Reverse thiophosphorylase activity of a glycoside phosphorylase in the synthesis of an unnatural Manβ1,4GlcNAc library

β-Mannosides are ubiquitous in nature, with diverse roles in many biological processes. Notably, Manβ1,4GlcNAc a constituent of the core N-glycan in eukaryotes was recently identified as an immune activator, highlighting its potential for use in immunotherapy. Despite their biological significance, the synthesis of β-mannosidic linkages remains one of the major challenges in glycoscience. Here we present a chemoenzymatic strategy that affords a series of novel unnatural Manβ1,4GlcNAc analogues using the β-1,4-d-mannosyl-N-acetyl-d-glucosamine phosphorylase, BT1033. We show that the presence of fluorine in the GlcNAc acceptor facilitates the formation of longer β-mannan-like glycans. We also pioneer a "reverse thiophosphorylase" enzymatic activity, favouring the synthesis of longer glycans by catalysing the formation of a phosphorolysis-stable thioglycoside linkage, an approach that may be generally applicable to other phosphorylases.

Multicolor Photoluminescent Carbon Dots à La Carte for Biomedical Applications

Dual-emission fluorescence probes that provide high sensitivity are key for biomedical diagnostic applications. Nontoxic carbon dots (CDs) are an emerging alternative to traditional fluorescent probes; however, robust and reproducible synthetic strategies are still needed to access materials with controlled emission profiles and improved fluorescence quantum yields (FQYs). Herein, we report a practical and general synthetic strategy to access dual-emission CDs with FQYs as high as 0.67 and green/blue, yellow/blue, or red/blue excitation-dependent emission profiles using common starting materials such as citric acid, cysteine, and co-dopants to bias the synthetic pathway. Structural and physicochemical analysis using nuclear magnetic resonance, absorbance and fluorescence spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy in addition to transmission electron and atomic force microscopy (TEM and AFM) is used to elucidate the material's composition which is responsible for the unique observed photoluminescence properties. Moreover, the utility of the probes is demonstrated in the clinical setting by the synthesis of green/blue emitting antibody-CD conjugates which are used for the immunohistochemical staining of human brain tissues of glioblastoma patients, showing detection under two different emission channels.

Molecular basis for bacterial N-glycosylation by a soluble HMW1C-like N-glycosyltransferase

Soluble HMW1C-like N-glycosyltransferases (NGTs) catalyze the glycosylation of Asn residues in proteins, a process fundamental for bacterial autoaggregation, adhesion and pathogenicity. However, our understanding of their molecular mechanisms is hindered by the lack of structures of enzymatic complexes. Here, we report structures of binary and ternary NGT complexes of Aggregatibacter aphrophilus NGT (AaNGT), revealing an essential dyad of basic/acidic residues located in the N-terminal all α-domain (AAD) that intimately recognizes the Thr residue within the conserved motif Asn0-X+1-Ser/Thr+2. Poor substrates and inhibitors such as UDP-galactose and UDP-glucose mimetics adopt non-productive conformations, decreasing or impeding catalysis. QM/MM simulations rationalize these results, showing that AaNGT follows a SN2 reaction mechanism in which the acceptor asparagine uses its imidic form for catalysis and the UDP-glucose phosphate group acts as a general base. These findings provide key insights into the mechanism of NGTs and will facilitate the design of structure-based inhibitors to treat diseases caused by non-typeable H. influenzae or other Gram-negative bacteria.

Carbon-based glyco-nanoplatforms: towards the next generation of glycan-based multivalent probes

Cell surface carbohydrates mediate a wide range of carbohydrate-protein interactions key to healthy and disease mechanisms. Many of such interactions are multivalent in nature and in order to study these processes at a molecular level, many glycan-presenting platforms have been developed over the years. Among those, carbon nanoforms such as graphene and their derivatives, carbon nanotubes, carbon dots and fullerenes, have become very attractive as biocompatible platforms that can mimic the multivalent presentation of biologically relevant glycosides. The most recent examples of carbon-based nanoplatforms and their applications developed over the last few years to study carbohydrate-mediate interactions in the context of cancer, bacterial and viral infections, among others, are highlighted in this review.

Reengineering of cancer cell surface charges can modulate cell migration

The ability to modulate the cell surface structure provides a powerful tool to understand fundamental processes and also to elicit desired cellular responses. Here we report the development of a new class of ‘clickable labels’ to reengineer the cell surface charges of live cells. The method relies on the use of metabolic oligosaccharide engineering (MOE) combined with chemo selective labeling of cell surface azido-containing sialic acids with dibenzocyclooctyne (DBCO) ionic-probes. Using this strategy, we demonstrate that reducing the negative charge induced by the overexpression of cell surface sialic acids in cancer cells leads to a reduction in cell migration without affecting drug supceptibility.

