Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

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.

Integrated Chemoenzymatic Approach to Streamline the Assembly of Complex Glycopeptides in the Liquid Phase

Glycosylation of proteins is a complicated post-translational modification. Despite the significant progress in glycoproteomics, accurate functions of glycoproteins are still ambiguous owing to the difficulty in obtaining homogeneous glycopeptides or glycoproteins. Here, we describe a streamlined chemoenzymatic method to prepare complex glycopeptides by integrating hydrophobic tag-supported chemical synthesis and enzymatic glycosylations. The hydrophobic tag is utilized to facilitate peptide chain elongation in the liquid phase and expeditious product separation. After removal of the tag, a series of glycans are installed on the peptides via efficient glycosyltransferase-catalyzed reactions. The general applicability and robustness of this approach are exemplified by efficient preparation of 16 well-defined SARS-CoV-2 O-glycopeptides, 4 complex MUC1 glycopeptides, and a 31-mer glycosylated glucagon-like peptide-1. Our developed approach will open up a new range of easy access to various complex glycopeptides of biological importance.

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)-promoted reduction of azides to amines under metal-free conditions

A simple and novel metal-free reduction of azides to amines in the presence of DBU is reported. This DBU-promoted transformation features good functional group tolerance and high chemo-selectivity.

Peptidyl ω-Asp Selenoesters Enable Efficient Synthesis of N-Linked Glycopeptides

Chemical synthesis is an attractive approach allows for the assembly of homogeneous complex N-linked glycopeptides and glycoproteins, but the limited coupling efficiency between glycans and peptides hampered the synthesis and research in the related field. Herein we developed an alternative glycosylation to construct N-linked glycopeptide via efficient selenoester-assisted aminolysis, which employs the peptidyl ω-asparagine selenoester and unprotected glycosylamine to perform rapid amide-bond ligation. This glycosylation strategy is highly compatible with the free carboxylic acids and hydroxyl groups of peptides and carbohydrates, and readily available for the assembly of structure-defined homogeneous N-linked glycopeptides, such as segments derived from glycoprotein EPO and IL-5.

Perfluorophenylboronic acid-catalyzed direct α-stereoselective synthesis of 2-deoxygalactosides from deactivated peracetylated d-galactal

Perfluorophenylboronic acid 1c catalyzes the direct stereoselective addition of alcohol nucleophiles to deactivated peracetylated d-galactal to give 2-deoxygalactosides in 55-88% yield with complete α-selectivity. The unprecedented results reported here also enable the synthesis of disaccharides containing the 2-deoxygalactose moiety directly from the deactivated peracetylated d-galactal. This convenient and metal-free glycosylation method works well with a wide range of alcohol nucleophiles as acceptors and tolerates a range of functional groups without the formation of the Ferrier byproduct and without the need for a large excess of nucleophiles or additives. The method is potentially useful for the synthesis of a variety of α-2-deoxygalactosides.

A Highly Efficient Ga(OTf)3/KI-Catalysed Transformation of aryl azides to aryl amines using low catalyst loading

A highly proficient transformation of aryl azides to aryl amines under Ga(OTf)3/KI catalysis is described.

Convergent Synthesis of Sialyl LewisX- O-Core-1 Threonine

Selectins are a class of cell adhesion molecules that play a critical role during the initial steps of inflammation. The N-terminal domain of P-selectin glycoprotein ligand-1 (PSGL-1) binds to all selectins, but with the highest affinity to P-selectin. Recent evidence suggests that the blockade of P-selectin/PSGL-1 interactions provides a viable therapeutic option for the treatment of many inflammatory diseases. Herein, we report the total synthesis of threonine bearing sialyl LewisX (sLeX) linked to a Core-1- O-hexasaccharide 1, as a key glycan of the N-terminal domain of PSGL-1. A convergent synthesis using α-selective sialylation and a regioselective [4+2] glycosylation are the key features of this synthesis.

Convergent Synthesis of N-Linked Glycopeptides via Aminolysis of ω-Asp p-Nitrophenyl Thioesters in Solution

An efficient N-linked glycosylation reaction between glycosylamines and p-nitrophenyl thioester peptides has been developed. The reaction conditions are mild and compatible with the C-terminal free carboxylic acid group and the unprotected N-linked sialyloligosaccharide. By means of this convergent strategy, a versatile N-glycopeptide fragment containing an N-terminal Thz and a C-terminal thioester was readily prepared, which is available for the synthesis of long glycopeptides and glycoproteins using the protocol of native chemical ligation.

