Lactose-Functionalized Carbosilane Glycodendrimers Are Highly Potent Multivalent Ligands for Galectin-9 Binding: Increased Glycan Affinity to Galectins Correlates with Aggregation Behavior

Galectins, the glycan binding proteins, and their respective carbohydrate ligands represent a unique fundamental regulatory network modulating a plethora of biological processes. The advances in galectin-targeted therapy must be based on a deep understanding of the mechanism of ligand-protein recognition. Carbosilane dendrimers, the well-defined and finely tunable nanoscaffolds with low toxicity, are promising for multivalent carbohydrate ligand presentation to target galectin receptors. The study discloses a synthetic method for two types of lactose-functionalized carbosilane glycodendrimers (Lac-CS-DDMs). Furthermore, we report their outstanding, dendritic effect-driven affinity to tandem-type galectins, especially Gal-9. In the enzyme-linked immunosorbent assay, the affinity of the third-generation multivalent dendritic ligand bearing 32 lactose units to Gal-9 reached nanomolar values (IC50 = 970 nM), being a 1400-fold more effective inhibitor than monovalent lactose for this protein. This demonstrates a game-changing impact of multivalent presentation on the inhibitory effect of a ligand as simple as lactose. Moreover, using DLS hydrodynamic diameter measurements, we correlated the increased affinity of the glycodendrimer ligands to Gal-3 and Gal-8 but especially to Gal-9 with the formation of relatively uniform and stable galectin/Lac-CS-DDM aggregates.

Multivalent glycans for biological and biomedical applications

Recognition of glycans by proteins plays a crucial role in a variety of physiological processes in cells and living organisms. In addition, interactions of glycans with proteins are involved in the development of diverse diseases, such as pathogen infection, inflammation and tumor metastasis. It is well-known that multivalent glycans bind to proteins much more strongly than do their monomeric counterparts. Owing to this property, numerous multivalent glycans have been utilized to elucidate glycan-mediated biological processes and to discover glycan-based biomedical agents. In this review, we discuss recent advances (2014-2020) made in the development and biological and biomedical applications of synthetic multivalent glycans, including neoglycopeptides, neoglycoproteins, glycodendrimers, glycopolymers, glyconanoparticles and glycoliposomes. We hope this review assists researchers in the design and development of novel multivalent glycans with predictable activities.

Synthesis of N-Acetyllactosamine and N-Acetyllactosamine-Based Bioactives

N-Acetyllactosamine (LacNAc) or more specifically β-d-galactopyranosyl-1,4-N-acetyl-d-glucosamine is a unique acyl-amino sugar and a key structural unit in human milk oligosaccharides, an antigen component of many glycoproteins, and an antiviral active component for the development of effective drugs against viruses. LacNAc is useful itself and as a basic building block for producing various bioactive oligosaccharides, notably because this synthesis may be used to add value to dairy lactose. Despite a significant amount of information in the literature on the benefits, structures, and types of different LacNAc-derived oligosaccharides, knowledge about their effective synthesis for large-scale production is still in its infancy. This work provides a comprehensive analysis of existing production strategies for LacNAc and important LacNAc-based structures, including sialylated LacNAc as well as poly- and oligo-LacNAc. We conclude that direct extraction from milk is too complex, while chemical synthesis is also impractical at an industrial scale. Microbial routes have application when multiple step reactions are needed, but the major route to large-scale biochemical production will likely lie with enzymatic routes, particularly those using β-galactosidases (for LacNAc synthesis), sialidases (for sialylated LacNAc synthesis), and β-N-acetylhexosaminidases (for oligo-LacNAc synthesis). Glycosyltransferases, especially for the biosynthesis of extended complex LacNAc structures, could also play a major role in the future. In these cases, immobilization of the enzyme can increase stability and reduce cost. Processing parameters, such as substrate concentration and purity, acceptor/donor ratio, water activity, and temperature, can affect product selectivity and yield. More work is needed to optimize these reaction parameters and in the development of robust, thermally stable enzymes to facilitate commercial production of these important bioactive substances.

Synthesis of Glycodendrimers with Antiviral and Antibacterial Activity

Glycodendrimers are an important class of synthetic macromolecules that can be used to mimic many structural and functional features of cell-surface glycoconjugates. Their carbohydrate moieties perform key important functions in bacterial and viral infections, often regulated by carbohydrate-protein interactions. Several studies have shown that the molecular structure, valency and spatial organisation of carbohydrate epitopes in glycoconjugates are key factors in the specificity and avidity of carbohydrate-protein interactions. Choosing the right glycodendrimers almost always helps to interfere with such interactions and blocks bacterial or viral adhesion and entry into host cells as an effective strategy to inhibit bacterial or viral infections. Herein, the state of the art in the design and synthesis of glycodendrimers employed for the development of anti-adhesion therapy against bacterial and viral infections is described.

