Chemo-enzymatic synthesis of poly-N-acetyllactosamine (poly-LacNAc) structures and their characterization for CGL2-galectin-mediated binding of ECM glycoproteins to biomaterial surfaces

Microgels with Immobilized Glycosyltransferases for Enzymatic Glycan Synthesis

Glycans, composed of linked monosaccharides, play crucial roles in biology and find diverse applications. Enhancing their enzymatic synthesis can be achieved by immobilizing enzymes on materials such as microgels. Here, we present microgels with immobilized glycosyltransferases, synthesized through droplet microfluidics, immobilizing enzymes either via encapsulation or postattachment. SpyTag-SpyCatcher interaction was used for enzyme binding, among others. Fluorescamine and permeability assays confirmed enzyme immobilization and microgel porosity, while enzymatic activities were determined using HPLC. The potential application of microgels in cascade reactions involving multiple enzymes was demonstrated by combining β4GalT and α3GalT in an enzymatic reaction with high yields. Moreover, a cascade of β4GalT and β3GlcNAcT was successfully implemented. These results pave the way toward a modular membrane bioreactor for automated glycan synthesis containing the presented biocatalytic microgels.

Impacts of β-1, 3-N-acetylglucosaminyltransferases (B3GNTs) in human diseases

Glycosylation modification of proteins is a common post-translational modification that exists in various organisms and has rich biological functions. It is usually catalyzed by multiple glycosyltransferases located in the Golgi apparatus. β-1,3-N-acetylglucosaminyltransferases (B3GNTs) are members of the glycosyltransferases and have been found to be involved in the occurrence and development of a variety of diseases including autoimmunity diseases, cancers, neurodevelopment, musculoskeletal system, and metabolic diseases. The functions of B3GNTs represent the glycosylation of proteins is a crucial and frequently life-threatening step in progression of most diseases. In this review, we give an overview about the roles of B3GNTs in tumor, nervous system, musculoskeletal and metabolic diseases, describing the recent results about B3GNTs, in order to provide a research direction and exploration value for the prevention, diagnosis and treatment of these diseases.

An Overview of Sugar Nucleotide-Dependent Glycosyltransferases for Human Milk Oligosaccharide Synthesis

Human milk oligosaccharides (HMOs) have received increasing attention because of their special effects on infant health and commercial value as the new generation of core components in infant formula. Currently, large-scale production of HMOs is generally based on microbial synthesis using metabolically engineered cell factories. Introduction of the specific glycosyltransferases is essential for the construction of HMO-producing engineered strains in which the HMO-producing glycosyltransferases are generally sugar nucleotide-dependent. Four types of glycosyltransferases have been used for typical glycosylation reactions to synthesize HMOs. Soluble expression, substrate specificity, and regioselectivity are common concerns of these glycosyltransferases in practical applications. Screening of specific glycosyltransferases is an important research topic to solve these problems. Molecular modification has also been performed to enhance the catalytic activity of various HMO-producing glycosyltransferases and to improve the substrate specificity and regioselectivity. In this article, various sugar nucleotide-dependent glycosyltransferases for HMO synthesis were overviewed, common concerns of these glycosyltransferases were described, and the future perspectives of glycosyltransferase-related studies were provided.

Site-Specific N- and O-Glycosylation Analysis of Human Plasma Fibronectin

Human plasma fibronectin is an adhesive protein that plays a crucial role in wound healing. Many studies had indicated that glycans might mediate the expression and functions of fibronectin, yet a comprehensive understanding of its glycosylation is still missing. Here, we performed a comprehensive N- and O-glycosylation mapping of human plasma fibronectin and quantified the occurrence of each glycoform in a site-specific manner. Intact N-glycopeptides were enriched by zwitterionic hydrophilic interaction chromatography, and N-glycosite sites were localized by the ¹⁸O-labeling method. O-glycopeptide enrichment and O-glycosite identification were achieved by an enzyme-assisted site-specific extraction method. An RP–LC–MS/MS system functionalized with collision-induced dissociation and stepped normalized collision energy (sNCE)-HCD tandem mass was applied to analyze the glycoforms of fibronectin. A total of 6 N-glycosites and 53 O-glycosites were identified, which were occupied by 38 N-glycoforms and 16 O-glycoforms, respectively. Furthermore, 77.31% of N-glycans were sialylated, and O-glycosylation was dominated by the sialyl-T antigen. These site-specific glycosylation patterns on human fibronectin can facilitate functional analyses of fibronectin and therapeutics development.

Electrochemical Impedance Spectroscopy Biosensor Enabling Kinetic Monitoring of Fucosyltransferase Activity