Chemoenzymatic synthesis of α-dystroglycan core M1 O-mannose glycans

The diversity-oriented chemoenzymatic synthesis of α-dystroglycan (α-DG) core M1 O-mannose glycans has been achieved via a three-step sequential one-pot multienzyme (OPME) glycosylation of a chemically prepared disaccharyl serine intermediate. The high flexibility and efficiency of this chemoenzymatic strategy was demonstrated for the synthesis of three more complex core M1 O-mannose glycans for the first time along with three previously reported core M1 structures.

A convergent strategy for the synthesis of type-1 elongated mucin cores 1-3 and the corresponding glycopeptides

Mucins are a class of highly O-glycosylated proteins found on the surface of cells in epithelial tissues. O-Glycosylation is crucial for the functionality of mucins and changes therein can have severe consequences for an organism. With that in mind, the elucidation of interactions of carbohydrate binding proteins with mucins, whether in morbidly altered or unaltered conditions, continue to shed light on mechanisms involved in diseases like chronic inflammations and cancer. Despite the known importance of type-1 and type-2 elongated mucin cores 1-4 in glycobiology, the corresponding type-1 structures are much less well studied. Here, the first chemical synthesis of extended mucin type-1 O-glycan core 1-3 amino acid structures based on a convergent approach is presented. By utilizing differentiation in acceptor reactivity, shared early stage Tn- and T-acceptor intermediates were elongated with a common type-1 [β-D-Gal-1,3-β-D-GlcNAc] disaccharide, which allows for straightforward preparation of diverse glycosylated amino acids carrying the type-1 mucin core 1-3 saccharides. The obtained glycosylated 9-fluorenylmethoxycarbonyl (Fmoc)-protected amino acid building blocks were employed in synthesis of type-1 mucin glycopeptides, which are useful in biological applications.

A unified strategy for the synthesis of mucin cores 1-4 saccharides and the assembled multivalent glycopeptides

By displaying different O-glycans in a multivalent mode, mucin and mucin-like glycoproteins are involved in a plethora of protein binding events. The understanding of the roles of the glycans and the identification of potential glycan binding proteins are major challenges. To enable future binding studies of mucin glycan and glycopeptide probes, a method that gives flexible and efficient access to all common mucin core-glycosylated amino acids was developed. Based on a convergent synthesis strategy starting from a shared early stage intermediate by differentiation in the glycoside acceptor reactivity, a common disaccharide building block allows for the creation of extended glycosylated amino acids carrying the mucin type-2 cores 1-4 saccharides. Formation of a phenyl-sulfenyl-N-Troc (Troc=trichloroethoxycarbonyl) byproduct during N-iodosuccinimide-promoted thioglycoside couplings was further characterized and a new methodology for the removal of the Troc group is described. The obtained glycosylated 9-fluorenylmethoxycarbonyl (Fmoc)-protected amino acid building blocks are incorporated into peptides for multivalent glycan display.

The development of synthetic antitumour vaccines from mucin glycopeptide antigens

Based on important cell-biological and biochemical results concerning the structural difference between membrane glycoproteins of normal epithelial cells and epithelial tumour cells, tumour-associated glycopeptide antigens have been chemically synthesised and structurally confirmed. Glycopeptide structures of the tandem repeat sequence of mucin MUC1 of epithelial tumour cells constitute the most promising tumour-associated antigens. In order to generate a sufficient immunogenicity of these endogenous structures, usually tolerated by the immune system, these synthetic glycopeptide antigens were conjugated to immune stimulating components: in fully synthetic two-component vaccines either with T-cell peptide epitopes or with Toll-like receptor2 lipopeptide ligands or in three-component vaccines with both these stimulants. Alternatively, the synthetic glycopeptide antigens were coupled to immune stimulating carrier proteins. In particular, MUC1 glycopeptide conjugates with Tetanus toxoid proved to be efficient vaccines inducing very strong immune responses in mice. The antibodies elicited with the fully synthetic vaccines showed selective recognition of the tumour-associated glycopeptides as was shown by neutralisation and micro-array binding experiments. After booster immunisations, most of the immune responses showed the installation of an immunological memory. Immunisation with fully synthetic three-component vaccines induced immune reactions with therapeutic effects in terms of reduction of the tumour burden in mice or in killing of tumour cells in culture, while MUC1 glycopeptide-Tetanus toxoid vaccines elicited antibodies in mice which recognised tumour cells in human tumour tissues. The results achieved so far are considered to be promising for the development of an active immunisation against tumours.

Chemical assembly of N-glycoproteins: a refined toolbox to address a ubiquitous posttranslational modification

Incremental developments in the chemistry of peptides, proteins and carbohydrates have enabled researchers to assemble entire glycoproteins with high precision. Based on sophisticated ligation chemistries pure glycoproteins bearing a single glycosylation pattern have become available. The impact of N-glycosylation on the function of glycoproteins is generally recognized but not well understood. Based on the recent advances in the synthesis of glycoproteins by chemical methods researchers can finally start to elucidate the various roles of carbohydrates in complex biomolecules in detail.

