Carbohydrate mimetics-based glycosyltransferase inhibitors

Mitigating candidiasis with acarbose by targeting Candida albicans α-glucosidase: in-silico, in-vitro and transcriptomic approaches

Biofilm-associated candidiasis poses a significant challenge in clinical settings due to the limited effectiveness of existing antifungal treatments. The challenges include increased pathogen virulence, multi-drug resistance, and inadequate penetration of antimicrobials into biofilm structures. One potential solution to this problem involves the development of novel drugs that can modulate fungal virulence and biofilm formation, which is essential for pathogenesis. Resistance in Candida albicans is initiated by morphological changes from yeast to hyphal form. This transition triggers a series of events such as cell wall elongation, increased adhesion, invasion of host tissues, pathogenicity, biofilm formation, and the initiation of an immune response. The cell wall is a critical interface for interactions with host cells, primarily through various cell wall proteins, particularly mannoproteins. Thus, cell wall proteins and enzymes are considered potential antifungal targets. In this regard, we explored α-glucosidase as our potential target which plays a crucial role in processing mannoproteins. Previous studies have shown that inhibition of α-glucosidase leads to defects in cell wall integrity, reduced adhesion, diminished secretion of hydrolytic enzymes, alterations in immune recognition, and reduced pathogenicity. Since α-glucosidase, primarily converts carbohydrates, our study focuses on FDA-approved carbohydrate mimic drugs (Glycomimetics) with well-documented applications in various biological contexts. Through virtual screening of 114 FDA-approved carbohydrate-based drugs, a pseudo-sugar Acarbose, emerged as a top hit. Acarbose is known for its pharmacological potential in managing type 2 diabetes mellitus by targeting α-glucosidase. Our preliminary investigations indicate that Acarbose effectively inhibits C. albicans biofilm formation, reduces virulence, impairs morphological switching, and hinders the adhesion and invasion of host cells, all at very low concentrations in the nanomolar range. Furthermore, transcriptomic analysis reveals the mechanism of action of Acarbose, highlighting its role in targeting α-glucosidase.

Expanding the scope of the successive ring expansion strategy for macrocycle and medium-sized ring synthesis: unreactive and reactive lactams

New methods are described that expand the scope of the Successive Ring Expansion (SuRE) with respect to synthetically challenging lactams. A protocol has been developed for use with 'unreactive' lactams, enabling SuRE reactions to be performed on subsrates that fail under previously established conditions. Ring expansion is also demonstarted on 'reactive' lactams derived from iminosugars for the first time. The new SuRE methods were used to prepare a diverse array of medium-sized and macrocyclic lactams and lactones, which were evaluted in an anti-bacterial assay against E. coli BW25113WT.

Palladium(II)-Catalyzed Heck Coupling: Direct Stereoselective Synthesis of C-Aryl Glycosides from Nonactivated Glycals and Thianthrenium Salts

Here, we report an efficient Pd(II)-catalyzed Heck coupling reaction utilizing modular and readily available thianthrenium salts. The tunability and ease of thianthrenium salts facilitated the integration of glycals with drugs, natural products, and peptides. This method allows the incorporation of diverse glycals into structurally varied aglycon components without directing groups or prefunctionalization and provides a practical method for synthesizing C-aryl glycosides, offering a new avenue for the production of complex glycosides with potential applications.

Specialized metabolites degradation by microorganisms

Secondary metabolites are specialized metabolic products synthesized by plants, insects, and bacteria, some of which exhibit significant physiological activities against other organisms. Plants containing bioactive secondary metabolites have been used in traditional medicine for centuries. In developed countries, one-fourth of medicines directly contain plant-derived compounds or indirectly contain them via semi-synthesis. These compounds have contributed considerably to the development of not only medicine but also molecular biology. Moreover, the biosynthesis of these physiologically active secondary metabolites has attracted substantial interest and has been extensively studied. However, in many cases, the degradation mechanisms of these secondary metabolites remain unclear. In this review, some unique microbial degradation pathways for lignans and C-glycosides are explored.

Drug Discovery Based on Fluorine-Containing Glycomimetics

Glycomimetics, which are synthetic molecules designed to mimic the structures and functions of natural carbohydrates, have been developed to overcome the limitations associated with natural carbohydrates. The fluorination of carbohydrates has emerged as a promising solution to dramatically enhance the metabolic stability, bioavailability, and protein-binding affinity of natural carbohydrates. In this review, the fluorination methods used to prepare the fluorinated carbohydrates, the effects of fluorination on the physical, chemical, and biological characteristics of natural sugars, and the biological activities of fluorinated sugars are presented.