Monitoring glycosyltransferases on biosensors is of great interest for pathogen and cancer diagnostics. As a proof of concept, we here demonstrate the layer-by-layer immobilization of a multivalent neoglycoprotein (NGP) as a substrate for a bacterial fucosyltransferase (FucT) and the subsequent binding of the fucose-specific Aleuria aurantia lectin (AAL) on an electrochemical impedance spectroscopy (EIS) sensor. We report for the first time the binding kinetics of a glycosyltransferase in real-time. Highly stable EIS measurements are obtained by the modification of counter and reference electrodes with polypyrrole: polystyrene sulfonate (PPy:PSS). In detail, the N-acetyllactosamine (LacNAc)-carrying NGP was covalently immobilized on the gold working electrode and served as a substrate for the FucT-catalyzed reaction. The LacNAc epitopes were converted to Lewis^x (Le^x) and detected by AAL. AAL binding to the Le^x epitope was further confirmed in a lectin displacement and a competitive lectin binding inhibition experiment. We monitored the individual kinetic processes via EIS. The time constant for covalent immobilization of the NGP was 653 s. The FucT kinetics was the slowest process with a time constant of 1121 s. In contrast, a short time constant of 11.8 s was determined for the interaction of AAL with the modified NGPs. When this process was competed by 400 mM fucose, the binding was significantly slowed down, as indicated by a time constant of 978 s. The kinetics for the displacement of bound AAL by free fucose was observed with a time constant of 424 s. We conclude that this novel EIS biosensor and the applied workflow has the potential to detect FucT and other GT activities in general and further monitor protein-glycan interactions, which may be useful for the detection of pathogenic bacteria and cancer cells in future biomedical applications.

Cell-free systems for accelerating glycoprotein expression and biomanufacturing

Protein glycosylation, the enzymatic modification of amino acid sidechains with sugar moieties, plays critical roles in cellular function, human health, and biotechnology. However, studying and producing defined glycoproteins remains challenging. Cell-free glycoprotein synthesis systems, in which protein synthesis and glycosylation are performed in crude cell extracts, offer new approaches to address these challenges. Here, we review versatile, state-of-the-art systems for biomanufacturing glycoproteins in prokaryotic and eukaryotic cell-free systems with natural and synthetic N-linked glycosylation pathways. We discuss existing challenges and future opportunities in the use of cell-free systems for the design, manufacture, and study of glycoprotein biomedicines.

Linear triazole-linked pseudo oligogalactosides as scaffolds for galectin inhibitor development

Galectins play key roles in numerous biological processes. Their mode of action depends on their localization which can be extracellular, cytoplasmic, or nuclear and is partly mediated through interactions with β-galactose containing glycans. Galectins have emerged as novel therapeutic targets notably for the treatment of inflammatory disorders and cancers. This has stimulated the design of carbohydrate-based inhibitors targeting the carbohydrate recognition domains (CRDs) of the galectins. Pursuing this approach, we reasoned that linear oligogalactosides obtained by straightforward iterative click chemistry could mimic poly-lactosamine motifs expressed at eukaryote cell surfaces which the extracellular form of galectin-3, a prominent member of the galectin family, specifically recognizes. Affinities toward galectin-3 consistently increased with the length of the representative oligogalactosides but without reaching that of oligo-lactosamines. Elucidation of the X-ray crystal structures of the galectin-3 CRD in complex with a synthesized di- and tri-galactoside confirmed that the compounds bind within the carbohydrate-binding site. The atomic structures revealed that binding interactions mainly occur with the galactose moiety at the non-reducing end, primarily with subsites C and D of the CRD, differing from oligo-lactosamine which bind more consistently across the whole groove formed by the five subsites (A-E) of the galectin-3 CRD.

Enzymatic Cascades for Tailored 13C6 and 15N Enriched Human Milk Oligosaccharides

Several health benefits, associated with human milk oligosaccharides (HMOS), have been revealed in the last decades. Further progress, however, requires not only the establishment of a simple "routine" method for absolute quantification of complex HMOS mixtures but also the development of novel synthesis strategies to improve access to tailored HMOS. Here, we introduce a combination of salvage-like nucleotide sugar-producing enzyme cascades with Leloir-glycosyltransferases in a sequential pattern for the convenient tailoring of stable isotope-labeled HMOS. We demonstrate the assembly of [13C6]galactose into lacto-N- and lacto-N-neo-type HMOS structures up to octaoses. Further, we present the enzymatic production of UDP-[15N]GlcNAc and its application for the enzymatic synthesis of [13C6/15N]lacto-N-neo-tetraose for the first time. An exemplary application was selected-analysis of tetraose in complex biological mixtures-to show the potential of tailored stable isotope reference standards for the mass spectrometry-based quantification, using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) as a fast and straightforward method for absolute quantification of HMOS. Together with the newly available well-defined tailored isotopic HMOS, this can make a crucial contribution to prospective research aiming for a more profound understanding of HMOS structure-function relations.

Identifying Efficient Clostridium difficile Toxin A Binders with a Multivalent Neo-Glycoprotein Glycan Library

Clostridium difficile infections cause gastrointestinal disorders and can lead to life-threatening conditions. The symptoms can vary from severe diarrhea to the formation of pseudomembranous colitis and therefore trigger a need for new therapies. The initial step of disease is the binding of the bacterial enterotoxins toxin A and B to the cell surface of epithelial intestinal cells. Scavenging of the toxins is crucial to inhibit their fatal effect in the human body and circumvent the administration of antibiotics. Cell surface glycans are common as ligands for TcdA. Although crucial for carbohydrate-protein interactions, a multivalent presentation of glycans for binding has been hardly considered. Here, we establish a neo-glycoprotein-based glycan library to identify an effective multivalent glycan ligand for TcdA. It comprises 40 different glycan epitopes based on N-acetyllactosamine precursors. Nine structures exhibit strong binding of the receptor domain. Among them, the Lewis^y-Lewis^x-epitope shows the best performance for binding both the receptor domain and the holotoxin. Therefore, the glycan was synthesized de novo and coupled to BSA as a scaffold for multivalent presentation. The corresponding neo-glycoprotein facilitates the proper scavenging of TcdA in vitro and effectively protects HT29 cells from TcdA-induced cell damage.

