Convergent chemo-enzymatic synthesis of mannosylated glycopeptides; targeting of putative vaccine candidates to antigen presenting cells

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

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

Immunomodulatory glycomedicine: Introducing next generation cancer glycovaccines

Cancer remains a leading cause of death worldwide due to the lack of safer and more effective therapies. Cancer vaccines developed from neoantigens are an emerging strategy to promote protective and therapeutic anti-cancer immune responses. Advances in glycomics and glycoproteomics have unveiled several cancer-specific glycosignatures, holding tremendous potential to foster effective cancer glycovaccines. However, the immunosuppressive nature of tumours poses a major obstacle to vaccine-based immunotherapy. Chemical modification of tumour associated glycans, conjugation with immunogenic carriers and administration in combination with potent immune adjuvants constitute emerging strategies to address this bottleneck. Moreover, novel vaccine vehicles have been optimized to enhance immune responses against otherwise poorly immunogenic cancer epitopes. Nanovehicles have shown increased affinity for antigen presenting cells (APCs) in lymph nodes and tumours, while reducing treatment toxicity. Designs exploiting glycans recognized by APCs have further enhanced the delivery of antigenic payloads, improving glycovaccine's capacity to elicit innate and acquired immune responses. These solutions show potential to reduce tumour burden, while generating immunological memory. Building on this rationale, we provide a comprehensive overview on emerging cancer glycovaccines, emphasizing the potential of nanotechnology in this context. A roadmap towards clinical implementation is also delivered foreseeing advances in glycan-based immunomodulatory cancer medicine.

N-Glycan Engineering: Constructing the N-GlcNAc Stump

N-Glycosylation is often essential for the structure and function of proteins. However, N-glycosylated proteins from natural sources exhibit considerable heterogeneity in the appended oligosaccharides, bringing daunting challenges to corresponding basic research and therapeutic applications. To address this issue, various synthetic, enzymatic, and chemoenzymatic approaches have been elegantly designed. Utilizing the endoglycosidase-catalyzed transglycosylation method, a single N-acetylglucosamine (N-GlcNAc, analogous to a tree stump) on proteins can be converted to various homogeneous N-glycosylated forms, thereby becoming the focus of research efforts. In this concept article, we briefly introduce the methods that allow the generation of N-GlcNAc and its close analogues on proteins and peptides and highlight the current challenges and opportunities the scientific community is facing.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Biocatalyzed Synthesis of Glycostructures with Anti-infective Activity

Molecules containing carbohydrate moieties play essential roles in fighting a variety of bacterial and viral infections. Consequently, the design of new carbohydrate-containing drugs or vaccines has attracted great attention in recent years as means to target several infectious diseases.Conventional methods to produce these compounds face numerous challenges because their current production technology is based on chemical synthesis, which often requires several steps and uses environmentally unfriendly reactants, contaminant solvents, and inefficient protocols. The search for sustainable processes such as the use of biocatalysts and eco-friendly solvents is of vital importance. Therefore, their use in a variety of reactions leading to the production of pharmaceuticals has increased exponentially in the last years, fueled by recent advances in protein engineering, enzyme directed evolution, combinatorial biosynthesis, immobilization techniques, and flow biocatalysis. In glycochemistry and glycobiology, enzymes belonging to the families of glycosidases, glycosyltransferases (Gtfs), lipases, and, in the case of nucleoside and nucleotide analogues, also nucleoside phosphorylases (NPs) are the preferred choices as catalysts.In this Account, on the basis of our expertise, we will discuss the recent biocatalytic and sustainable approaches that have been employed to synthesize carbohydrate-based drugs, ranging from antiviral nucleosides and nucleotides to antibiotics with antibacterial activity and glycoconjugates such as neoglycoproteins (glycovaccines, GCVs) and glycodendrimers that are considered as very promising tools against viral and bacterial infections.In the first section, we will report the use of NPs and N-deoxyribosyltransferases for the development of transglycosylation processes aimed at the synthesis of nucleoside analogues with antiviral activity. The use of deoxyribonucleoside kinases and hydrolases for the modification of the sugar moiety of nucleosides has been widely investigated.Next, we will describe the results obtained using enzymes for the chemoenzymatic synthesis of glycoconjugates such as GCVs and glycodendrimers with antibacterial and antiviral activity. In this context, the search for efficient enzymatic syntheses represents an excellent strategy to produce structure-defined antigenic or immunogenic oligosaccharide analogues with high purity. Lipases, glycosidases, and Gtfs have been used for their preparation.Interestingly, many authors have proposed the use Gtfs originating from the biosynthesis of natural glycosylated antibiotics such as glycopeptides, macrolides, and aminoglycosides. These have been used in the chemoenzymatic semisynthesis of novel antibiotic derivatives by modification of the sugar moiety linked to their complex scaffold. These contributions will be described in the last section of this review because of their relevance in the fight against the spreading phenomenon of antibiotic resistance. In this context, the pioneering in vivo synthesis of novel derivatives obtained by genetic manipulation of producer strains (combinatorial biosynthesis) will be shortly described as well.All of these strategies provide a useful and environmentally friendly synthetic toolbox. Likewise, the field represents an illustrative example of how biocatalysis can contribute to the sustainable development of complex glycan-based therapies and how problems derived from the integration of natural tools in synthetic pathways can be efficiently tackled to afford high yields and selectivity. The use of enzymatic synthesis is becoming a reality in the pharmaceutical industry and in drug discovery to rapidly afford collections of new antibacterial or antiviral molecules with improved specificity and better metabolic stability.