Preference of Bacterial Rhamnosyltransferases for 6-Deoxysugars Reveals a Strategy To Deplete O-Antigens

Bacteria synthesize hundreds of bacteria-specific or "rare" sugars that are absent in mammalian cells and enriched in 6-deoxy monosaccharides such as l-rhamnose (l-Rha). Across bacteria, l-Rha is incorporated into glycans by rhamnosyltransferases (RTs) that couple nucleotide sugar substrates (donors) to target biomolecules (acceptors). Since l-Rha is required for the biosynthesis of bacterial glycans involved in survival or host infection, RTs represent potential antibiotic or antivirulence targets. However, purified RTs and their unique bacterial sugar substrates have been difficult to obtain. Here, we use synthetic nucleotide rare sugar and glycolipid analogs to examine substrate recognition by three RTs that produce cell envelope components in diverse species, including a known pathogen. We find that bacterial RTs prefer pyrimidine nucleotide-linked 6-deoxysugars, not those containing a C6-hydroxyl, as donors. While glycolipid acceptors must contain a lipid, isoprenoid chain length, and stereochemistry can vary. Based on these observations, we demonstrate that a 6-deoxysugar transition state analog inhibits an RT in vitro and reduces levels of RT-dependent O-antigen polysaccharides in Gram-negative cells. As O-antigens are virulence factors, bacteria-specific sugar transferase inhibition represents a novel strategy to prevent bacterial infections.

Iminosugar-Based Nicotinamide Phosphoribosyltransferase (NAMPT) Inhibitors as Potential Anti-Pancreatic Cancer Agents

The nicotinamide phosphoribosyltransferase (NAMPT) is considered a very promising therapeutic target because it is overexpressed in pancreatic cancer. Although many inhibitors have been prepared and tested, clinical trials have shown that NAMPT inhibition may result in severe haematological toxicity. Therefore, the development of conceptually new inhibitors is an important and challenging task. We synthesized ten β-d-iminoribofuranosides bearing various heterocycle-based chains carbon-linked to the anomeric position starting from non-carbohydrate derivatives. They were then submitted to NAMPT inhibition assays, as well as to pancreatic tumor cells viability and intracellular NAD⁺ depletion evaluation. The biological activity of the compounds was compared to that of the corresponding analogues lacking the carbohydrate unit to assess, for the first time, the contribution of the iminosugar moiety to the properties of these potential antitumor agents.

One-pot synthesis of heteroaryl diketoalkynyl C-glycoside and dialkynyl di-C-glycoside analogues by three-component successive coupling reaction

The synthesis of heteroaryl diketoalkynyl C-glycoside and dialkynyl di-C-glycoside analogues has been accomplished by successive coupling of heteroaromatics, oxalyl chloride and terminal sugar alkynes in one pot. The three-component coupling reaction catalyzed by CuI gives heteroaryl diketoalkynyl C-glycosides. The same three-component coupling in the presence of n-BuLi produces dialkynyl di-C-glycosides, and the 1:1 of molar ratio of heteroaromatics to terminal sugar alkynes affords the corresponding esters of dialkynyl di-C-glycosides. The desired products have been obtained in good to excellent yields. This sequential one-pot method is mild and efficient, suitable for different heteroaromatics and terminal sugar alkynes. The sugar alkynes include furanosides, pyranosides, and acyclic sugars. Twenty-seven examples have been given. The mechanism for the formation of the desired products has been elucidated.

Structural mechanism of a dual-functional enzyme DgpA/B/C as both a C-glycoside cleaving enzyme and an O- to C-glycoside isomerase

The C-glycosidic bond that connects the sugar moiety with aglycone is difficult to be broken or made due to its inert nature. The knowledge of C-glycoside breakdown and synthesis is very limited. Recently, the enzyme DgpA/B/C cascade from a human intestinal bacterium PUE was identified to specifically cleave the C-glycosidic bond of puerarin (daidzein-8-C-glucoside). Here we investigated how puerarin is recognized and oxidized by DgpA based on crystal structures of DgpA with or without substrate and biochemical characterization. More strikingly, we found that apart from being a C-glycoside cleaving enzyme, DgpA/B/C is capable of efficiently converting O- to C-glycoside showing the activity as a structure isomerase. A possible mechanistic model was proposed dependently of the simulated complex structure of DgpB/C with 3″-oxo-daidzin and structure-based mutagenesis. Our findings not only shed light on understanding the enzyme-mediated C-glycosidic bond breakage and formation, but also may help to facilitate stereospecific C-glycoside synthesis in pharmaceutical industry.

Recent advances in palladium-catalyzed C(sp3)/C(sp2)-H bond functionalizations: access to C-branched glycosides

Over the recent decades, tremendous interest has developed in the transformation of complex substrates by C-H activation and functionalization. In particular, palladium-catalyzed directing and non-directing group-assisted C-H functionalization has emerged as a powerful avenue to access C-branched glycosides. Due to the extreme complexity, delicate functionalities, and high stability of C-H bonds, site-selective functionalization of carbohydrate under mild conditions is highly desirable. The purpose of this review is to cover most of the recent advances in palladium-catalyzed C(sp³) and C(sp²)-H bond functionalizations for the synthesis of C-branched glycosides along with future directions.