An advance in the chemical synthesis of homogeneous N-linked glycopolypeptides by convergent aspartylation

We describe a useful advance in glycopeptide synthesis. We have developed a one-flask aspartylation/deprotection method, wherein long peptide fragments, bearing proximal pseudoproline functionality are merged with complex glycan domains. Following aspartylation, acidmediated global deprotection reveals the elaborated glycopeptide. The temporary pseudoproline functionality serves to suppress formation of aspartimide side products during solid phase peptide synthesis and aspartylation.

Structural basis of multivalent binding to wheat germ agglutinin

The inhibition of carbohydrate-protein interactions by tailored multivalent ligands is a powerful strategy for the treatment of many human diseases. Crucial for the success of this approach is an understanding of the molecular mechanisms as to how a binding enhancement of a multivalent ligand is achieved. We have synthesized a series of multivalent N-acetylglucosamine (GlcNAc) derivatives and studied their interaction with the plant lectin wheat germ agglutinin (WGA) by an enzyme-linked lectin assay (ELLA) and X-ray crystallography. The solution conformation of one ligand was determined by NMR spectroscopy. Employing a GlcNAc carbamate motif with alpha-configuration and by systematic variation of the spacer length, we were able to identify divalent ligands with unprecedented high WGA binding potency. The best divalent ligand has an IC(50) value of 9.8 microM (ELLA) corresponding to a relative potency of 2350 (1170 on a valency-corrected basis, i.e., per mol sugar contained) compared to free GlcNAc. X-ray crystallography of the complex of WGA and the second best, closely related divalent ligand explains this activity. Four divalent molecules simultaneously bind to WGA with each ligand bridging adjacent binding sites. This shows for the first time that all eight sugar binding sites of the WGA dimer are simultaneously functional. We also report a tetravalent neoglycopeptide with an IC(50) value of 0.9 microM being 25,500 times higher than that of GlcNAc (6400 times per contained sugar) and the X-ray structure analysis of its complex with glutaraldehyde-cross-linked WGA. Comparison of the crystal structure and the solution NMR structure of the neoglycopeptide as well as results from the ELLA suggest that the conformation of the glycopeptide in solution is already preorganized in a way supporting multivalent binding to the protein. Our findings show that bridging adjacent protein binding sites by multivalent ligands is a valid strategy to find high-affinity protein ligands and that even subtle changes of the linker structure can have a significant impact on the binding affinity.

Synthesis of novel DC-SIGN ligands with an alpha-fucosylamide anchor

The dendritic cell-specific intercellular adhesion molecule (ICAM) 3-grabbing nonintegrin (DC-SIGN) is a C-type lectin that appears to perform several different functions. Besides mediating adhesion between dendritic cells and T lymphocytes, DC-SIGN recognizes several pathogens some of which, including HIV, appear to exploit it to invade host organisms. The intriguing diversity of the roles attributed to DC-SIGN and their therapeutic implications have stimulated the search for new ligands that could be used as biological probes and possibly as lead compounds for drug development. The natural ligands of DC-SIGN consist of mannose oligosaccharides or fucose-containing Lewis-type determinants. Using the known 3D structure of the Lewis-x trisaccharide, we have identified some monovalent alpha-fucosylamides that bind to DC-SIGN with inhibitory constants 0.4-0.5 mM, as determined by SPR, and have characterized their interaction with the protein by STD NMR spectroscopy. This work establishes for the first time alpha-fucosylamides as functional mimics of chemically and enzymatically unstable alpha-fucosides and describes interesting candidates for the preparation of multivalent systems able to block the receptor DC-SIGN with high affinity and with potential biomedical applications.

Chemical preparation of sialyl Lewis x using an enzymatically synthesized sialoside building block

The sialyl Lewis x tetrasaccharide with a propylamine aglycon was assembled by chemoselective glycosylation from a p-tolyl thioglycosyl donor obtained from an enzymatically synthesized sialodisaccharide. Combining the advantages of highly efficient enzymatic synthesis of sialoside building blocks, and diverse chemical glycosylation, this chemoenzymatic approach is practical for obtaining complex sialosides and their analogues.