Synthesis of glycopeptides and glycopeptide conjugates

Protein glycosylation is a key post-translational modification important to many facets of biology. Glycosylation can have critical effects on protein conformation, uptake and intracellular routing. In immunology, glycosylation of antigens has been shown to play a role in self/non-self distinction and the effective uptake of antigens. Improperly glycosylated proteins and peptide fragments, for instance those produced by cancerous cells, are also prime candidates for vaccine design. To study these processes, access to peptides bearing well-defined glycans is of critical importance. In this review, the key approaches towards synthetic, well-defined glycopeptides, are described, with a focus on peptides useful for and used in immunological studies. Special attention is given to the glycoconjugation approaches that have been developed in recent years, as these enable rapid synthesis of various (unnatural) glycopeptides, enabling powerful carbohydrate structure/activity studies. These techniques, combined with more traditional total synthesis and chemoenzymatic methods for the production of glycopeptides, should help unravel some of the complexities of glycobiology in the near future.

Controlled Decoration of [60]Fullerene with Polymannan Analogues and Amino Acid Derivatives through Malondiamide-Based Linkers

In the last few years, nanomaterials based on fullerene have begun to be considered promising tools in the development of efficient adjuvant/delivery systems for vaccination, thanks to their several advantages such as biocompatibility, size, and easy preparation and modification. In this work we reported the chemoenzymatic synthesis of natural polymannan analogues (di- and tri-mannan oligosaccharides characterized by α1,6man and/or α1,2man motifs) endowed with an anomeric propargyl group. These sugar derivatives were submitted to 1,3 Huisgen dipolar cycloaddition with a malondiamide-based chain equipped with two azido terminal groups. The obtained sugar-modified malondiamide derivatives were used to functionalize the surface of Buckminster fullerene (C₆₀) in a highly controlled fashion, and yields (11–41%) higher than those so far reported by employing analogue linkers. The same strategy has been exploited to obtain C₆₀ endowed with natural and unnatural amino acid derivatives. Finally, the first double functionalization of fullerene with both sugar- and amino acid-modified malondiamide chains was successfully performed, paving the way to the possible derivatization of fullerenes with immunogenic sugars and more complex antigenic peptides.

Protecting group free glycosylation: one-pot stereocontrolled access to 1,2-trans glycosides and (1→6)-linked disaccharides of 2-acetamido sugars

Unprotected 2-acetamido sugars may be directly converted into their oxazolines using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and a suitable base, in aqueous solution. Freeze drying and acid catalysed reaction with an alcohol as solvent produces the corresponding 1,2-trans-glycosides in good yield. Alternatively, dissolution in an aprotic solvent system and acidic activation in the presence of an excess of an unprotected glycoside as a glycosyl acceptor, results in the stereoselective formation of the corresponding 1,2-trans linked disaccharides without any protecting group manipulations. Reactions using aryl glycosides as acceptors are completely regioselective, producing only the (1→6)-linked disaccharides.