Advances in Pd-catalyzed C-C bond formation in carbohydrates and their applications in the synthesis of natural products and medicinally relevant molecules

Advances in the Pd-catalyzed synthesis of C-glycosides and branched sugars are summarized herein and the strategies are categorized based on named reactions or types of sugar moieties involved in the reactions. These include cross-coupling reactions, C-H activations, and carbonylative cross-coupling reactions. Applications of Pd-catalyzed C-glycosylation reactions are discussed in the synthesis of natural products and biologically active molecules such as bergenin, papulacandin D, and SGLT2-inhibitors. Important mechanistic cycles are drawn and the mechanisms for how Pd-activates the sugar moieties for various coupling partners are discussed. The directing group-assisted C-glycosylation and some intramolecular C-H activation reactions are also included.

Chemical Biology Tools for Modulating and Visualizing Gram-Negative Bacterial Surface Polysaccharides

Bacterial cells present a wide diversity of saccharides that decorate the cell surface and help mediate interactions with the environment. Many Gram-negative cells express O-antigens, which are long sugar polymers that makeup the distal portion of lipopolysaccharide (LPS) that constitutes the surface of the outer membrane. This review highlights chemical biology tools that have been developed in recent years to facilitate the modulation of O-antigen synthesis and composition, as well as related bacterial polysaccharide pathways, and the detection of unique glycan sequences. Advances in the biochemistry and structural biology of O-antigen biosynthetic machinery are also described, which provide guidance for the design of novel chemical and biomolecular probes. Many of the tools noted here have not yet been utilized in biological systems and offer researchers the opportunity to investigate the complex sugar architecture of Gram-negative cells.

Opportunities and Challenges of Bacterial Glycosylation for the Development of Novel Antibacterial Strategies

Glycosylation is a ubiquitous process that is universally conserved in nature. The various products of glycosylation, such as polysaccharides, glycoproteins, and glycolipids, perform a myriad of intra- and extracellular functions. The multitude of roles performed by these molecules is reflected in the significant diversity of glycan structures and linkages found in eukaryotes and prokaryotes. Importantly, glycosylation is highly relevant for the virulence of many bacterial pathogens. Various surface-associated glycoconjugates have been identified in bacteria that promote infectious behavior and survival in the host through motility, adhesion, molecular mimicry, and immune system manipulation. Interestingly, bacterial glycosylation systems that produce these virulence factors frequently feature rare monosaccharides and unusual glycosylation mechanisms. Owing to their marked difference from human glycosylation, bacterial glycosylation systems constitute promising antibacterial targets. With the rise of antibiotic resistance and depletion of the antibiotic pipeline, novel drug targets are urgently needed. Bacteria-specific glycosylation systems are especially promising for antivirulence therapies that do not eliminate a bacterial population, but rather alleviate its pathogenesis. In this review, we describe a selection of unique glycosylation systems in bacterial pathogens and their role in bacterial homeostasis and infection, with a focus on virulence factors. In addition, recent advances to inhibit the enzymes involved in these glycosylation systems and target the bacterial glycan structures directly will be highlighted. Together, this review provides an overview of the current status and promise for the future of using bacterial glycosylation to develop novel antibacterial strategies.

Iminosugars as glycosyltransferase inhibitors

Iminosugars are naturally occurring carbohydrate analogues known since 1967. These natural compounds and hundreds of their synthetic derivatives prepared over five decades have been mainly exploited to inhibit the glycosidases, the enzymes catalysing the glycosidic bond cleavage, in order to find new drugs for the treatment of type 2 diabetes and other diseases. However, iminosugars are also inhibitors of glycosyltransferases, the enzymes responsible for the synthesis of oligosaccharides and glycoconjugates. The selective inhibition of specific glycosyltransferases involved in cancer or bacterial infections could lead to innovative therapeutic agents. The synthesis and biological properties of all the iminosugars assayed to date as glycosyltransferase inhibitors are reviewed in the present article.

Approaches to the Synthesis of Perfluoroalkyl-Modified Carbohydrates and Derivatives: Thiosugars, Iminosugars, and Tetrahydro(thio)pyrans

Fluoroalkyl-substituted carbohydrates play relevant roles in diverse areas such as supramolecular chemistry, glycoconjugation, liquid crystals, and surfactants, with direct applications as wetting, antifreeze, and coating agents. In light of these promising applications, new methodologies for the late-stage incorporation of fluoroalkyl R_F groups into carbohydrates and derivatives are herein presented as they are relevant to the synthetic carbohydrate community. Previously reviewed protocols for the installation of R_F groups onto carbohydrates and derivatives will be succinctly summarized in the light of the new achievements. Fluoroalkyl-substituted iminosugars, on the other hand, are also interesting glycomimetic derivatives with prominent roles as glycosidases and glycosyltransferases inhibitors, as has recently been demonstrated. Also, they positively contribute to the study of sugar-protein interactions and enzyme mechanisms. New advances in the syntheses of fluoroalkyl-substituted iminosugars will also be presented here.