Preparation of glycosylated amino acids suitable for Fmoc solid-phase assembly

Many biological interactions and functions are mediated by glycans, consequently leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. Despite the challenges that carbohydrate moieties bring into the synthesis of glycopeptides and glycoproteins, considerable progress has been made during recent decades. Glycopeptides carrying many simple glycans have been chemically synthesized, enzymatic approaches have been utilized to introduce more complex glycans, and most recently native chemical ligation has enabled synthesis of glycoproteins from well-designed peptide and glycopeptide building blocks. Currently, general synthetic methodology for glycopeptides relies on preformed glycosylated amino acids for the stepwise solid-phase peptide synthesis. The formation of glycosidic bonds is of fundamental importance in the assembly of glycopeptides. As such, every glycosylation has to be regarded as a unique problem, demanding considerable systematic research. In this chapter we will summarize the most common chemical methods for the stereoselective synthesis of N- and O-glycosylated amino acids. The particular emphasis will be given to the preparation of building blocks for use in solid-phase glycopeptide synthesis based on the 9-fluorenylmethoxycarbonyl (Fmoc) protective group strategy.

A stereoselective synthesis of digitoxin and digitoxigen mono- and bisdigitoxoside from digitoxigenin via a palladium-catalyzed glycosylation

A convergent and stereocontrolled route to trisaccharide natural product digitoxin has been developed. The route is amenable to the preparation of both the digitoxigen mono- and bisdigitoxoside. This route featured the iterative application of the palladium-catalyzed glycosylation reaction, reductive 1,3-transposition, diastereoselective dihydroxylation, and regioselective protection. The natural product digitoxin was fashioned in 15 steps starting from digitoxigenin 2 and pyranone 8a or 18 steps from achiral acylfuran.

Spacer-separated sialyl LewisX cyclopeptide conjugates as potential E-selectin ligands

Completely protected sialyl LewisX azide was synthesized from a neolactosamine azide precursor carrying a 3-O-allyloxycarbonyl group as the temporary protecting group. After its Pd(0)-catalyzed deprotection and stereoselective alpha-fucosylation, the obtained LewisX azide was subjected to O-deacetylation in the galactose unit and subsequent regio- and stereoselective sialylation. Reduction of the anomeric azido group afforded the sialyl LewisX amine building block. Two molecules of this tetrasaccharide ligand were conjugated to a preformed cyclooctapeptide containing two equidistant l-asparagine units equipped with carboxy-terminated tetraethyleneglycol side chains to give, after deprotection, the target glycopeptide conjugate. Preliminary biological evaluation of the synthesized bivalent sialyl LewisX cyclopeptide conjugate showed only slightly enhanced inhibition of E-selectin binding in spite of the given flexibility of the two linked saccharide determinants.

Highly regioselective synthesis of a 3-O-sulfonated arabino Lewisa asparagine building block suitable for glycopeptide synthesis

Using the stannylene method, the trisaccharide 2-acetamido-3-O-[6-O-benzyl-beta-D-galactopyranosyl]-4-O-[2,3,4-tri-O-benzyl-beta-D-arabinopyranosyl]-6-O-benzyl-2-deoxy-beta-D-glucopyranosyl azide was regioselectively sulfonated and, after reduction of the anomeric azide, coupled to Fmoc alpha-allyl aspartate. After Pd(0)-catalyzed deallylation, the sulfatyl Lewis(a) asparagine building block was obtained, suitable for solid-phase glycopeptide synthesis applying the fluoride labile PTMSEL linker system.

Glycopeptides as versatile tools for glycobiology

This review describes the recent advances in the field of glycopeptide and small glycoprotein synthesis. The strategies covered include chemical and chemoenzymatic synthesis, native chemical ligation (NCL), and expressed chemical ligation. The importance of glycopeptide synthesis is exemplified by giving the reader an overview of how versatile and important these well-defined glycopeptides are as tools in glycobiology.

Advances in glycoprotein synthesis

The development of chemical and enzymatic methods for the synthesis of homogeneous glycoproteins is a fascinating challenge at the interface between chemistry and biology. Discussed here are the currently available methods for preparation of homogeneous glycoproteins. These methods include (1) glycopeptide ligation; (2) glycoprotein remodeling; and (3) in vivo suppressor tRNA technology.

Glycopeptide and glycoprotein synthesis involving unprotected carbohydrate building blocks

This review summarizes the chemical and chemoenzymatic synthesis of glycopeptides and glycoproteins using unprotected carbohydrates as key intermediates. The synthetic methods covered herein include the convergent synthesis of glycopeptides by chemoselective ligation of peptides and free glycans, solution- and solid-phase synthesis of glycopeptides by sequential peptide elongation with unprotected glycosyl amino acids or short glycopeptides as building blocks, and the synthesis of glycopeptides by enzymatic and/or chemical elongation of the free glycans. The use of unprotected carbohydrates in these syntheses can circumvent the final-stage carbohydrate deprotection, lead to highly convergent synthetic designs, and more significantly, take advantage of the commercially available free glycans isolated from nature, which could considerably facilitate the synthesis of complex glycopeptides and glycoproteins.