Emerging glyco-based strategies to steer immune responses

Glycan structures are common posttranslational modifications of proteins, which serve multiple important structural roles (for instance in protein folding), but also are crucial participants in cell-cell communications and in the regulation of immune responses. Through the interaction with glycan-binding receptors, glycans are able to affect the activation status of antigen-presenting cells, leading either to induction of pro-inflammatory responses or to suppression of immunity and instigation of immune tolerance. This unique feature of glycans has attracted the interest and spurred collaborations of glyco-chemists and glyco-immunologists to develop glycan-based tools as potential therapeutic approaches in the fight against diseases such as cancer and autoimmune conditions. In this review, we highlight emerging advances in this field, and in particular, we discuss on how glycan-modified conjugates or glycoengineered cells can be employed as targeting devices to direct tumor antigens to lectin receptors on antigen-presenting cells, like dendritic cells. In addition, we address how glycan-based nanoparticles can act as delivery platforms to enhance immune responses. Finally, we discuss some of the latest developments in glycan-based therapies, including chimeric antigen receptor (CAR)-T cells to achieve targeting of tumor-associated glycan-specific epitopes, as well as the use of glycan moieties to suppress ongoing immune responses, especially in the context of autoimmunity.

Chemical (neo)glycosylation of biological drugs

Biological drugs, specifically proteins and peptides, are a privileged class of medicinal agents and are characterized with high specificity and high potency of therapeutic activity. However, biologics are fragile and require special care during storage, and are often modified to optimize their pharmacokinetics in terms of proteolytic stability and blood residence half-life. In this review, we showcase glycosylation as a method to optimize biologics for storage and application. Specifically, we focus on chemical glycosylation as an approach to modify biological drugs. We present case studies that illustrate the success of this methodology and specifically address the highly important question: does connectivity within the glycoconjugate have to be native or not? We then present the innovative methods of chemical glycosylation of biologics and specifically highlight the emerging and established protecting group-free methodologies of glycosylation. We discuss thermodynamic origins of protein stabilization via glycosylation, and analyze in detail stabilization in terms of proteolytic stability, aggregation upon storage and/or heat treatment. Finally, we present a case study of protein modification using sialic acid-containing glycans to avoid hepatic clearance of biological drugs. This review aims to spur interest in chemical glycosylation as a facile, powerful tool to optimize proteins and peptides as medicinal agents.

Synthesis of Asparagine Derivatives Harboring a Lewis X Type DC-SIGN Ligand and Evaluation of their Impact on Immunomodulation in Multiple Sclerosis

The protein myelin oligodendrocyte glycoprotein (MOG) is a key component of myelin and an autoantigen in the disease multiple sclerosis (MS). Post‐translational N‐glycosylation of Asn₃₁ of MOG seems to play a key role in modulating the immune response towards myelin. This is mediated by the interaction of Lewis‐type glycan structures in the N‐glycan of MOG with the DC‐SIGN receptor on dendritic cells (DCs). Here, we report the synthesis of an unnatural Lewis X (Le^X)‐containing Fmoc‐SPPS‐compatible asparagine building block (SPPS=solid‐phase peptide synthesis), as well as asparagine building blocks containing two Le^X‐derived oligosaccharides: LacNAc and Fucα1‐3GlcNAc. These building blocks were used for the glycosylation of the immunodominant portion of MOG (MOG_31‐55) and analyzed with respect to their ability to bind to DC‐SIGN in different biological setups, as well as their ability to inhibit the citrullination‐induced aggregation of MOG_31‐55. Finally, a cytokine secretion assay was carried out on human monocyte‐derived DCs, which showed the ability of the neoglycopeptide decorated with a single Le^X to alter the balance of pro‐ and anti‐inflammatory cytokines, inducing a tolerogenic response.