The Tumour Microenvironment and Circulating Tumour Cells: A Partnership Driving Metastasis and Glycan-Based Opportunities for Cancer Control

Circulating tumour cells (CTC) are rare cells that actively detach or are shed from primary tumours into the lymph and blood. Some CTC subpopulations gain the capacity to survive, home and colonize distant locations, forming metastasis. This results from a multifactorial process in which cancer cells optimize motility, invasion, immune escape and cooperative relationships with microenvironmental cues. Here we present evidences of a self-fuelling molecular crosstalk between cancer cells and the tumour stroma supporting the main milestones leading to metastasis. We discuss how the tumour microenvironment supports pre-metastatic niches and CTC development and ultimately dictates CTC fate in targeted organs. Finally, we highlight the key role played by protein glycosylation in metastasis development, its prompt response to microenvironmental stimuli and the tremendous potential of glycan-based molecular signatures for liquid biopsies and targeted therapeutics.

Cobalt-Catalyzed Cross-Couplings between Alkyl Halides and Grignard Reagents

Metal-catalyzed cross-couplings have emerged as essential tools for the construction of C-C bonds. The identification of efficient catalytic systems as well as large substrate scope made these cross-couplings key reactions to access valuable molecules ranging from materials, agrochemicals to active pharmaceutical ingredients. They have been increasingly integrated in retrosynthetic plans, allowing shorter and original route development. Palladium-catalyzed cross-couplings still largely rule the field, with the most popular reactions in industrial processes being the Suzuki and Sonogashira couplings. However, the extensive use of palladium complexes raises several problems such as limited resources, high cost, environmental impact, and frequent need for sophisticated ligands. As a consequence, the use of nonprecious and cheap metal catalysts has appeared as a new horizon in cross-coupling development. Over the last three decades, a growing interest has thus been devoted to Fe-, Co-, Cu-, or Ni-catalyzed cross-couplings. Their natural abundance makes them cost-effective, allowing the conception of more sustainable and less expensive chemical processes, especially for large-scale production of active molecules. In addition to these economical and environmental considerations, the 3d metal catalysts also exhibit complementary reactivity with palladium complexes, facilitating the use of alkyl halide partners due to the decrease of β-elimination side reactions. In particular, by using cobalt catalysts, numerous cross-couplings between alkyl halides and organometallics have been described. However, cobalt catalysis still stays far behind palladium catalysis in terms of popularity and applications, and the expansion of the substrate scope as well as the development of simple and robust catalytic systems remains an important challenge.In 2012, our group entered the cobalt catalysis field by developing a cobalt-catalyzed cross-coupling between C-bromo glycosides and Grignard reagents. The generality of the coupling allowed the preparation of a range of valuable C-aryl and C-vinyl glycoside building blocks. We then focused on the functionalization of saturated N-heterocycles, and a variety of halo-azetidines, -pyrrolidines, and -piperidines were successfully reacted with aryl and alkenyl Grignard reagents under cobalt catalysis. With the objective of preparing valuable α-aryl amides, a cobalt-catalyzed cross-coupling applied to α-bromo amides was studied and then extended to α-bromo lactams. Recently, we also reported an efficient and general cross-coupling involving cyclopropyl- and cyclobutyl-magnesium bromides. This method allows the alkylation of functionalized small strained rings by a range of primary and secondary alkyl halides.

From Heterocycles to Carbacycles: Synthesis of Carbocyclic Nucleoside Analogues from Enals and Hydroxylamines

Substituted and stereochemically dense carbacycles can be prepared by sequential dipolar cycloaddition and reductive cleavage from enals and hydroxylamines. The reaction sequence proceeds with high efficiency for a wide variety of enals and hydroxylamines. The reaction is regio- and diastereoselective for the initial formation of a bridged bisisoxazolidine intermediate, which then undergoes quantitative double N-O cleavage to produce carbacycles as single diastereomers.

Leloir glycosyltransferases of natural product C-glycosylation: structure, mechanism and specificity