Protecting-group-free S-glycosylation towards thioglycosides and thioglycopeptides in water

A facile and green S-glycosylation method has been developed featuring protecting-group-free and proceeding-in-water like enzymatic synthesis. Glycosylation of fluoride donors with thiol sugar acceptors using Ca(OH)2 as a promoter afforded various thioglycosides in good yields with exclusive stereoselectivity. This method also enabled the successful production of S-linked oligosaccharides and S-linked glycopeptides.

Optimization of the Microwave Assisted Glycosylamines Synthesis Based on a Statistical Design of Experiments Approach

Glycans carry a vast range of functions in nature. Utilizing their properties and functions in form of polymers, coatings or glycan derivatives for various applications makes the synthesis of modified glycans crucial. Since amines are easy to modify for subsequent reactions, we investigated regioselective amination conditions of different saccharides. Amination reactions were performed according to Kochetkov and Likhoshertov and accelerated by microwave irradiation. We optimized the synthesis of glycosylamines for N-acetyl-d-galactosamine, d-lactose, d-glucuronic acid and l-(-)-fucose using the design of experiments (DoE) approach. DoE enables efficient optimization with limited number of experimental data. A DoE software generated a set of experiments where reaction temperature, concentration of carbohydrate, nature of aminating agent and solvent were investigated. We found that the synthesis of glycosylamines significantly depends on the nature of the carbohydrate and on the reaction temperature. There is strong indication that high temperatures are favored for the amination reaction.

Biocatalysis - Key enabling tools from biocatalytic one-step and multi-step reactions to biocatalytic total synthesis

In the area of human-made innovations to improve the quality of life, biocatalysis has already had a great impact and contributed enormously to a growing number of catalytic transformations aimed at the detection and analysis of compounds, the bioconversion of starting materials and the preparation of target compounds at any scale, from laboratory small scale to industrial large scale. The key enabling tools which have been developed in biocatalysis over the last decades also provide great opportunities for further development and numerous applications in various sectors of the global bioeconomy. Systems biocatalysis is a modular, bottom-up approach to designing the architecture of enzyme-catalyzed reaction steps in a synthetic route from starting materials to target molecules. The integration of biocatalysis and sustainable chemistry in vitro aims at ideal conversions with high molecular economy and their intensification. Retrosynthetic analysis in the chemical and biological domain has been a valuable tool for target-oriented synthesis while, on the other hand, diversity-oriented synthesis builds on forward-looking analysis. Bioinformatic tools for rapid identification of the required enzyme functions, efficient enzyme production systems, as well as generalized bioprocess design tools, are important for rapid prototyping of the biocatalytic reactions. The tools for enzyme engineering and the reaction engineering of each enzyme-catalyzed one-step reaction are also valuable for coupling reactions. The tools to overcome interaction issues with other components or enzymes are of great interest in designing multi-step reactions as well as in biocatalytic total synthesis.

Aberrant glycosylation patterns on cancer cells: Therapeutic opportunities for glycodendrimers/metallodendrimers oncology

Despite exciting discoveries and progresses in drug design against cancer, its cure is still rather elusive and remains one of the humanities major challenges in health care. The safety profiles of common small molecule anti-cancer therapeutics are less than at acceptable levels and limiting deleterious side-effects have to be urgently addressed. This is mainly caused by their incapacity to differentiate healthy cells from cancer cells; hence, the use of high dosage becomes necessary. One possible solution to improve the therapeutic windows of anti-cancer agents undoubtedly resides in modern nanotechnology. This review presents a discussion concerning multivalent carbohydrate-protein interactions as this topic pertains to the fundamental aspects that lead glycoscientists to tackle glyconanoparticles. The second section describes the detailed properties of cancer cells and how their aberrant glycan surfaces differ from those of healthy cells. The third section briefly describes the immune systems, both innate and adaptative, because the numerous displays of cell surface protein receptors necessitate to be addressed from the multivalent angles, a strength full characteristic of nanoparticles. The next chapter presents recent advances in glyconanotechnologies, including glycodendrimers in particular, as they apply to glycobiology and carbohydrate-based cancer vaccines. This was followed by an overview of metallodendrimers and how this rapidly evolving field may contribute to our arsenal of therapeutic tools to fight cancer. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