Developing a Library of Mannose-Based Mono- and Disaccharides: A General Chemoenzymatic Approach to Monohydroxylated Building Blocks

Regioselective deprotection of acetylated mannose-based mono- and disaccharides differently functionalized in anomeric position was achieved by enzymatic hydrolysis. Candida rugosa lipase (CRL) and Bacillus pumilus acetyl xylan esterase (AXE) were immobilized on octyl-Sepharose and glyoxyl-agarose, respectively. The regioselectivity of the biocatalysts was affected by the sugar structure and functionalization in anomeric position. Generally, CRL was able to catalyze regioselective deprotection of acetylated monosaccharides in C6 position. When acetylated disaccharides were used as substrates, AXE exhibited a marked preference for the C2, or C6 position when C2 was involved in the glycosidic bond. By selecting the best enzyme for each substrate in terms of activity and regioselectivity, we prepared a small library of differently monohydroxylated building blocks that could be used as intermediates for the synthesis of mannosylated glycoconjugate vaccines targeting mannose receptors of antigen presenting cells.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross-talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll-like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C-type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

Going Native: Synthesis of Glycoproteins and Glycopeptides via Native Linkages To Study Glycan-Specific Roles in the Immune System

Glycosylation plays a myriad of roles in the immune system: Certain glycans can interact with specific immune receptors to kickstart a pro-inflammatory response, whereas other glycans can do precisely the opposite and ameliorate the immune response. Specific glycans and glycoforms can themselves become the targets of the adaptive immune system, leading to potent antiglycan responses that can lead to the killing of altered self- or pathogenic species. This hydra-like set of roles glycans play is of particular importance in cancer immunity, where it influences the anticancer immune response, likely playing pivotal roles in tumor survival or clearance. The complexity of carbohydrate biology requires synthetic access to glycoproteins and glycopeptides that harbor homogeneous glycans allowing the probing of these systems with high precision. One particular complicating factor in this is that these synthetic structures are required to be as close to the native structures as possible, as non-native linkages can themselves elicit immune responses. In this Review, we discuss examples and current strategies for the synthesis of natively linked single glycoforms of peptides and proteins that have enabled researchers to gain new insights into glycoimmunology, with a particular focus on the application of these reagents in cancer immunology.

Systematic Dual Targeting of Dendritic Cell C-Type Lectin Receptor DC-SIGN and TLR7 Using a Trifunctional Mannosylated Antigen

Dendritic cells (DCs) are important initiators of adaptive immunity, and they possess a multitude of Pattern Recognition Receptors (PRR) to generate an adequate T cell mediated immunity against invading pathogens. PRR ligands are frequently conjugated to tumor-associated antigens in a vaccination strategy to enhance the immune response toward such antigens. One of these PPRs, DC-SIGN, a member of the C-type lectin receptor (CLR) family, has been extensively targeted with Lewis structures and mannose glycans, often presented in multivalent fashion. We synthesized a library of well-defined mannosides (mono-, di-, and tri-mannosides), based on known "high mannose" structures, that we presented in a systematically increasing number of copies (n = 1, 2, 3, or 6), allowing us to simultaneously study the effect of mannoside configuration and multivalency on DC-SIGN binding via Surface Plasmon Resonance (SPR) and flow cytometry. Hexavalent presentation of the clusters showed the highest binding affinity, with the hexa-α1,2-di-mannoside being the most potent ligand. The four highest binding hexavalent mannoside structures were conjugated to a model melanoma gp100-peptide antigen and further equipped with a Toll-like receptor 7 (TLR7)-agonist as adjuvant for DC maturation, creating a trifunctional vaccine conjugate. Interestingly, DC-SIGN affinity of the mannoside clusters did not directly correlate with antigen presentation enhancing properties and the α1,2-di-mannoside cluster with the highest binding affinity in our library even hampered T cell activation. Overall, this systematic study has demonstrated that multivalent glycan presentation can improve DC-SIGN binding but enhanced binding cannot be directly translated into enhanced antigen presentation and the sole assessment of binding affinity is thus insufficient to determine further functional biological activity. Furthermore, we show that well-defined antigen conjugates combining two different PRR ligands can be generated in a modular fashion to increase the effectiveness of vaccine constructs.