The β-N-Acetylhexosaminidase in the Synthesis of Bioactive Glycans: Protein and Reaction Engineering

N-Acetylhexosamine oligosaccharides terminated with GalNAc act as selective ligands of galectin-3, a biomedically important human lectin. Their synthesis can be accomplished by β-N-acetylhexosaminidases (EC 3.2.1.52). Advantageously, these enzymes tolerate the presence of functional groups in the substrate molecule, such as the thiourea linker useful for covalent conjugation of glycans to a multivalent carrier, affording glyconjugates. β-N-Acetylhexosaminidases exhibit activity towards both N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc) moieties. A point mutation of active-site amino acid Tyr into other amino acid residues, especially Phe, His, and Asn, has previously been shown to strongly suppress the hydrolytic activity of β-N-acetylhexosaminidases, creating enzymatic synthetic engines. In the present work, we demonstrate that Tyr470 is an important mutation hotspot for altering the ratio of GlcNAcase/GalNAcase activity, resulting in mutant enzymes with varying affinity to GlcNAc/GalNAc substrates. The enzyme selectivity may additionally be manipulated by altering the reaction medium upon changing pH or adding selected organic co-solvents. As a result, we are able to fine-tune the β-N-acetylhexosaminidase affinity and selectivity, resulting in a high-yield production of the functionalized GalNAcβ4GlcNAc disaccharide, a selective ligand of galectin-3.

Machine-Driven Enzymatic Oligosaccharide Synthesis by Using a Peptide Synthesizer

For decades, researchers have endeavored to develop a general automated system to synthesize oligosaccharides that is comparable to the preparation of oligonucleotides and oligopeptides by commercially available machines. Inspired by the success of automated oligosaccharide synthesis through chemical glycosylation, a fully automated system is reported for oligosaccharides synthesis through enzymatic glycosylation in aqueous solution. The designed system is based on the use of a thermosensitive polymer and a commercially available peptide synthesizer. This study represents a proof-of-concept demonstration that the enzymatic synthesis of oligosaccharides can be achieved in an automated manner using a commercially available peptide synthesizer.

A Compartmented Flow Microreactor System for Automated Optimization of Bioprocesses Applying Immobilized Enzymes

In the course of their development, industrial biocatalysis processes have to be optimized in small-scale, e. g., within microfluidic bioreactors. Recently, we introduced a novel microfluidic reactor device, which can handle defined reaction compartments of up to 250 μL in combination with magnetic micro carriers. By transferring the magnetic carriers between subsequent compartments of differing compositions, small scale synthesis, and bioanalytical assays can be conducted. In the current work, this device is modified and extended to broaden its application range to the screening and optimization of bioprocesses applying immobilized enzymes. Besides scaling the maximum compartment volume up to 3 mL, a temperature control module, as well as a focused infrared spot were integrated. By adjusting the pump rate, compartment volumes can be accurately dosed with an error <5% and adjusted to the requested temperature within less than a minute. For demonstration of bioprocess parameter optimization within such compartments, the influence of pH, temperature, substrate concentration, and enzyme carrier loading was automatically screened for the case of transferring UDP-Gal onto N-acetylglucosamine linked to a tert-butyloxycarbonyl protected amino group using immobilized β1,4-galactosyltransferase-1. In addition, multiple recycling of the enzyme carriers and the use of increased compartment volumes also allows the simple production of preparative amounts of reaction products.

Enzymatic Cascade Synthesis Provides Novel Linear Human Milk Oligosaccharides as Reference Standards for xCGE-LIF Based High-Throughput Analysis

A rising amount of known health benefits leads to an increased attention of science and nutrient industry to human milk oligosaccharides (HMOS). The unique diversity of HMOS includes several rare, complex, and high molecular weight structures. Therefore, identification and elucidation of complex structures, which may occur only in traces, poses a daunting analytical challenge, further complicated by the limited access to suitable standards. Regarding this, inherent diversity of HMOS and their structural complexity make them difficult to synthesize. The use of recombinant Leloir-glycosyltransferases offers a common strategy to overcome the latter issues. In this study, linear long-chained Lacto-N-biose-type (LNT) and Lacto-N-neo-type (LNnT) HMOS are tailored far beyond the known naturally occurring length. Thereby novel well-defined reference standards for screening HMOS composition by high performance and high throughput analytics are provided. It is shown here for the first time the synthesis of LNT oligomers up to 26 and LNnT oligomers up to 30 sugar units in a semi-sequential one-pot synthesis as analyzed by high performance multiplexed capillary gel electrophoresis with laser-induced fluorescence detection (xCGE-LIF). While being a high-throughput method, xCGE-LIF can also handle long chained linkage isomers of challenging similarity, some of them even present only in trace amounts.