Reducing the negative charges induced by the overexpression of cell surface sialic acids using cationic clickable labels leads to a reduction in cancer cell migration without affecting drug supceptibility.

Carbon dot-based fluorescent antibody nanoprobes as brain tumour glioblastoma diagnostics

The development of efficient and sensitive tools for the detection of brain cancer in patients is of the utmost importance particularly because many of these tumours go undiagnosed until the disease has advanced and when treatment is less effective. Current strategies employ antibodies (Abs) to detect Glial Fibrillary Acid Protein (GFAP) in tissue samples, since GFAP is unique to the brain and not present in normal peripheral blood, and it relies on fluorescent reporters. Herein we describe a low cost, practical and general method for the labelling of proteins and antibodies with fluorescent carbon dots (CD) to generate diagnostic probes that are robust, photostable and applicable to the clinical setting. The two-step protocol relies on the conjugation of a dibenzocyclooctyne (DBCO)-functionalised CD with azide functionalised proteins by combining amide conjugation and strain promoted alkyne-azide cycloaddition (SPAAC) ligation chemistry. The new class of Ab-CD conjugates developed using this strategy was successfully used for the immunohistochemical staining of human brain tissues of patients with glioblastoma (GBM) validating the approach. Overall, these novel fluorescent probes offer a promising and versatile strategy in terms of costs, photostability and applicability which can be extended to other Abs and protein systems.

Glioma Stem-Like Cells and Metabolism: Potential for Novel Therapeutic Strategies

Glioma stem-like cells (GSCs) were first described as a population which may in part be resistant to traditional chemotherapeutic therapies and responsible for tumour regrowth. Knowledge of the underlying metabolic complexity governing GSC growth and function may point to potential differences between GSCs and the tumour bulk which could be harnessed clinically. There is an increasing interest in the direct/indirect targeting or reprogramming of GSC metabolism as a potential novel therapeutic approach in the adjuvant or recurrent setting to help overcome resistance which may be mediated by GSCs. In this review we will discuss stem-like models, interaction between metabolism and GSCs, and potential current and future strategies for overcoming GSC resistance.

Benefits of Chemical Sugar Modifications Introduced by Click Chemistry for Glycoproteomic Analyses

Mucin-type O-glycosylation is among the most complex post-translational modifications. Despite mediating many physiological processes, O-glycosylation remains understudied compared to other modifications, simply because the right analytical tools are lacking. In particular, analysis of intact O-glycopeptides by mass spectrometry is challenging for several reasons; O-glycosylation lacks a consensus motif, glycopeptides have low charge density which impairs ETD fragmentation, and the glycan structures modifying the peptides are unpredictable. Recently, we introduced chemically modified monosaccharide analogues that allowed selective tracking and characterization of mucin-type O-glycans after bioorthogonal derivatization with biotin-based enrichment handles. In doing so, we realized that the chemical modifications used in these studies have additional benefits that allow for improved analysis by tandem mass spectrometry. In this work, we built on this discovery by generating a series of new GalNAc analogue glycopeptides. We characterized the mass spectrometric signatures of these modified glycopeptides and their signature residues left by bioorthogonal reporter reagents. Our data indicate that chemical methods for glycopeptide profiling offer opportunities to optimize attributes such as increased charge state, higher charge density, and predictable fragmentation behavior.

Carbon Dots as an Emergent Class of Antimicrobial Agents

Antimicrobial resistance is a recognized global challenge. Tools for bacterial detection can combat antimicrobial resistance by facilitating evidence-based antibiotic prescribing, thus avoiding their overprescription, which contributes to the spread of resistance. Unfortunately, traditional culture-based identification methods take at least a day, while emerging alternatives are limited by high cost and a requirement for skilled operators. Moreover, photodynamic inactivation of bacteria promoted by photosensitisers could be considered as one of the most promising strategies in the fight against multidrug resistance pathogens. In this context, carbon dots (CDs) have been identified as a promising class of photosensitiser nanomaterials for the specific detection and inactivation of different bacterial species. CDs possess exceptional and tuneable chemical and photoelectric properties that make them excellent candidates for antibacterial theranostic applications, such as great chemical stability, high water solubility, low toxicity and excellent biocompatibility. In this review, we will summarize the most recent advances on the use of CDs as antimicrobial agents, including the most commonly used methodologies for CD and CD/composites syntheses and their antibacterial properties in both in vitro and in vivo models developed in the last 3 years.

Imidazolium labelling permits the sensitive mass-spectrometric detection of N-glycosides directly from serum

A novel imidazolium derivative (GITag) shows superior ionisation and consequently allows increased mass spectrometric detection capabilities of oligosaccharides and N-glycans. Here we demonstrate that human serum samples can be directly labelled by GITag on a MALDI target plate, abrogating prevalently required sample pretreatment or clean-up steps.