High-Affinity N-(2-Hydroxypropyl)methacrylamide Copolymers with Tailored N-Acetyllactosamine Presentation Discriminate between Galectins

N-Acetyllactosamine (LacNAc; Galβ4GlcNAc) is a typical disaccharide ligand of galectins. The most abundant members of these human lectins, galectin-1 (Gal-1) and galectin-3 (Gal-3), participate in a number of pathologies including cancerogenesis and metastatic formation. In this study, we synthesized a series of fifteen N-(2-hydroxypropyl)methacrylamide (HPMA)-based glycopolymers with varying LacNAc amounts and presentations and evaluated the impact of their architecture on the binding affinity to Gal-1 and Gal-3. The controlled radical reversible addition-fragmentation chain transfer copolymerization technique afforded linear polymer precursors with comparable molecular weight (M_n ≈ 22,000 g mol^-1) and narrow dispersity (D̵ ≈ 1.1). The precursors were conjugated with the functionalized LacNAc disaccharide (4-22 mol % content in glycopolymer) prepared by enzymatic synthesis under catalysis by β-galactosidase from Bacillus circulans. The structure-affinity relationship study based on the enzyme-linked immunosorbent assay revealed that the type of LacNAc presentation, individual or clustered on bi- or trivalent linkers, brings a clear discrimination (almost 300-fold) between Gal-1 and Gal-3, reaching avidity to Gal-1 in the nanomolar range. Whereas Gal-1 strongly preferred a dense presentation of individually distributed LacNAc epitopes, Gal-3 preferred a clustered LacNAc presentation. Such a strong galectin preference based just on the structure of a multivalent glycopolymer type is exceptional. The prepared nontoxic, nonimmunogenic, and biocompatible glycopolymers are prospective for therapeutic applications requiring selectivity for one particular galectin.

Glycan Array Technology

Glycan (or carbohydrate) arrays have become an essential tool in glycomics, providing fast and high-throughput data on protein-carbohydrate interactions with small amounts of carbohydrate ligands. The general concepts of glycan arrays have been adopted from other microarray technologies such as those used for nucleic acid and proteins. However, carbohydrates have presented their own challenges, in particular in terms of access to glycan probes, linker attachment chemistries and analysis, which will be reviewed in this chapter. As more and more glycan probes have become available through chemical and enzymatic synthesis and robust linker chemistries have been developed, the applications of glycan arrays have dramatically increased over the past 10 years, which will be illustrated with recent examples.

Carbohydrate supramolecular chemistry: beyond the multivalent effect

(Hetero)multivalency acts as a multichannel switch that shapes the supramolecular properties of carbohydrates in an intrinsically multifactorial biological context.

β-N-Acetylhexosaminidases-the wizards of glycosylation

β-N-Acetylhexosaminidases (EC 3.2.1.52) are a unique family of glycoside hydrolases with dual substrate specificity and a particular reaction mechanism. Though hydrolytic enzymes per se, their good stability, easy recombinant production, absolute stereoselectivity, and a broad substrate specificity predestine these enzymes for challenging applications in carbohydrate synthesis. This mini-review aims to demonstrate the catalytic potential of β-N-acetylhexosaminidases in a range of unusual reactions, processing of unnatural substrates, formation of unexpected products, and demanding reaction designs. The use of unconventional media can considerably alter the progress of transglycosylation reactions. By means of site-directed mutagenesis, novel catalytic machineries can be constructed. Glycosylation of difficult substrates such as sugar nucleotides was accomplished, and the range of afforded glycosidic bonds comprises unique non-reducing sugars. Specific functional groups may be tolerated in the substrate molecule, which makes β-N-acetylhexosaminidases invaluable allies in difficult synthetic problems.