Efficient synthesis and enzymatic extension of an N-GlcNAz asparagine building block

N-Azidoacetyl-d-glucosamine (GlcNAz) is a particularly useful tool in chemical biology as the azide is a metabolically stable yet accessible handle within biological systems. Herein, we report a practical synthesis of FmocAsn(N-Ac₃GlcNAz)OH, a building block for solid phase peptide synthesis (SPPS). Protecting group manipulations are minimised by taking advantage of the inherent chemoselectivity of phosphine-mediated azide reduction, and the resulting glycosyl amine is employed directly in the opening of Fmoc protected aspartic anhydride. We show potential application of the building block by establishing it as a substrate for enzymatic glycan extension using sugar oxazolines of varying size and biological significance with several endo-β-N-acetylglucosaminidases (ENGases). The added steric bulk resulting from incorporation of the azide is shown to have no or a minor impact on the yield of enzymatic glycan extension.

Meet the Board of ChemistryOpen: Antony J. Fairbanks

Antony J. Fairbanks is a Professor in the Department of Chemistry at the University of Canterbury in New Zealand. The research of his group focuses on the broad areas of organic synthesis, particularly applied to carbohydrates. He currently serves as an active Editorial Board member for ChemistryOpen.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Rational design, preparation and characterization of recombinant Ag85B variants and their glycoconjugates with T-cell antigenic activity against Mycobacterium tuberculosis

Tuberculosis is the deadliest infectious disease in the world. The variable efficacy of the current treatments highlights the need for more effective agents against this disease. In the past few years, we focused on the investigation of antigenic glycoconjugates starting from recombinant Ag85B (rAg85B), a potent protein antigen from Mycobacterium tuberculosis. In this paper, structural modifications were rationally designed in order to obtain a rAg85B variant protein able to maintain its immunogenicity after glycosylation. Lysine residues involved in the main T-epitope sequences (namely, K30 and K282) have been substituted with arginine to prevent their glycosylation by a lysine-specific reactive linker. The effectiveness of the mutation strategy and the detailed structure of resulting neo-glycoconjugates have been studied by intact mass spectrometry, followed by peptide and glycopeptide mapping. The effect of K30R and K282R mutations on the T-cell activity of rAg85B has also been investigated with a preliminary immunological evaluation performed by enzyme-linked immunospotting on the different variant proteins and their glycosylation products. After glycosylation, the two variant proteins with an arginine in position 30 completely retain the original T-cell activity, thus representing adequate antigenic carriers for the development of efficient glycoconjugate vaccines against tuberculosis.

The ENGases: versatile biocatalysts for the production of homogeneous N-linked glycopeptides and glycoproteins

The endo-β-N-acetylglucosaminidases (ENGases) are an enzyme class (EC 3.2.1.96) produced by a range of organisms, ranging from bacteria, through fungi, to higher order species, including humans, comprising two-sub families of glycosidases which all cleave the chitobiose core of N-linked glycans. Synthetic applications of these enzymes, i.e. to catalyse the reverse of their natural hydrolytic mode of action, allow the attachment of N-glycans to a wide variety of substrates which contain an N-acetylglucosamine (GlcNAc) residue to act as an 'acceptor' handle. The use of N-glycan oxazolines, high energy intermediates on the hydrolytic pathway, as activated donors allows their high yielding attachment to almost any amino acid, peptide or protein that contains a GlcNAc residue as an acceptor. The synthetic effectiveness of these biocatalysts has been significantly increased by the production of mutant glycosynthases; enzymes which can still catalyse synthetic processes using oxazolines as donors, but which do not hydrolyse the reaction products. ENGase biocatalysts are now finding burgeoning application for the production of biologically active glycopeptides and glycoproteins, including therapeutic monoclonal antibodies (mAbs) for which the oligosaccharides have been remodelled to optimise effector functions.

Synthetic and semi-synthetic approaches to unprotected N-glycan oxazolines

N-Glycan oxazolines have found widespread use as activated donor substrates for endo-β-N-acetylglucosaminidase (ENGase) enzymes, an important application that has correspondingly stimulated interest in their production, both by total synthesis and by semi-synthesis using oligosaccharides isolated from natural sources. Amongst the many synthetic approaches reported, the majority rely on the fabrication (either by total synthesis, or semi-synthesis from locust bean gum) of a key Manβ(1-4)GlcNAc disaccharide, which can then be elaborated at the 3- and 6-positions of the mannose unit using standard glycosylation chemistry. Early approaches subsequently relied on the Lewis acid catalysed conversion of peracetylated N-glycan oligosaccharides produced in this manner into their corresponding oxazolines, followed by global deprotection. However, a key breakthrough in the field has been the development by Shoda of 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and related reagents, which can direct convert an oligosaccharide with a 2-acetamido sugar at the reducing terminus directly into the corresponding oxazoline in water. Therefore, oxazoline formation can now be achieved in water as the final step of any synthetic sequence, obviating the need for any further protecting group manipulations, and simplifying synthetic strategies. As an alternative to total synthesis, significant quantities of several structurally complicated N-glycans can be isolated from natural sources, such as egg yolks and soy bean flour. Enzymatic transformations of these materials, in concert with DMC-mediated oxazoline formation as a final step, allow access to a selection of N-glycan oxazoline structures both in larger quantities and in a more expedient fashion than is achievable by total synthesis.