A prominent attribute of chemical structure in microbial and plant natural products is aromatic C-glycosylation. In plants, various flavonoid natural products have a β-C-d-glucosyl moiety attached to their core structure. Natural product C-glycosides have attracted significant attention for their own unique bioactivity as well as for representing non-hydrolysable analogs of the canonical O-glycosides. The biosynthesis of natural product C-glycosides is accomplished by sugar nucleotide-dependent (Leloir) glycosyltransferases. Here, we provide an overview on the C-glycosyltransferases of microbial, plant and insect origin that have been biochemically characterized. Despite sharing basic evolutionary relationships, as evidenced by their common membership to glycosyltransferase family GT-1 and conserved GT-B structural fold, the known C-glycosyltransferases are diverse in the structural features that govern their reactivity, selectivity and specificity. Bifunctional glycosyltransferases can form C- and O-glycosides dependent on the structure of the aglycon acceptor. Recent crystal structures of plant C-glycosyltransferases and di-C-glycosyltransferases complement earlier structural studies of bacterial enzymes and provide important molecular insight into the enzymatic discrimination between C- and O-glycosylation. Studies of enzyme structure and mechanism converge on the view of a single displacement (SN2)-like mechanism of enzymatic C-glycosyl transfer, largely analogous to O-glycosyl transfer. The distinction between reactions at the O- or C-acceptor atom is achieved through the precise positioning of the acceptor relative to the donor substrate in the binding pocket. Nonetheless, C-glycosyltransferases may differ in the catalytic strategy applied to induce nucleophilic reactivity at the acceptor carbon. Evidence from the mutagenesis of C-glycosyltransferases may become useful in engineering these enzymes for tailored reactivity.

Selectins-The Two Dr. Jekyll and Mr. Hyde Faces of Adhesion Molecules-A Review

Selectins belong to a group of adhesion molecules that fulfill an essential role in immune and inflammatory responses and tissue healing. Selectins are glycoproteins that decode the information carried by glycan structures, and non-covalent interactions of selectins with these glycan structures mediate biological processes. The sialylated and fucosylated tetrasaccharide sLe^x is an essential glycan recognized by selectins. Several glycosyltransferases are responsible for the biosynthesis of the sLe^x tetrasaccharide. Selectins are involved in a sequence of interactions of circulated leukocytes with endothelial cells in the blood called the adhesion cascade. Recently, it has become evident that cancer cells utilize a similar adhesion cascade to promote metastases. However, like Dr. Jekyll and Mr. Hyde’s two faces, selectins also contribute to tissue destruction during some infections and inflammatory diseases. The most prominent function of selectins is associated with the initial stage of the leukocyte adhesion cascade, in which selectin binding enables tethering and rolling. The first adhesive event occurs through specific non-covalent interactions between selectins and their ligands, with glycans functioning as an interface between leukocytes or cancer cells and the endothelium. Targeting these interactions remains a principal strategy aimed at developing new therapies for the treatment of immune and inflammatory disorders and cancer. In this review, we will survey the significant contributions to and the current status of the understanding of the structure of selectins and the role of selectins in various biological processes. The potential of selectins and their ligands as therapeutic targets in chronic and acute inflammatory diseases and cancer will also be discussed. We will emphasize the structural characteristic of selectins and the catalytic mechanisms of glycosyltransferases involved in the biosynthesis of glycan recognition determinants. Furthermore, recent achievements in the synthesis of selectin inhibitors will be reviewed with a focus on the various strategies used for the development of glycosyltransferase inhibitors, including substrate analog inhibitors and transition state analog inhibitors, which are based on knowledge of the catalytic mechanism.

Utility of bacterial peptidoglycan recycling enzymes in the chemoenzymatic synthesis of valuable UDP sugar substrates

Uridine diphosphate (UDP) sugars are essential precursors for glycosylation reactions in all forms of life. Reactions that transfer the carbohydrate from the UDP donor are catalyzed by glycosyltransferases (Gtfs). While the stereochemistry and negative physiological charge of UDP-sugars are essential for their biochemical function in the cell, these characteristics make them challenging molecules to synthesize and purify on scale in the laboratory. This chapter focuses on the utilization of a chemoenzymatic synthesis of muramyl UDP-sugars, key building blocks in the bacterial cell peptidoglycan. A scalable strategy to obtain UDP-N-acetyl muramic acid derivatives (UDP-NAM), the first committed intermediate used solely in peptidoglycan biosynthesis, is described herein. This methodology utilizes two enzymes involving the cell wall recycling enzymes MurNAc/GlcNAc anomeric kinase (AmgK) and NAM α-1-phosphate uridylyl transferase (MurU), respectively. The promiscuity of these enzymes allows for the unique chemical functionality to be embedded in bacterial peptidoglycan both in vitro and in whole bacterial cells for subsequent structural and functional studies of this important biopolymer.

Palladium-Catalysed C(sp3 )-H Glycosylation for the Synthesis of C-Alkyl Glycoamino Acids

We have developed a highly efficient and practical approach for palladium-catalyzed trifluoroacetate-promoted N-quinolylcarboxamide-directed glycosylation of inert β-C(sp³ )-H bonds of N-phthaloyl α-amino acids with glycals under mild conditions. For the first time, C(sp³ )-H activation for glycosylation was achieved to build C-alkyl glycosides. This method facilitates the synthesis of various β-substituted C-alkyl glycoamino acids and offers a tool for glycopeptide synthesis.