Galectin-Carbohydrate Interactions in Biomedicine and Biotechnology

Cellular communication events are mediated by interactions between cell-surface sugars and lectins, which are carbohydrate-binding proteins. Galectins are β-galactosyl-binding lectins that bridge molecules by their sugar moieties, forming a signaling and adhesion network. Severe changes in glycosylation and galectin expression accompany major processes in oncogenesis, cardiovascular disorders, and other pathologies, making galectins attractive therapeutic targets. Here we discuss advanced strategies of chemo-enzymatic carbohydrate synthesis for creating lead glycomimetics and (neo-)glycoconjugates for galectin-1 and -3 targeting in biomedicine and biotechnology. We will describe the challenges and bottlenecks on the route into biomedical and biotechnological practice and present the first clinical candidates. The coming era will see an exciting translation of selective well-defined high-affinity galectin ligands from bench to bedside.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

"Sweet Flavonoids": Glycosidase-Catalyzed Modifications

Natural flavonoids, especially in their glycosylated forms, are the most abundant phenolic compounds found in plants, fruit, and vegetables. They exhibit a large variety of beneficial physiological effects, which makes them generally interesting in a broad spectrum of scientific areas. In this review, we focus on recent advances in the modifications of the glycosidic parts of various flavonoids employing glycosidases, covering both selective trimming of the sugar moieties and glycosylation of flavonoid aglycones by natural and mutant glycosidases. Glycosylation of flavonoids strongly enhances their water solubility and thus increases their bioavailability. Antioxidant and most biological activities are usually less pronounced in glycosides, but some specific bioactivities are enhanced. The presence of l-rhamnose (6-deoxy-α-l-mannopyranose) in rhamnosides, rutinosides (rutin, hesperidin) and neohesperidosides (naringin) plays an important role in properties of flavonoid glycosides, which can be considered as "pro-drugs". The natural hydrolytic activity of glycosidases is widely employed in biotechnological deglycosylation processes producing respective aglycones or partially deglycosylated flavonoids. Moreover, deglycosylation is quite commonly used in the food industry aiming at the improvement of sensoric properties of beverages such as debittering of citrus juices or enhancement of wine aromas. Therefore, natural and mutant glycosidases are excellent tools for modifications of flavonoid glycosides.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

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.

Applications of Nonenzymatic Catalysts to the Alteration of Natural Products

The application of small molecules as catalysts for the diversification of natural product scaffolds is reviewed. Specifically, principles that relate to the selectivity challenges intrinsic to complex molecular scaffolds are summarized. The synthesis of analogues of natural products by this approach is then described as a quintessential "late-stage functionalization" exercise wherein natural products serve as the lead scaffolds. Given the historical application of enzymatic catalysts to the site-selective alteration of complex molecules, the focus of this Review is on the recent studies of nonenzymatic catalysts. Reactions involving hydroxyl group derivatization with a variety of electrophilic reagents are discussed. C-H bond functionalizations that lead to oxidations, aminations, and halogenations are also presented. Several examples of site-selective olefin functionalizations and C-C bond formations are also included. Numerous classes of natural products have been subjected to these studies of site-selective alteration including polyketides, glycopeptides, terpenoids, macrolides, alkaloids, carbohydrates, and others. What emerges is a platform for chemical remodeling of naturally occurring scaffolds that targets virtually all known chemical functionalities and microenvironments. However, challenges for the design of very broad classes of catalysts, with even broader selectivity demands (e.g., stereoselectivity, functional group selectivity, and site-selectivity) persist. Yet, a significant spectrum of powerful, catalytic alterations of complex natural products now exists such that expansion of scope seems inevitable. Several instances of biological activity assays of remodeled natural product derivatives are also presented. These reports may foreshadow further interdisciplinary impacts for catalytic remodeling of natural products, including contributions to SAR development, mode of action studies, and eventually medicinal chemistry.

Multivalency as an action principle in multimodal lectin recognition and glycosidase inhibition: a paradigm shift driven by carbon-based glyconanomaterials

The last decade has witnessed a series of discoveries that question the traditional paradigm of multivalency as a "safe" strategy to enhance the binding affinity of a lectin receptor to its cognate carbohydrate ligand. Upon following the initial reports on the supplementary effects operating in the presence of a third carbohydrate species (heteromultivalent effect), the observation of functional promiscuity of glyco(mimetic)ligands elicited by (hetero)multivalency, spreading from lectins to glycoprocessing enzymes (inhibitory multivalent effect), has raised concerns about the potential consequences of glyconanomaterials binding to non-cognate proteins and creating messiness or noise in the processes they participate in. Carbon-based glycomaterials, specifically glyconanodiamonds and glycofullerenes, have been instrumental in increasing our awareness of the frequency of these lectin-enzyme crosstalk behaviours elicited by multivalency, driving a reformulation of the rules and concepts in glycoscience towards a "generalized multivalency" scenario.