Synthesis of Glycosides by Glycosynthases

The many advances in glycoscience have more and more brought to light the crucial role of glycosides and glycoconjugates in biological processes. Their major influence on the functionality and stability of peptides, cell recognition, health and immunity and many other processes throughout biology has increased the demand for simple synthetic methods allowing the defined syntheses of target glycosides. Additional interest in glycoside synthesis has arisen with the prospect of producing sustainable materials from these abundant polymers. Enzymatic synthesis has proven itself to be a promising alternative to the laborious chemical synthesis of glycosides by avoiding the necessity of numerous protecting group strategies. Among the biocatalytic strategies, glycosynthases, genetically engineered glycosidases void of hydrolytic activity, have gained much interest in recent years, enabling not only the selective synthesis of small glycosides and glycoconjugates, but also the production of highly functionalized polysaccharides. This review provides a detailed overview over the glycosylation possibilities of the variety of glycosynthases produced until now, focusing on the transfer of the most common glucosyl-, galactosyl-, xylosyl-, mannosyl-, fucosyl-residues and of whole glycan blocks by the different glycosynthase enzyme variants.

Glycosylation of Recombinant Antigenic Proteins from Mycobacterium tuberculosis: In Silico Prediction of Protein Epitopes and Ex Vivo Biological Evaluation of New Semi-Synthetic Glycoconjugates

Tuberculosis is still one of the most deadly infectious diseases worldwide, and the use of conjugated antigens, obtained by combining antigenic oligosaccharides, such as the lipoarabinomannane (LAM), with antigenic proteins from Mycobacterium tuberculosis (MTB), has been proposed as a new strategy for developing efficient vaccines. In this work, we investigated the effect of the chemical glycosylation on two recombinant MTB proteins produced in E. coli with an additional seven-amino acid tag (recombinant Ag85B and TB10.4). Different semi-synthetic glycoconjugated derivatives were prepared, starting from mannose and two disaccharide analogs. The glycans were activated at the anomeric position with a thiocyanomethyl group, as required for protein glycosylation by selective reaction with lysines. The glycosylation sites and the ex vivo evaluation of the immunogenic activity of the different neo-glycoproteins were investigated. Glycosylation does not modify the immunological activity of the TB10.4 protein. Similarly, Ag85B maintains its B-cell activity after glycosylation while showing a significant reduction in the T-cell response. The results were correlated with the putative B- and T-cell epitopes, predicted using a combination of in silico systems. In the recombinant TB10.4, the unique lysine is not included in any T-cell epitope. Lys30 of Ag85B, identified as the main glycosylation site, proved to be the most important site involved in the formation of T-cell epitopes, reasonably explaining why its glycosylation strongly influenced the T-cell activity. Furthermore, additional lysines included in different epitopes (Lys103, -123 and -282) are also glycosylated. In contrast, B-cell epitopic lysines of Ag85B were found to be poorly glycosylated and, thus, the antibody interaction of Ag85B was only marginally affected after coupling with mono- or disaccharides.

Chemo-enzymatic synthesis of a glycosylated peptide containing a complex N-glycan based on unprotected oligosaccharides by using DMT-MM and Endo-M

For chemo-enzymatic synthesis of a glycosylated peptide, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) was used for the synthesis of a N-acetylglucosaminyl peptide and a pseudoglycopeptide by solid-phase peptide synthesis without the requirement of protecting groups on the carbohydrate. We also performed transglycosylation of an N-glycan to the N-acetylglucosaminyl peptide using endo-β-N-acetylglucosaminidase from Mucor hiemalis (Endo-M) to synthesize a glycopeptide containing a complex N-glycan.