Sugared biomaterial binding lectins: achievements and perspectives

Lectins, a distinct group of glycan-binding proteins, play a prominent role in the immune system ranging from pathogen recognition and tuning of inflammation to cell adhesion or cellular signalling. The possibilities of their detailed study expanded along with the rapid development of biomaterials in the last decade. The immense knowledge of all aspects of glycan-lectin interactions both in vitro and in vivo may be efficiently used in bioimaging, targeted drug delivery, diagnostic and analytic biological methods. Practically applicable examples comprise photoluminescence and optical biosensors, ingenious three-dimensional carbohydrate microarrays for high-throughput screening, matrices for magnetic resonance imaging, targeted hyperthermal treatment of cancer tissues, selective inhibitors of bacterial toxins and pathogen-recognising lectin receptors, and many others. This review aims to present an up-to-date systematic overview of glycan-decorated biomaterials promising for interactions with lectins, especially those applicable in biology, biotechnology or medicine. The lectins of interest include galectin-1, -3 and -7 participating in tumour progression, bacterial lectins from Pseudomonas aeruginosa (PA-IL), E. coli (Fim-H) and Clostridium botulinum (HA33) or DC-SIGN, receptors of macrophages and dendritic cells. The spectrum of lectin-binding biomaterials covered herein ranges from glycosylated organic structures, calixarene and fullerene cores over glycopeptides and glycoproteins, functionalised carbohydrate scaffolds of cyclodextrin or chitin to self-assembling glycopolymer clusters, gels, micelles and liposomes. Glyconanoparticles, glycan arrays, and other biomaterials with a solid core are described in detail, including inorganic matrices like hydroxyapatite or stainless steel for bioimplants.

Poly-N-Acetyllactosamine Neo-Glycoproteins as Nanomolar Ligands of Human Galectin-3: Binding Kinetics and Modeling

Galectin-3 (Gal-3) is recognized as a prognostic marker in several cancer types. Its involvement in tumor development and proliferation makes this lectin a promising target for early cancer diagnosis and anti-cancer therapies. Gal-3 recognizes poly-N-acetyllactosamine (LacNAc)-based carbohydrate motifs of glycoproteins and glycolipids with a high specificity for internal LacNAc epitopes. This study analyzes the mode and kinetics of binding of Gal-3 to a series of multivalent neo-glycoproteins presenting complex poly-LacNAc-based oligosaccharide ligands on a scaffold of bovine serum albumin. These neo-glycoproteins rank among the strongest Gal-3 ligands reported, with K_d reaching sub-nanomolar values as determined by surface plasmon resonance. Significant differences in the binding kinetics were observed within the ligand series, showing the tetrasaccharide capped with N,N'-diacetyllactosamine (LacdiNAc) as the strongest ligand of Gal-3 in this study. A molecular model of the Gal-3 carbohydrate recognition domain with docked oligosaccharide ligands is presented that shows the relations in the binding site at the molecular level. The neo-glycoproteins presented herein may be applied for selective recognition of Gal-3 both on the cell surface and in blood serum.

A robust protocol for directed aryl sulfotransferase evolution toward the carbohydrate building block GlcNAc

Bacterial aryl sulfotransferases (AST) utilize p-nitrophenylsulfate (pNPS) as a phenolic donor to sulfurylate typically a phenolic acceptor. Interest in aryl sulfotransferases is growing because of their broad variety of acceptors and cost-effective sulfuryl-donors. For instance, aryl sulfotransferase A (ASTA) from Desulfitobacterium hafniense was recently reported to sulfurylate d-glucose. In this study, a directed evolution protocol was developed and validated for aryl sulfotransferase B (ASTB). Thereby the well-known pNPS quantification system was advanced to operate efficiently as a continuous screening system in 96-well MTP format with a true coefficient of variation of 14.3%. A random mutagenesis library (SeSaM library) of ASTB was screened (1,760 clones) to improve sulfurylation of the carbohydrate building block N-acetylglucosamine (GlcNAc). The beneficial variant ASTB-V1 (Val579Asp) showed an up to 3.4-fold increased specific activity toward GlcNAc when compared to ASTB-WT. HPLC- and MS-analysis confirmed ASTB-V1's increased GlcNAc monosulfurylation (2.4-fold increased product formation) representing the validation of the first successful directed evolution round of an AST for a saccharide substrate.

Enzymatic Synthesis of N-Acetyllactosamine (LacNAc) Type 1 Oligomers and Characterization as Multivalent Galectin Ligands

Repeats of the disaccharide unit N-acetyllactosamine (LacNAc) occur as type 1 (Galβ1, 3GlcNAc) and type 2 (Galβ1, 4GlcNAc) glycosylation motifs on glycoproteins and glycolipids. The LacNAc motif acts as binding ligand for lectins and is involved in many biological recognition events. To the best of our knowledge, we present, for the first time, the synthesis of LacNAc type 1 oligomers using recombinant β1,3-galactosyltransferase from Escherichia coli and β1,3-N-acetylglucosaminyltranferase from Helicobacter pylori. Tetrasaccharide glycans presenting LacNAc type 1 repeats or LacNAc type 1 at the reducing or non-reducing end, respectively, were conjugated to bovine serum albumin as a protein scaffold by squarate linker chemistry. The resulting multivalent LacNAc type 1 presenting neo-glycoproteins were further studied for specific binding of the tumor-associated human galectin 3 (Gal-3) and its truncated counterpart Gal-3∆ in an enzyme-linked lectin assay (ELLA). We observed a significantly increased affinity of Gal-3∆ towards the multivalent neo-glycoprotein presenting LacNAc type 1 repeating units. This is the first evidence for differences in glycan selectivity of Gal-3∆ and Gal-3 and may be further utilized for tracing Gal-3∆ during tumor progression and therapy.