Nanotechnology in Glycomics: Applications in Diagnostics, Therapy, Imaging, and Separation Processes

This review comprehensively covers the most recent achievements (from 2013) in the successful integration of nanomaterials in the field of glycomics. The first part of the paper addresses the beneficial properties of nanomaterials for the construction of biosensors, bioanalytical devices, and protocols for the detection of various analytes, including viruses and whole cells, together with their key characteristics. The second part of the review focuses on the application of nanomaterials integrated with glycans for various biomedical applications, that is, vaccines against viral and bacterial infections and cancer cells, as therapeutic agents, for in vivo imaging and nuclear magnetic resonance imaging, and for selective drug delivery. The final part of the review describes various ways in which glycan enrichment can be effectively done using nanomaterials, molecularly imprinted polymers with polymer thickness controlled at the nanoscale, with a subsequent analysis of glycans by mass spectrometry. A short section describing an active glycoprofiling by microengines (microrockets) is covered as well.

Engineered N-acetylhexosamine-active enzymes in glycoscience

BACKGROUND

In recent years, enzymes modifying N-acetylhexosamine substrates have emerged in numerous theoretical studies as well as practical applications from biology, biomedicine, and biotechnology. Advanced enzyme engineering techniques converted them into potent synthetic instruments affording a variety of valuable glycosides.

SCOPE OF REVIEW

This review presents the diversity of engineered enzymes active with N-acetylhexosamine carbohydrates: from popular glycoside hydrolases and glycosyltransferases to less known oxidases, epimerases, kinases, sulfotransferases, and acetylases. Though hydrolases in natura, engineered chitinases, β-N-acetylhexosaminidases, and endo-β-N-acetylglucosaminidases were successfully employed in the synthesis of defined natural and derivatized chitooligomers and in the remodeling of N-glycosylation patterns of therapeutic antibodies. The genes of various N-acetylhexosaminyltransferases were cloned into metabolically engineered microorganisms for producing human milk oligosaccharides, Lewis X structures, and human-like glycoproteins. Moreover, mutant N-acetylhexosamine-active glycosyltransferases were applied, e.g., in the construction of glycomimetics and complex glycostructures, industrial production of low-lactose milk, and metabolic labeling of glycans. In the synthesis of biotechnologically important compounds, several innovative glycoengineered systems are presented for an efficient bioproduction of GlcNAc, UDP-GlcNAc, N-acetylneuraminic acid, and of defined glycosaminoglycans.

MAJOR CONCLUSIONS

The above examples demonstrate that engineering of N-acetylhexosamine-active enzymes was able to solve complex issues such as synthesis of tailored human-like glycoproteins or industrial-scale production of desired oligosaccharides. Due to the specific catalytic mechanism, mutagenesis of these catalysts was often realized through rational solutions.

GENERAL SIGNIFICANCE

Specific N-acetylhexosamine glycosylation is crucial in biological, biomedical and biotechnological applications and a good understanding of its details opens new possibilities in this fast developing area of glycoscience.

Glycan-decorated HPMA copolymers as high-affinity lectin ligands

New conjugates of N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers tethered with chitooligosaccharidic epitopes of varying lengths are potent ligands of wheat germ agglutinin, reaching subnanomolar binding affinities.

Complete tetraglycosylation of a calix[4]arene by a chemo-enzymatic approach

It was demonstrated that a calixarene can be a substrate for glycosyltransferases and thanks to an exhaustive glycosylation a multivalent tetralactosaminyl calix[4]arene was obtained.

Assessing the effect of different shapes of glyco-gold nanoparticles on bacterial adhesion and infections

Achieving selective and sensitive carbohydrate-protein interactions (CPIs) using nanotechnology is an intriguing area of research. Here we demonstrate that the different shapes of gold nanoparticles (AuNPs) functionalized with monosaccharides tune the bacterial aggregations. The mechanism of aggregation revealed that the large number of surface interactions of rod shaped mannose-AuNPs with E. coli ORN 178 compared with spherical and star-shaped AuNPs exhibited higher avidity and sensitivity. Moreover, such sensitive binding can be used for effective inhibition of bacterial infection of cells.

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.

Aqueous Glycosylation of Unprotected Sucrose Employing Glycosyl Fluorides in the Presence of Calcium Ion and Trimethylamine

We report a synthetic glycosylation reaction between sucrosyl acceptors and glycosyl fluoride donors to yield the derived trisaccharides. This reaction proceeds at room temperature in an aqueous solvent mixture. Calcium salts and a tertiary amine base promote the reaction with high site-selectivity for either the 3'-position or 1'-position of the fructofuranoside unit. Because nonenzymatic aqueous oligosaccharide syntheses are underdeveloped, mechanistic studies were carried out in order to identify the origin of the selectivity, which we hypothesized was related to the structure of the hydroxyl group array in sucrose. The solution conformation of various monodeoxysucrose analogs revealed the co-operative nature of the hydroxyl groups in mediating both this aqueous glycosyl bond-forming reaction and the site-selectivity at the same time.