Advances in Enzymatic Glycoside Synthesis

A robust platform for facile defined glycan synthesis does not exist. Yet the need for such technology has never been greater as researchers seek to understand the full scope of carbohydrate function, stretching beyond the classical roles of structure and energy storage to encompass highly nuanced cell signaling events. To comprehensively explore and exploit the full diversity of carbohydrate functions, we must first be able to synthesize them in a controlled manner. Toward this goal, traditional chemical syntheses are inefficient while nature's own synthetic enzymes, the glycosyl transferases, can be challenging to express and expensive to employ on scale. Glycoside hydrolases represent a pool of glycan processing enzymes that can be either used in a transglycosylation mode or, better, engineered to function as "glycosynthases," mutant enzymes capable of assembling glycosides. Glycosynthases grant access to valuable glycans that act as functional and structural probes or indeed as inhibitors and therapeutics in their own right. The remodelling of glycosylation patterns in therapeutic proteins via glycoside hydrolases and their mutants is an exciting frontier in both basic research and industrial scale processes.

Glycosylation, an effective synthetic strategy to improve the bioavailability of therapeutic peptides

Glycosylation of peptides is a promising strategy for modulating the physicochemical properties of peptide drugs and for improving their absorption through biological membranes. This review highlights various methods for the synthesis of glycoconjugates and recent progress in the development of glycosylated peptide therapeutics. Furthermore, the impacts of glycosylation in overcoming the existing barriers that restrict oral and brain delivery of peptides are described herein.

Synthesis of a hybrid type N-glycan heptasaccharide oxazoline for Endo M catalysed glycosylation

Endo-β-N-acetylglucosaminidases (ENGases) are versatile biocatalysts that allow access to a wide variety of defined homogenous N-linked glycoconjugates in a convergent manner. A hybrid-type N-glycan was accessed by total synthesis, converted to an oxazoline, and used as a donor substrate with both wild type Endo M and an N175Q glycosynthase Endo M mutant allowing the convergent synthesis of a glycosylated amino acid bearing a hybrid N-glycan structure.

Peptide-based synthetic vaccines

Classically all vaccines were produced using live or attenuated microorganisms or parts of them. However, the use of whole organisms, their components or the biological process for vaccine production has several weaknesses. The presence of immunologically redundant biological components or biological impurities in such vaccines might cause major problems. All the disadvantageous of traditional vaccines might be overcome via the development of fully synthetic peptide-based vaccines. However, once minimal antigenic epitopes only are applied for immunisation, the immune responses are poor. The use of an adjuvant can overcome this obstacle; however, it may raise new glitches. Here we briefly summarise the current stand on peptide-based vaccines, discuss epitope and adjuvant design, and multi-epitope and nanoparticle-based vaccine approaches. This mini review discusses also the disadvantages and benefits associated with peptide-based vaccines. It proposes possible methods to overcome the weaknesses of the synthetic vaccine strategy and suggests future directions for its development.

New chemosynthetic route to linear ε-poly-lysine

ε-Poly-lysine (ε-PL) is an uncommon cationic, naturally-occurring homopolymer produced by the fermentation process. Due to its significant antimicrobial activity and nontoxicity to humans, ε-PL is now industrially produced as an additive, e.g. for food and cosmetics. However, the biosynthetic route can only make polymers with a molecular weight of about 3 kDa. Here, we report a new chemical strategy based on ring-opening polymerization (ROP) to obtain ε-PL from lysine. The 2,5-dimethylpyrrole protected α-amino-ε-caprolactam monomer was prepared through cyclization of lysine followed by protection. ROP of this monomer, followed by the removal of the protecting group, 2,5-dimethylpyrrole, ultimately yielded ε-PL with varying molecular weights. The structure of this chemosynthetic ε-PL has been fully characterized by 1H NMR, 13C NMR, and MALDI-TOF MS analyses. This chemosynthetic ε-PL exhibited a similar pKa value and low cytotoxicity as the biosynthetic analogue. Using this new chemical strategy involving ROP without the need for phosgene may enable a more cost effective production of ε-PL on a larger-scale, facilitating the design of more advanced biomaterials.