Identification of the binding roles of terminal and internal glycan epitopes using enzymatically synthesized N-glycans containing tandem epitopes

Glycans play diverse roles in a wide range of biological processes. Research on glycan-binding events is essential for learning their biological and pathological functions. However, the functions of terminal and internal glycan epitopes exhibited during binding with glycan-binding proteins (GBPs) and/or viruses need to be further identified. Therefore, a focused library of 36 biantennary asparagine (Asn)-linked glycans with some presenting tandem glycan epitopes was synthesized via a combined Core Isolation/Enzymatic Extension (CIEE) and one-pot multienzyme (OPME) synthetic strategy. These N-glycans include those containing a terminal sialyl N-acetyllactosamine (LacNAc), sialyl Lewis x (sLex) and Siaα2-8-Siaα2-3/6-R structures with N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc) sialic acid form, LacNAc, Lewis x (Lex), α-Gal, and Galα1-3-Lex; and tandem epitopes including α-Gal, Lex, Galα1-3-Lex, LacNAc, and sialyl LacNAc, presented with an internal sialyl LacNAc or 1-2 repeats of an internal LacNAc or Lex component. They were synthesized in milligram-scale, purified to over 98% purity, and used to prepare a glycan microarray. Binding studies using selected plant lectins, antibodies, and viruses demonstrated, for the first time, that when interpreting the binding between glycans and GBPs/viruses, not only the structure of the terminal glycan epitopes, but also the internal epitopes and/or modifications of terminal epitopes needs to be taken into account.

Sequential One-Pot Multienzyme Chemoenzymatic Synthesis of Glycosphingolipid Glycans

Glycosphingolipids are a diverse family of biologically important glycolipids. In addition to variations on the lipid component, more than 300 glycosphingolipid glycans have been characterized. These glycans are directly involved in various molecular recognition events. Several naturally occurring sialic acid forms have been found in sialic acid-containing glycosphingolipids, namely gangliosides. However, ganglioside glycans containing less common sialic acid forms are currently not available. Herein, highly effective one-pot multienzyme (OPME) systems are used in sequential for high-yield and cost-effective production of glycosphingolipid glycans, including those containing different sialic acid forms such as N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), 2-keto-3-deoxy-d-glycero-d-galacto-nononic acid (Kdn), and 8-O-methyl-N-acetylneuraminic acid (Neu5Ac8OMe). A library of 64 structurally distinct glycosphingolipid glycans belonging to ganglio-series, lacto-/neolacto-series, and globo-/isoglobo-series glycosphingolipid glycans is constructed. These glycans are essential standards and invaluable probes for bioassays and biomedical studies.

Multiplexed Capillary Electrophoresis as Analytical Tool for Fast Optimization of Multi-Enzyme Cascade Reactions - Synthesis of Nucleotide Sugars: Dedicated to Prof. Dr. Vladimir Křen on the occasion of his 60th birthday

Nucleotide sugars are considered as bottleneck and expensive substrates for enzymatic glycan synthesis using Leloir-glycosyltransferases. Synthesis from cheap substrates such as monosaccharides is accomplished by multi-enzyme cascade reactions. Optimization of product yields in such enzyme modules is dependent on the interplay of multiple parameters of the individual enzymes and governed by a considerable time effort when convential analytic methods like capillary electrophoresis (CE) or HPLC are applied. We here demonstrate for the first time multiplexed CE (MP-CE) as fast analytical tool for the optimization of nucleotide sugar synthesis with multi-enzyme cascade reactions. We introduce a universal separation method for nucleotides and nucleotide sugars enabling us to analyze the composition of six different enzyme modules in a high-throughput format. Optimization of parameters (T, pH, inhibitors, kinetics, cofactors and enzyme amount) employing MP-CE analysis is demonstrated for enzyme modules for the synthesis of UDP-α-D-glucuronic acid (UDP-GlcA) and UDP-α-D-galactose (UDP-Gal). In this way we achieve high space-time-yields: 1.8 g/L⋆h for UDP-GlcA and 17 g/L⋆h for UDP-Gal. The presented MP-CE methodology has the impact to be used as general analytical tool for fast optimization of multi-enzyme cascade reactions.

Donor substrate promiscuity of bacterial β1-3-N-acetylglucosaminyltransferases and acceptor substrate flexibility of β1-4-galactosyltransferases

β1-3-N-Acetylglucosaminyltransferases (β3GlcNAcTs) and β1-4-galactosyltransferases (β4GalTs) have been broadly used in enzymatic synthesis of N-acetyllactosamine (LacNAc)-containing oligosaccharides and glycoconjugates including poly-LacNAc, and lacto-N-neotetraose (LNnT) found in the milk of human and other mammals. In order to explore oligosaccharides and derivatives that can be synthesized by the combination of β3GlcNAcTs and β4GalTs, donor substrate specificity studies of two bacterial β3GlcNAcTs from Helicobacter pylori (Hpβ3GlcNAcT) and Neisseria meningitidis (NmLgtA), respectively, using a library of 39 sugar nucleotides were carried out. The two β3GlcNAcTs have complementary donor substrate promiscuity and 13 different trisaccharides were produced. They were used to investigate the acceptor substrate specificities of three β4GalTs from Neisseria meningitidis (NmLgtB), Helicobacter pylori (Hpβ4GalT), and bovine (Bβ4GalT), respectively. Ten of the 13 trisaccharides were shown to be tolerable acceptors for at least one of these β4GalTs. The application of NmLgtA in one-pot multienzyme (OPME) synthesis of two trisaccharides including GalNAcβ1-3Galβ1-4GlcβProN3 and Galβ1-3Galβ1-4Glc was demonstrated. The study provides important information for using these glycosyltransferases as powerful catalysts in enzymatic and chemoenzymatic syntheses of oligosaccharides and derivatives which can be useful probes and reagents.