Recent advances in multivalent carbon nanoform-based glycoconjugates

Multivalent carbohydrate-mediated interactions are fundamental to many biological processes, including disease mechanisms. To study these significant glycan-mediated interactions at a molecular level, carbon nanoforms such as fullerenes, carbon nanotubes, or graphene and their derivatives have been identified as promising biocompatible scaffolds that can mimic the multivalent presentation of biologically relevant glycans. In this minireview, we will summarize the most relevant examples of the last few years in the context of their applications.

Dissecting the Structural and Chemical Determinants of the "Open-to-Closed" Motion in the Mannosyltransferase PimA from Mycobacteria

The phosphatidyl-myo-inositol mannosyltransferase A (PimA) is an essential peripheral membrane glycosyltransferase that initiates the biosynthetic pathway of phosphatidyl-myo-inositol mannosides (PIMs), key structural elements and virulence factors of Mycobacterium tuberculosis. PimA undergoes functionally important conformational changes, including (i) α-helix-to-β-strand and β-strand-to-α-helix transitions and (ii) an "open-to-closed" motion between the two Rossmann-fold domains, a conformational change that is necessary to generate a catalytically competent active site. In previous work, we established that GDP-Man and GDP stabilize the enzyme and facilitate the switch to a more compact active state. To determine the structural contribution of the mannose ring in such an activation mechanism, we analyzed a series of chemical derivatives, including mannose phosphate (Man-P) and mannose pyrophosphate-ribose (Man-PP-RIB), and additional GDP derivatives, such as pyrophosphate ribose (PP-RIB) and GMP, by the combined use of X-ray crystallography, limited proteolysis, circular dichroism, isothermal titration calorimetry, and small angle X-ray scattering methods. Although the β-phosphate is present, we found that the mannose ring, covalently attached to neither phosphate (Man-P) nor PP-RIB (Man-PP-RIB), does promote the switch to the active compact form of the enzyme. Therefore, the nucleotide moiety of GDP-Man, and not the sugar ring, facilitates the "open-to-closed" motion, with the β-phosphate group providing the high-affinity binding to PimA. Altogether, the experimental data contribute to a better understanding of the structural determinants involved in the "open-to-closed" motion not only observed in PimA but also visualized and/or predicted in other glycosyltransfeases. In addition, the experimental data might prove to be useful for the discovery and/or development of PimA and/or glycosyltransferase inhibitors.

Chemo-selective Rh-catalysed hydrogenation of azides into amines

Rh/Al₂O₃ can be used as an effective chemo-selective reductive catalyst that combines the mild conditions of catalytic hydrogenation with high selectivity for azide moieties in the presence of other hydrogenolysis labile groups such as benzyl and benzyloxycarbonyl functionalities. The practicality of this strategy is exemplified with a range of azide-containing carbohydrate and amino acid derivatives.

Exploiting structure-activity relationships of QS-21 in the design and synthesis of streamlined saponin vaccine adjuvants

We report the design, synthesis, immunological evaluation, and conformational analysis of new saponin variants as promising vaccine adjuvants. These studies have provided expedient synthetic access to streamlined adjuvant-active saponins and yielded molecular-level insights into saponin conformation that correlated with their in vivo adjuvant activities.

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.

Glycomimetics Targeting Glycosyltransferases: Synthetic, Computational and Structural Studies of Less-Polar Conjugates

The Leloir donors are nucleotide sugars essential for a variety of glycosyltransferases (GTs) involved in the transfer of a carbohydrate to an acceptor substrate, typically a protein or an oligosaccharide. A series of less-polar nucleotide sugar analogues derived from uridine have been prepared by replacing one phosphate unit with an alkyl chain. The methodology is based on the radical hydrophosphonylation of alkenes, which allows coupling of allyl glycosyl compounds with a phosphate unit suitable for conjugation to uridine. Two of these compounds, the GalNAc and galactose derivatives, were further tested on a model GT, such as GalNAc-T2 (an important GT widely distributed in human tissues), to probe that both compounds bound in the medium-high micromolar range. The crystal structure of GalNAc-T2 with the galactose derivative traps the enzyme in an inactive form; this suggests that compounds only containing the β-phosphate could be efficient ligands for the enzyme. Computational studies with GalNAc-T2 corroborate these findings and provide further insights into the mechanism of the catalytic cycle of this family of enzymes.

Recent advances on the enantioselective synthesis of C-nucleosides inhibitors of inosine monophosphate dehydrogenase (IMPDH)

This review will describe the recent advances in the synthesis of C-nucleosides with inhibitory activity of inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the biosynthesis of guanine nucleotides. The review will cover synthetic approaches of structural analogues showing modifications in the furanose ring as well as in the heterocyclic base. Heterocyclic sugar nucleoside analogues in which the furanose ring has been replaced by a different heterocyclic ring including aza analogues, thioanalogues as well as dioxolanyl and isoxazolidinyl analogues are also considered.