Fighting Cholera One-on-One: The Development and Efficacy of Multivalent Cholera-Toxin-Binding Molecules

A series of diseases, ranging from cholera via travelers' diarrhea to hamburger disease, are caused by bacterially produced toxic proteins. In particular, a toxic protein unit is brought into the host cell upon binding to specific membrane-bound oligosaccharides on the host cell membrane. For example, the protein that causes cholera, cholera toxin (CT), has five identical, symmetrically placed binding pockets (B proteins), on top of which the toxic A protein resides. A promising strategy to counteract the devastating biological effects of this AB5 protein involves the development of inhibitors that can act as mimics of membrane-bound GM1 molecules, i.e., that can bind CT strongly and selectively. To reach this goal, two features are essential: First of all, the inhibitor should display oligosaccharides that resemble as much as possible the naturally occurring cell-surface pentasaccharide onto which CT normally binds, the so-called GM1 sugar (the oligosaccharide part of which is then labeled GM1os). Second, the inhibitor should be able to bind CT via multivalent interactions so as to bind CT as strongly as possible to allow for a real competition with the cell-membrane-bound GM1 molecules. In this Account, we present elements of the path that leads to strong CT inhibition by outlining the roles of multivalency and the development and use of GM1 mimics. First, multivalency effects were investigated using "sugar-coated" platforms, ranging from dendritic structures with up to eight oligosaccharides to platforms that mimicked the fivefold symmetry of CT itself. The latter goal was reached either via synthetic scaffolds like corannulene or calix[5]arene or via the development of a neolectin CT mimic that itself carries five GM1os groups. Second, the effect of the nature of the oligosaccharide appended to this platform was investigated via the use of oligosaccharides of increasing complexity, from galactose and lactose to the tetrasaccharide GM2os and eventually to GM1os itself. The combination of these threads gives rise to a series of inhibitors that can strongly bind CT, with IC50 values below 100 pM, and in some cases can even bind one-on-one.

Efficient and selective glucosylation of prenylated phenolic compounds by Mucor hiemalis

Mucor hiemaliscould efficiently and selectively catalyze β-O-glucosylation of the isoprenyl-neighboring hydroxyl group of prenylated phenolic compounds.

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.

Dendritic glycopolymers based on dendritic polyamine scaffolds: view on their synthetic approaches, characteristics and potential for biomedical applications

The potential of dendritic glycopolymers based on dendritic polyamine scaffolds for biomedical applications is presented and compared with that of the structurally related anti-adhesive dendritic glycoconjugates.

Chiral resolution through stereoselective transglycosylation by sucrose phosphorylase: application to the synthesis of a new biomimetic compatible solute, (R)-2-O-α-D-glucopyranosyl glyceric acid amide

Sucrose phosphorylase catalysed glycosylation of glyceric acid amide with complete regio- and diastereo-selectivity is studied. (R)-2-O-α-D-Glucopyranosyl glyceric acid amide was obtained in high yield from single-step transformation of racemic glyceric acid amide and sucrose. Non-productive binding of (S)-glyceric acid amide appeared to underlie strict enantiodiscrimination by the enzyme, thus supporting chiral resolutions based on stereoselective transglycosylation.

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.

Angling for uniqueness in enzymatic preparation of glycosides

In the early days of biocatalysis, limitations of an enzyme modeled the enzymatic applications; nowadays the enzyme can be engineered to be suitable for the process requirements. This is a general bird's-eye view and as such cannot be specific for articulated situations found in different classes of enzymes or for selected enzymatic processes. As far as the enzymatic preparation of glycosides is concerned, recent scientific literature is awash with examples of uniqueness related to the features of the biocatalyst (yield, substrate specificity, regioselectivity, and resistance to a particular reaction condition). The invention of glycosynthases is just one of the aspects that has thrust forward the research in this field. Protein engineering, metagenomics and reaction engineering have led to the discovery of an expanding number of novel enzymes and to the setting up of new bio-based processes for the preparation of glycosides. In this review, new examples from the last decade are compiled with attention both to cases in which naturally present, as well as genetically inserted, characteristics of the catalysts make them attractive for biocatalysis.