity of these glycosylation reactions was realized by utilizing remote participation. Two asymmetrically deuterium labeled sialyl N-glycans were also synthesized by the same strategy. The synthesized N-glycans were used to probe the molecular basis of H1N1 neuraminidase recognition. The asymmetrically deuterated N-glycans revealed a difference in the recognition of sialic acid on each branch. Meanwhile, the tetraantennary N-glycan was used to evaluate the effects of multivalency and steric hinderance by forming branching structures.

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.

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Capillary Nanogel Electrophoresis for the Determination of the β1-4 Galactosyltransferase Michaelis-Menten Constant and Real-Time Addition of Galactose Residues to N-Glycans and Glycoprotein

A thermally reversible nanogel is used in capillary electrophoresis to create discrete regions for a galactosyltransferase reaction and separation. The β1-4 galactosyltransferase enzyme, donor, and co-factor were patterned in the capillary. The substrate was driven through these zones and converted to galactosylated products, which were separated and identified. Using this capillary electrophoresis method, the degree of glycosylation was discernible for a pentasaccharide and for biantennary N-glycans. With the ability to distinguish between reaction products for which either one or two galactose residues were transferred, the capillary nanogel electrophoresis system was used to determine the Michaelis-Menten value, KM. For the β1-4 galactosyltransferase, the KM value obtained for a pentasaccharide substrate was 1.23 ± 0.08 mM. Once KM was established, the enzyme/substrate ratio was evaluated to add a single galactose residue or to fully galactosylate a biantennary N-glycan. Additionally, capillary nanogel electrophoresis was adapted to transfer galactose residues to protein. The applicability of the method for real-time online modification of whole protein was demonstrated with the Herceptin glycoprotein. Complete retardation by Erythrina cristagalli lectin after enzymatic modification confirmed the addition of galactose residues to the Herceptin. This demonstrated the potential of the method to be used for online modification of other glycoproteins.

In-depth characterization of non-human sialic acid (Neu5Gc) in human serum using label-free ZIC-HILIC/MRM-MS

Sialic acid Neu5Gc, a non-human glycan, is recognized as a new harmful substance that can cause vascular disease and cancer. Humans are unable to synthesize Neu5Gc due to a genetic defect that converts Neu5Ac to Neu5Gc, but Neu5Gc is often observed in human biological samples. Therefore, the demand for accurately measuring the amount of Neu5Gc present in human blood or tissues is rapidly increasing, but there is still no method to reliably quantify trace amounts of a non-human sugar. In particular, selective isolation and detection of Neu5Gc from human serum is analytically challenging due to the presence of excess sialic acid Neu5Ac, which has physicochemical properties very similar to Neu5Gc. Herein, we developed the label-free approach based on ZIC-HILIC/MRM-MS that can enrich sialic acids released from human serum and simultaneously monitor Neu5Ac and Neu5Gc. The combination of complete separation of Neu5Gc from abundant Neu5Ac by hydrophilic and electrostatic interactions with selective monitoring of structure-specific cross-ring cleavage ions generated by negative CID-MS/MS was remarkably effective for quantification of Neu5Ac and Neu5Gc at the femtomole level. Indeed, we were able to successfully determine the absolute quantitation of Neu5Gc from 30 healthy donors in the range of 3.336 ± 1.252 pg/μL (mean ± SD), 10,000 times lower than Neu5Ac. In particular, analysis of sialic acids in protein-free serum revealed that both Neu5Ac and Neu5G are mostly bound to proteins and/or lipids, but not in free form. In addition, the correlation between expression level of Neu5Gc and biological factors such as BMI, age, and sex was investigated. This method can be widely used in studies requiring sialic acid-related measurements such as disease diagnosis or prediction of immunogenicity in biopharmaceuticals as it is both fast and highly sensitive.

The Abnormal Glycopatterns of Salivary Glycoproteins in Esophageal Squamous Cell Carcinoma Patients

Glycosylation is one of the most crucial posttranslational modifications of proteins, containing a remarkable amount of biological information. The alteration of glycosylation is closely associated with certain diseases. Exploring glyco-code in the development of diseases is a hot topic in recent years. Esophageal squamous cell carcinoma (ESCC) is the primary pathological histology in developing countries and a severe threat to human health. Although the glycan profiles in the blood samples of ESCC patients were analyzed using glycomic and glycoproteomic methods, the difference of salivary glycopatterns between healthy subjects and ESCC patients is not explicit yet. In the present study, ESCC patients (n = 16) and healthy volunteers (HVs, n = 25) were enrolled. The glycomic strategy combining lectin microarray and lectin blotting was employed to investigate and confirm the altered salivary glycopatterns. Datura stramonium (DSA) was selected to isolate the GlcNAc or Galβ1-4GlcNA-containing glycoproteins due to the distinct difference between ESCC patients and HVs. The N-glycans from DSA-enriched glycoproteins were released by PNGase F and further identified by MALDI-TOF/TOF-MS to obtain the precise structural information of the altered glycans. As a result, the glycopatterns recognized by 13 lectins (e.g., ECA, RCA120, and DSA) showed significant alterations in ESCC patients' saliva. The ESCC patients showed higher levels of GalNAc and Gal, sialic acid, and GlcNAc expression profiles and lower levels of mannose and fucose expression profiles. The MALDI-TOF/TOF-MS results indicated that the proportion of the GlcNAc or Galβ1-4GlcNAc-containing N-glycans was increased in ESCC patients (79.04%) compared with HV (63.20%), which was consistent with the results of lectin microarrays. Our findings provide comprehensive information to understand the complex physiological changes in ESCC patients. And the altered salivary glycopatterns such as GlcNAc or Galβ1-4GlcNAc-containing N-glycans recognized by DSA might serve as potential biomarkers for the diagnosis of ESCC patients.

Recent Advances in the Chemical Biology of N-Glycans

Asparagine-linked N-glycans on proteins have diverse structures, and their functions vary according to their structures. In recent years, it has become possible to obtain high quantities of N-glycans via isolation and chemical/enzymatic/chemoenzymatic synthesis. This has allowed for progress in the elucidation of N-glycan functions at the molecular level. Interaction analyses with lectins by glycan arrays or nuclear magnetic resonance (NMR) using various N-glycans have revealed the molecular basis for the recognition of complex structures of N-glycans. Preparation of proteins modified with homogeneous N-glycans revealed the influence of N-glycan modifications on protein functions. Furthermore, N-glycans have potential applications in drug development. This review discusses recent advances in the chemical biology of N-glycans.

Nucleotide Sugars in Chemistry and Biology

Nucleotide sugars have essential roles in every living creature. They are the building blocks of the biosynthesis of carbohydrates and their conjugates. They are involved in processes that are targets for drug development, and their analogs are potential inhibitors of these processes. Drug development requires efficient methods for the synthesis of oligosaccharides and nucleotide sugar building blocks as well as of modified structures as potential inhibitors. It requires also understanding the details of biological and chemical processes as well as the reactivity and reactions under different conditions. This article addresses all these issues by giving a broad overview on nucleotide sugars in biological and chemical reactions. As the background for the topic, glycosylation reactions in mammalian and bacterial cells are briefly discussed. In the following sections, structures and biosynthetic routes for nucleotide sugars, as well as the mechanisms of action of nucleotide sugar-utilizing enzymes, are discussed. Chemical topics include the reactivity and chemical synthesis methods. Finally, the enzymatic in vitro synthesis of nucleotide sugars and the utilization of enzyme cascades in the synthesis of nucleotide sugars and oligosaccharides are briefly discussed.