Microwave-Assisted Synthesis of Glycoconjugates by Transgalactosylation with Recombinant Thermostable β-Glycosidase from Pyrococcus

The potential of the hyperthermophilic β-glycosidase from Pyrococcus woesei (DSM 3773) for the synthesis of glycosides under microwave irradiation (MWI) at low temperatures was investigated. Transgalactosylation reactions with β-N-acetyl-d-glucosamine as acceptor substrate (GlcNAc-linker-tBoc) under thermal heating (TH, 85 °C) and under MWI at 100 and 300 W resulted in the formation of (Galβ(1,4)GlcNAc-linker-tBoc) as the main product in all reactions. Most importantly, MWI at temperatures far below the temperature optimum of the hyperthermophilic glycosidase led to higher product yields with only minor amounts of side products β(1,6-linked disaccharide and trisaccharides). At high acceptor concentrations (50 mM), transgalactosylation reactions under MWI at 300 W gave similar product yields when compared to TH at 85 °C. In summary, we demonstrate that MWI is useful as a novel experimental set-up for the synthesis of defined galacto-oligosaccharides. In conclusion, glycosylation reactions under MWI at low temperatures have the potential as a general strategy for regioselective glycosylation reactions of hyperthermophilic glycosidases using heat-labile acceptor or donor substrates.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

Applying Acylated Fucose Analogues to Metabolic Glycoengineering

Manipulations of cell surface glycosylation or glycan decoration of selected proteins hold immense potential for exploring structure-activity relations or increasing glycoprotein quality. Metabolic glycoengineering describes the strategy where exogenously supplied sugar analogues intercept biosynthetic pathways and are incorporated into glycoconjugates. Low membrane permeability, which so far limited the large-scale adaption of this technology, can be addressed by the introduction of acylated monosaccharides. In this work, we investigated tetra-O-acetylated, -propanoylated and -polyethylene glycol (PEG)ylated fucoses. Concentrations of up to 500 µM had no substantial effects on viability and recombinant glycoprotein production of human embryonic kidney (HEK)-293T cells. Analogues applied to an engineered Chinese hamster ovary (CHO) cell line with blocked fucose de novo synthesis revealed an increase in cell surface and recombinant antibody fucosylation as proved by lectin blotting, mass spectrometry and monosaccharide analysis. Significant fucose incorporation was achieved for tetra-O-acetylated and -propanoylated fucoses already at 20 µM. Sequential fucosylation of the recombinant glycoprotein, achieved by the application of increasing concentrations of PEGylated fucose up to 70 µM, correlated with a reduced antibody’s binding activity in a Fcγ receptor IIIa (FcγRIIIa) binding assay. Our results provide further insights to modulate fucosylation by exploiting the salvage pathway via metabolic glycoengineering.

Galectin Binding to Neo-Glycoproteins: LacDiNAc Conjugated BSA as Ligand for Human Galectin-3

Carbohydrate-lectin interactions are relatively weak. As they play an important role in biological recognition processes, multivalent glycan ligands are designed to enhance binding affinity and inhibitory potency. We here report on novel neo-glycoproteins based on bovine serum albumin as scaffold for multivalent presentation of ligands for galectins. We prepared two kinds of tetrasaccharides (N-acetyllactosamine and N,N-diacetyllactosamine terminated) by multi-step chemo-enzymatic synthesis utilizing recombinant glycosyltransferases. Subsequent conjugation of these glycans to lysine groups of bovine serum albumin via squaric acid diethyl ester yielded a set of 22 different neo-glycoproteins with tuned ligand density. The neo-glycoproteins were analyzed by biochemical and chromatographic methods proving various modification degrees. The neo-glycoproteins were used for binding and inhibition studies with human galectin-3 showing high affinity. Binding strength and inhibition potency are closely related to modification density and show binding enhancement by multivalent ligand presentation. At galectin-3 concentrations comparable to serum levels of cancer patients, we detect the highest avidities. Selectivity of N,N-diacetyllactosamine terminated structures towards galectin-3 in comparison to galectin-1 is demonstrated. Moreover, we also see strong inhibitory potency of our scaffolds towards galectin-3 binding. These novel neo-glycoproteins may therefore serve as selective and strong galectin-3 ligands in cancer related biomedical research.

Self-assembled glycopeptide nanofibers as modulators of galectin-1 bioactivity

Galectins are carbohydrate-binding proteins that act as extracellular signaling molecules in various normal and pathological processes. Galectin bioactivity is mediated by specific non-covalent interactions with cell-surface and extracellular matrix (ECM) glycoproteins, which can enhance or inhibit signaling events that influence various cellular behaviors, including adhesion, proliferation, differentiation, and apoptosis. Here, we developed a materials approach to modulate galectin bioactivity by mimicking natural galectin-glycoprotein interactions. Specifically, we created a variant of a peptide that self-assembles into β-sheet nanofibers under aqueous conditions, QQKFQFQFEQQ (Q11), which has an asparagine residue modified with the monosaccharide N-acetylglucosamine (GlcNAc) at its N-terminus (GlcNAc-Q11). GlcNAc-Q11 self-assembled into β-sheet nanofibers under similar conditions as Q11. Nanofibrillar GlcNAc moieties were efficiently converted to the galectin-binding disaccharide N-acetyllactosamine (LacNAc) via the enzyme β-1,4-galactosyltransferase and the sugar donor UDP-galactose, while retaining β-sheet structure and nanofiber morphology. LacNAc-Q11 nanofibers bound galectin-1 and -3 in a LacNAc concentration-dependent manner, although nanofibers bound galectin-1 with higher affinity than galectin-3. In contrast, galectin-1 bound weakly to GlcNAc-Q11 nanofibers, while no galectin-3 binding to these nanofibers was observed. Galectin-1 binding to LacNAc-Q11 nanofibers was specific because it could be inhibited by excess soluble β-lactose, a galectin-binding carbohydrate. LacNAc-Q11 nanofibers inhibited galectin-1-mediated apoptosis of Jurkat T cells in a LacNAc concentration-dependent manner, but were unable to inhibit galectin-3 activity, consistent with galectin-binding affinity of the nanofibers. We envision that glycopeptide nanofibers capable of modulating galectin-1 bioactivity will be broadly useful as biomaterials for various medical applications, including cancer therapeutics, immunotherapy, tissue regeneration, and viral prophylaxis.