Synthesis of C-glycosyl phosphonate derivatives of 4-amino-4-deoxy-α-ʟ-arabinose

The incorporation of basic substituents into the structurally conserved domains of cell wall lipopolysaccharides has been identified as a major mechanism contributing to antimicrobial resistance of Gram-negative pathogenic bacteria. Inhibition of the corresponding enzymatic steps, specifically the transfer of 4-amino-4-deoxy-ʟ-arabinose, would thus restore the activity of cationic antimicrobial peptides and several antimicrobial drugs. C-glycosidically-linked phospholipid derivatives of 4-amino-4-deoxy-ʟ-arabinose have been prepared as hydrolytically stable and chain-shortened analogues of the native undecaprenyl donor. The C-phosphonate unit was installed via a Wittig reaction of benzyl-protected 1,5-arabinonic acid lactone with the lithium salt of dimethyl methylphosphonate followed by an elimination step of the resulting hemiketal, leading to the corresponding exo- and endo-glycal derivatives. The ensuing selective monodemethylation and hydrogenolysis of the benzyl groups and reduction of the 4-azido group gave the α-ʟ-anomeric arabino- and ribo-configured methyl phosphonate esters. In addition, the monomethyl phosphonate glycal intermediates were converted into n-octyl derivatives followed by subsequent selective removal of the methyl phosphonate ester group and hydrogenation to give the octylphosphono derivatives. These intermediates will be of value for their future conversion into transition state analogues as well as for the introduction of various lipid extensions at the anomeric phosphonate moiety.

High-Throughput Approaches in Carbohydrate-Active Enzymology: Glycosidase and Glycosyl Transferase Inhibitors, Evolution, and Discovery

Carbohydrates are attached and removed in living systems through the action of carbohydrate-active enzymes such as glycosyl transferases and glycoside hydrolases. The molecules resulting from these enzymes have many important roles in organisms, such as cellular communication, structural support, and energy metabolism. In general, each carbohydrate transformation requires a separate catalyst, and so these enzyme families are extremely diverse. To make this diversity manageable, high-throughput approaches look at many enzymes at once. Similarly, high-throughput approaches can be a powerful way of finding inhibitors that can be used to tune the reactivity of these enzymes, either in an industrial, a laboratory, or a medicinal setting. In this review, we provide an overview of how these enzymes and inhibitors can be sought using techniques such as high-throughput natural product and combinatorial library screening, phage and mRNA display of (glyco)peptides, fluorescence-activated cell sorting, and metagenomics.

Iminosugars: Effects of Stereochemistry, Ring Size, and N-Substituents on Glucosidase Activities

N-substituted iminosugar analogues are potent inhibitors of glucosidases and glycosyltransferases with broad therapeutic applications, such as treatment of diabetes and Gaucher disease, immunosuppressive activities, and antibacterial and antiviral effects against HIV, HPV, hepatitis C, bovine diarrhea (BVDV), Ebola (EBOV) and Marburg viruses (MARV), influenza, Zika, and dengue virus. Based on our previous work on functionalized isomeric 1,5-dideoxy-1,5-imino-D-gulitol (L-gulo-piperidines, with inverted configuration at C-2 and C-5 in respect to glucose or deoxynojirimycin (DNJ)) and 1,6-dideoxy-1,6-imino-D-mannitol (D-manno-azepane derivatives) cores N-linked to different sites of glucopyranose units, we continue our studies on these alternative iminosugars bearing simple N-alkyl chains instead of glucose to understand if these easily accessed scaffolds could preserve the inhibition profile of the corresponding glucose-based N-alkyl derivatives as DNJ cores found in miglustat and miglitol drugs. Thus, a small library of iminosugars (14 compounds) displaying different stereochemistry, ring size, and N-substitutions was successfully synthesized from a common precursor, D-mannitol, by utilizing an SN2 aminocyclization reaction via two isomeric bis-epoxides. The evaluation of the prospective inhibitors on glucosidases revealed that merely D-gluco-piperidine (miglitol, 41a) and L-ido-azepane (41b) DNJ-derivatives bearing the N-hydroxylethyl group showed inhibition towards α-glucosidase with IC50 41 µM and 138 µM, respectively, using DNJ as reference (IC50 134 µM). On the other hand, β-glucosidase inhibition was achieved for glucose-inverted configuration (C-2 and C-5) derivatives, as novel L-gulo-piperidine (27a) and D-manno-azepane (27b), preserving the N-butyl chain, with IC50 109 and 184 µM, respectively, comparable to miglustat with the same N-butyl substituent (40a, IC50 172 µM). Interestingly, the seven-membered ring L-ido-azepane (40b) displayed near twice the activity (IC50 80 µM) of the corresponding D-gluco-piperidine miglustat drug (40a). Furthermore, besides α-glucosidase inhibition, both miglitol (41a) and L-ido-azepane (41b) proved to be the strongest β-glucosidase inhibitors of the series with IC50 of 4 µM.

Protein Glycosylation and Tumor Microenvironment Alterations Driving Cancer Hallmarks

Decades of research have disclosed a plethora of alterations in protein glycosylation that decisively impact in all stages of disease and ultimately contribute to more aggressive cell phenotypes. The biosynthesis of cancer-associated glycans and its reflection in the glycoproteome is driven by microenvironmental cues and these events act synergistically toward disease evolution. Such intricate crosstalk provides the molecular foundations for the activation of relevant oncogenic pathways and leads to functional alterations driving invasion and disease dissemination. However, it also provides an important source of relevant glyco(neo)epitopes holding tremendous potential for clinical intervention. Therefore, we highlight the transversal nature of glycans throughout the currently accepted cancer hallmarks, with emphasis on the crosstalk between glycans and the tumor microenvironment stromal components. Focus is also set on the pressing need to include glycans and glycoconjugates in comprehensive panomics models envisaging molecular-based precision medicine capable of improving patient care. We foresee that this may provide the necessary rationale for more comprehensive studies and molecular-based intervention.

Construction of C-glycosides of heterocycles containing the pyrimidin-2-amine or the 1H-pyrazolo[3,4-b]pyridine moiety and their biological evaluation for anticancer activities

A series of novel C-glycosides of heterocyclic derivatives containing a pyrimidin-2-amine or a 1 H-pyrazolo[3,4- b]pyridine moiety were synthesized using condensation reactions of the substituted puerarin with guanidine or 3-amino-5-hydroxypyrazole in methyl alcohol. Their chemical structures were characterized by Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and high-resolution mass spectrometry. In addition, their biological activity has been demonstrated by in vitro evaluation against the human leukemia cells K562 and human prostate cancer cells PC-3 by MTT-based assays, using the commercially available standard drug of cis-platin as a positive control. The results also demonstrated that most of the compounds showed considerable cytotoxicity to these two cell lines of K562 and PC-3, and indicated that novel C-glycosides of heterocyclic derivatives may be potential leads for further biological screenings and may generate drug-like molecules.

Synthesis and Preliminary Evaluation of Biological Activity of Glycoconjugates Analogues of Acyclic Uridine Derivatives

Herein we present the methodology for obtaining glycosyltransferase inhibitors, analogues of natural enzyme substrates of donor-type: UDP-glucose and UDP-galactose. The synthesis concerned glycoconjugates, nucleoside analogues containing an acyclic ribose mimetic linked to a uracil moiety in their structure. The biological activity of the synthesised compounds was determined on the basis of their ability to inhibit the model enzyme action of β-1,4-galactosyltransferase from bovine milk. The obtained results allowed to expand and supplement the existing library of synthetic compounds that are able to regulate the biological activity of enzymes from the GT class.

Regio- and stereospecific synthesis of rac-carbasugar-based cyclohexane pentols; Investigations of their α- and β-glucosidase inhibitions

In the present study, (3aR,7aS)-1,3,3a,4,7,7a-hexahydroisobenzofuran was submitted to photooxygenation and two isomeric hydroperoxides were successfully obtained. Without any further purification, reduction of the hydroperoxides with titanium tetraisopropoxide catalyzed by dimethyl sulfide gave two alcohol isomers in high yields. After acetylation of alcohol with Ac₂O in pyridine, epoxidation reaction of formed monoacetates with m-CPBA, then chromatographed and followed by hydrolysis of the acetate groups with NH₃ in CH₃OH resulted in the formation of epoxy alcohol isomers respectively. These epoxy alcohol isomers were subjected to trans-dihydroxylation reaction with acid (H₂SO₄) in the presence of water to afford triols. Acetylation of the free hydroxyl groups produced benzofuran triacetates in high yields. Ring-opening reaction of furan triacetates with sulfamic acid catalyzed in the presence of acetic acid/acetic anhydrate and subsequently hydrolysis of the acetate groups with ammonia gave the targeted cyclohexane carbasugar-based pentols. All products were separated and purified by chromatographic and crystallographic methods. Structural analyses of all compounds were conducted by spectral techniques including NMR and X-ray analyses. The biological inhibition activity of the target compounds was tested against glycosidase enzymes, α- and β-glucosidase.