A multifunctional anomeric linker for the chemoenzymatic synthesis of complex oligosaccharides

A new anomeric linker has been developed that facilitates the purification of glycans prepared by chemoenzymatic approaches and can readily give compounds that are appropriately modified for microarray development or glycan derivatives with a free reducing end that are needed as standards for the development of analytical protocols.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Rational design of a glycosynthase by the crystal structure of β-galactosidase from Bacillus circulans (BgaC) and its use for the synthesis of N-acetyllactosamine type 1 glycan structures

The crystal structure of β-galactosidase from Bacillus circulans (BgaC) was determined at 1.8Å resolution. The overall structure of BgaC consists of three distinct domains, which are the catalytic domain with a TIM-barrel structure and two all-β domains (ABDs). The main-chain fold and steric configurations of the acidic and aromatic residues at the active site were very similar to those of Streptococcus pneumoniae β(1,3)-galactosidase BgaC in complex with galactose. The structure of BgaC was used for the rational design of a glycosynthase. BgaC belongs to the glycoside hydrolase family 35. The essential nucleophilic amino acid residue has been identified as glutamic acid at position 233 by site-directed mutagenesis. Construction of the active site mutant BgaC-Glu233Gly gave rise to a galactosynthase transferring the sugar moiety from α-d-galactopyranosyl fluoride (αGalF) to different β-linked N-acetylglucosamine acceptor substrates in good yield (40-90%) with a remarkably stable product formation. Enzymatic syntheses with BgaC-Glu233Gly afforded the stereo- and regioselective synthesis of β1-3-linked key galactosides like galacto-N-biose or lacto-N-biose.

A general strategy for the chemoenzymatic synthesis of asymmetrically branched N-glycans

A systematic, efficient means of producing diverse libraries of asymmetrically branched N-glycans is needed to investigate the specificities and biology of glycan-binding proteins. To that end, we describe a core pentasaccharide that at potential branching positions is modified by orthogonal protecting groups to allow selective attachment of specific saccharide moieties by chemical glycosylation. The appendages were selected so that the antenna of the resulting deprotected compounds could be selectively extended by glycosyltransferases to give libraries of asymmetrical multi-antennary glycans. The power of the methodology was demonstrated by the preparation of a series of complex oligosaccharides that were printed as microarrays and screened for binding to lectins and influenza-virus hemagglutinins, which showed that recognition is modulated by presentation of minimal epitopes in the context of complex N-glycans.

Glycoengineering of host mimicking type-2 LacNAc polymers and Lewis X antigens on bacterial cell surfaces

Bacterial carbohydrate structures play a central role in mediating a variety of host-pathogen interactions. Glycans can either elicit protective immune response or lead to escape of immune surveillance by mimicking host structures. Lipopolysaccharide (LPS), a major component on the surface of Gram-negative bacteria, is composed of a lipid A-core and the O-antigen polysaccharide. Pathogens like Neisseria meningitidis expose a lipooligosaccharide (LOS), which outermost glycans mimick mammalian epitopes to avoid immune recognition. Lewis X (Galβ1-4(Fucα1-3)GlcNAc) antigens of Helicobacter pylori or of the helminth Schistosoma mansoni modulate the immune response by interacting with receptors on human dendritic cells. In a glycoengineering approach we generate human carbohydrate structures on the surface of recombinant Gram-negative bacteria, such as Escherichia coli and Salmonella enterica sv. Typhimurium that lack O-antigen. A ubiquitous building block in mammalian N-linked protein glycans is Galβ1-4GlcNAc, referred to as a type-2 N-acetyllactosamine, LacNAc, sequence. Strains displaying polymeric LacNAc were generated by introducing a combination of glycosyltransferases that act on modified lipid A-cores, resulting in efficient expression of the carbohydrate epitope on bacterial cell surfaces. The poly-LacNAc scaffold was used as an acceptor for fucosylation leading to polymers of Lewis X antigens. We analysed the distribution of the carbohydrate epitopes by FACS, microscopy and ELISA and confirmed engineered LOS containing LacNAc and Lewis X repeats by MALDI-TOF and NMR analysis. Glycoengineered LOS induced pro-inflammatory response in murine dendritic cells. These bacterial strains can thus serve as tools to analyse the role of defined carbohydrate structures in different biological processes.