Characterization of the single-subunit oligosaccharyltransferase STT3A from Trypanosoma brucei using synthetic peptides and lipid-linked oligosaccharide analogs

The initial transfer of a complex glycan in protein N-glycosylation is catalyzed by oligosaccharyltransferase (OST), which is generally a multisubunit membrane protein complex in the endoplasmic reticulum but a single-subunit enzyme (ssOST) in some protists. To investigate the reaction mechanism of ssOST, we recombinantly expressed, purified and characterized the STT3A protein from Trypanosoma brucei (TbSTT3A). We analyzed the in vitro activity of TbSTT3A by synthesizing fluorescently labeled acceptor peptides as well as lipid-linked oligosaccharide (LLO) analogs containing a chitobiose moiety coupled to oligoprenyl carriers of distinct lengths (C₁₀, C₁₅, C₂₀ and C₂₅) and with different double bond stereochemistry. We found that in addition to proline, charged residues at the +1 position of the sequon inhibited glycan transfer. An acidic residue at the −2 position significantly increased catalytic turnover but was not essential, in contrast to the bacterial OST. While all synthetic LLO analogs were processed by TbSTT3A, the length of the polyprenyl tail, but not the stereochemistry of the double bonds, determined their apparent affinity. We also synthesized phosphonate analogs of the LLOs, which were found to be competitive inhibitors of the reaction, although with lower apparent affinity to TbSTT3A than the active pyrophosphate analogs.

UDP-GlcNAc Analogues as Inhibitors of O-GlcNAc Transferase (OGT): Spectroscopic, Computational, and Biological Studies

A series of glycomimetics of UDP-GlcNAc, in which the β-phosphate has been replaced by either an alkyl chain or a triazolyl ring and the sugar moiety has been replaced by a pyrrolidine ring, has been synthesized by the application of different click-chemistry procedures. Their affinities for human O-GlcNAc transferase (hOGT) have been evaluated and studied both spectroscopically and computationally. The binding epitopes of the best ligands have been determined in solution by means of saturation transfer difference (STD) NMR spectroscopy. Experimental, spectroscopic, and computational results are in agreement, pointing out the essential role of the binding of β-phosphate. We have found that the loss of interactions from the β-phosphate can be counterbalanced by the presence of hydrophobic groups at a pyrroline ring acting as a surrogate of the carbohydrate unit. Two of the prepared glycomimetics show inhibition at a micromolar level.

Inhibitory properties of 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) derivatives acting on glycogen metabolising enzymes

Glycogen synthase (GS) and glycogen phosphorylase (GP) are the key enzymes that control, respectively, the synthesis and degradation of glycogen, a multi-branched glucose polymer that serves as a form of energy storage in bacteria, fungi and animals. An abnormal glycogen metabolism is associated with several human diseases. Thus, GS and GP constitute adequate pharmacological targets to modulate cellular glycogen levels by means of their selective inhibition. The compound 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) is a known potent inhibitor of GP. We studied the inhibitory effect of DAB, its enantiomer LAB, and 29 DAB derivatives on the activity of rat muscle glycogen phosphorylase (RMGP) and E. coli glycogen synthase (EcGS). The isoform 4 of sucrose synthase (SuSy4) from Solanum tuberosum L. was also included in the study for comparative purposes. Although these three enzymes possess highly conserved catalytic site architectures, the DAB derivatives analysed showed extremely diverse inhibitory potential. Subtle changes in the positions of crucial residues in their active sites are sufficient to discriminate among the structural differences of the tested inhibitors. For the two Leloir-type enzymes, EcGS and SuSy4, which use sugar nucleotides as donors, the inhibitory potency of the compounds analysed was synergistically enhanced by more than three orders of magnitude in the presence of ADP and UDP, respectively. Our results are consistent with a model in which these compounds bind to the subsite in the active centre of the enzymes that is normally occupied by the glucosyl residue which is transferred between donor and acceptor substrates. The ability to selectively inhibit the catalytic activity of the key enzymes of the glycogen metabolism may represent a new approach for the treatment of disorders of the glycogen metabolism.

ZnBr2-catalyzed directC-glycosylation of glycosyl acetates with terminal alkynes

C-Alkynyl glycosides were synthesizedviaZnBr₂-catalyzed cross-coupling of glycosyl acetates with terminal alkynes.

Molecular basis of lipid-linked oligosaccharide recognition and processing by bacterial oligosaccharyltransferase

Oligosaccharyltransferase (OST) is a membrane-integral enzyme that catalyzes the transfer of glycans from lipid-linked oligosaccharides (LLOs) onto asparagine side chains, the first step in protein N-glycosylation. Here, we report the X-ray structure of a single-subunit OST, PglB from Campylobacter lari, trapped in an intermediate state bound to an acceptor peptide and a synthetic LLO analog. The structure reveals the role of the external loop EL5, present in all OST enzymes, in substrate recognition. Whereas the N-terminal half of EL5 binds LLO, the C-terminal half interacts with the acceptor peptide. The glycan moiety of LLO must thread under EL5 to access the active site. Reducing EL5 mobility decreases the catalytic rate of OST when full-size heptasaccharide LLO is provided, but not for a monosaccharide-containing LLO analog. Our results define the chemistry of a ternary complex state, assign functional roles to conserved OST motifs, and provide opportunities for glycoengineering by rational design of PglB.

Square sugars: challenges and synthetic strategies

The synthesis of square sugars requires innovative strategies based on efficient stereoselective methodologies, from organocatalysis to metal carbene insertion.