Helicobacter pylori β1,3-N-acetylglucosaminyltransferase for versatile synthesis of type 1 and type 2 poly-LacNAcs on N-linked, O-linked and I-antigen glycans

Poly-N-acetyllactosamine extensions on N- and O-linked glycans are increasingly recognized as biologically important structural features, but access to these structures has not been widely available. Here, we report a detailed substrate specificity and catalytic efficiency of the bacterial β3-N-acetylglucosaminyltransferase (β3GlcNAcT) from Helicobacter pylori that can be adapted to the synthesis of a rich diversity of glycans with poly-LacNAc extensions. This glycosyltransferase has surprisingly broad acceptor specificity toward type-1, -2, -3 and -4 galactoside motifs on both linear and branched glycans, found commonly on N-linked, O-linked and I-antigen glycans. This finding enables the production of complex ligands for glycan-binding studies. Although the enzyme shows preferential activity for type 2 (Galβ1-4GlcNAc) acceptors, it is capable of transferring N-acetylglucosamine (GlcNAc) in β1-3 linkage to type-1 (Galβ1-3GlcNAc) or type-3/4 (Galβ1-3GalNAcα/β) sequences. Thus, by alternating the use of the H. pylori β3GlcNAcT with galactosyltransferases that make the β1-4 or β1-3 linkages, various N-linked, O-linked and I-antigen acceptors could be elongated with type-2 and type-1 LacNAc repeats. Finally, one-pot incubation of di-LacNAc biantennary N-glycopeptide with the β3GlcNAcT and GalT-1 in the presence of uridine diphosphate (UDP)-GlcNAc and UDP-Gal, yielded products with 15 additional LacNAc units on the precursor, which was seen as a series of sequential ion peaks representing alternative additions of GlcNAc and Gal residues, on matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. Overall, our data demonstrate a broader substrate specificity for the H. pylori β3GlcNAcT than previously recognized and demonstrate its ability as a potent resource for preparative chemo-enzymatic synthesis of complex glycans.

Synthesis of α-galactosyl epitopes by metabolically engineered Escherichia coli

The α-Gal epitope is a carbohydrate structure, Galα-3Galβ-4GlcNAc-R, expressed on glycoconjuguates in many mammals, but not in humans. Species that do not express this epitope have present in their serum large amounts of natural anti-Gal antibodies, which contribute to organ hyperacute rejection during xenotransplantation. We first describe the efficient conversion of lactose into isoglobotriaose (Galα-3Galβ-4Glc) using high cell density cultures of a genetically engineered Escherichia coli strain expressing the bovine gene for α-1,3-galactosyltransferase. Attempts to produce the Galili pentasaccharide (Galα-3Galβ-4GlcNAcβ-3Galβ-4Glc) by additionally expressing the Neisseria meningitis lgtA gene for β-1,3-N-acetylglucosaminyltransferase and the Helicobacter pylori gene for β-1,4-galactosyltransferase were unsuccessful and led to the formation of a series of long chain oligosaccharides formed by the repeated addition of the trisaccharide motif [Galβ-4GlcNAcβ-3Galα-3] onto a lacto-N-neotetraose primer. The replacement of LgtA by a more specific β-1,3-N-acetylglucosaminyltransferase from H. pylori, which was unable to glycosylate α-galactosides, prevented the formation of these unwanted compounds and allowed the successful formation of the Galili pentasaccharide and longer α-Gal epitopes.

Recognition of sialylated poly-N-acetyllactosamine chains on N- and O-linked glycans by human and avian influenza A virus hemagglutinins

Human influenza viruses are proposed to recognize sialic acids (pink diamonds) on glycans extended with poly-LacNAc chains (LacNAc=(yellow circle+blue square)). N- and O-linked glycans were extended with different poly-LacNAc chains with α2-3- and α2-6-linked sialic acids recognized by human and avian influenza viruses, respectively. The specificity of recombinant hemagglutinins (receptors in green) was investigated by using glycan microarray technology.

Chemo-enzymatic modification of poly-N-acetyllactosamine (LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) based on galactose oxidase treatment

The importance of glycans in biological systems is highlighted by their various functions in physiological and pathological processes. Many glycan epitopes on glycoproteins and glycolipids are based on N-acetyllactosamine units (LacNAc; Galβ1,4GlcNAc) and often present on extended poly-LacNAc glycans ([Galβ1,4GlcNAc](n)). Poly-LacNAc itself has been identified as a binding motif of galectins, an important class of lectins with functions in immune response and tumorigenesis. Therefore, the synthesis of natural and modified poly-LacNAc glycans is of specific interest for binding studies with galectins as well as for studies of their possible therapeutic applications. We present the oxidation by galactose oxidase and subsequent chemical or enzymatic modification of terminal galactose and N-acetylgalactosamine residues of poly-N-acetyllactosamine (poly-LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) by galactose oxidase. Product formation starting from different poly-LacNAc oligomers was characterised and optimised regarding formation of the C6-aldo product. Further modification of the aldehyde containing glycans, either by chemical conversion or enzymatic elongation, was established. Base-catalysed β-elimination, coupling of biotin-hydrazide with subsequent reduction to the corresponding hydrazine linkage, and coupling by reductive amination to an amino-functionalised poly-LacNAc oligomer were performed and the products characterised by LC-MS and NMR analysis. Remarkably, elongation of terminally oxidised poly-LacNAc glycans by β3GlcNAc- and β4Gal-transferase was also successful. In this way, a set of novel, modified poly-LacNAc oligomers containing terminally and/or internally modified galactose residues were obtained, which can be used for binding studies and various other applications.