Synthesis of neoglycoproteins containing O-methylated trisaccharides related to excretory/secretory antigens of Toxocara larvae

The disulfide bond as a key motif for the construction of multivalent glycoclusters

S-Glycosylated dendrons having a thioacetate group in their focal points led to multivalent glycoclusters by spontaneous O2-oxidation of sulfides.

Pondering the Structural Factors that Affect 1,2-trans-Galactosylation: A Lesson Learnt from 3-O-β-Galactosylation of Galactosamine

Stereoselective formation of glycosidic bonds remains one of the most challenging topics in carbohydrate chemistry. The predominant method for stereoselective construction of 1,2-trans-glycosidic bonds is through the neighboring group participation effect (NGPE), which proved to be less successful in synthesizing Galβ(1→3)GalNAc disaccharide. The steric effect that overshadows NGPE and the impacts of substituents at the 3-O- and 2-N-positions of donors and acceptors, respectively, on this synthesis were systematically examined to lead to some practical guidelines for choosing protecting groups towards the successful synthesis of Galβ(1→3)GalNAc and similar disaccharides.

A protocol for metal triflate catalyzed direct glycosylations with GalNAc 1-OPiv donors

Herein we report on the development of novel glycosylation methodology for the concise synthesis of naturally occurring glycoconjugate motifs containing N-acetylgalactosamine (GalNAc) from the cheaper and commercially available N-acetylglucosamine (GlcNAc). The stereoselective glycosylations proceed with catalytic amounts of a promoter and without the need for N-protection other than the biologically relevant N-acetyl group. Among the catalysts explored, both Bi(OTf)3 and Fe(OTf)3 were found to be highly active Lewis acids for this reaction. It was also found that other less reactive metal triflates such as those of Cu(II) and Yb(III) can be beneficial in glycosylation reactions on more demanding glycosyl acceptors. We have furthermore demonstrated that it is possible to control the anomeric stereoselectivity in the glycosylation via postglycosylation in situ anomerization to obtain good yields of α-galactosides. The present protocol was used to prepare important naturally occurring carbohydrate motifs, including a trisaccharide fragment of the naturally occurring marine sponge clarhamnoside.

Synthesis, antigenicity against human sera and structure-activity relationships of carbohydrate moieties from Toxocara larvae and their analogues

Stereocontrolled syntheses of biotin-labeled oligosaccharide portions containing the Galβ1-3GalNAc core of the TES-glycoprotein antigen obtained from larvae of the parasite Toxocara and their analogues have been accomplished. Trisaccharides Fuc2Meα1-2Gal4Meβ1-3GalNAcα1-OR (A), Fucα1-2Gal4Meβ1-3GalNAcα1-OR (B), Fuc2Meα1-2Galβ1-3GalNAcα1-OR (C), Fucα1-2Galβ1-3GalNAcα1-OR (D) and a disaccharide Fuc2Meα1-2Gal4Meβ1-OR (E) (R = biotinylated probe) were synthesized by block synthesis using 5-(methoxycarbonyl)pentyl-2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl-(1→3)-2-azide-4-O-benzyl-2-deoxy-α-D-galactopyranoside as a common glycosyl acceptor. We examined the antigenicity of these five oligosaccharides by enzyme linked immunosorbent assay (ELISA). Our results demonstrate that the O-methyl groups in these oligosaccharides are important for their antigenicity and the biotinylated oligosaccharides A, B, C and E have high serodiagnostic potential to detect infections caused by Toxocara larvae.

Chemoenzymatic synthesis of multivalent neoglycoconjugates carrying the helminth glycan antigen LDNF

Several parasitic helminthes, such as the human parasite Schistosoma mansoni, express glycoconjugates that contain terminal GalNAc beta1-4(Fuc alpha1-3)GlcNAc beta-R (LDNF) moieties. These LDNF glycans are dominant antigens of the parasite and are recognized by human dendritic cells via the C-type lectin DC-SIGN. To study the functional role of the LDNF antigen in interaction with the immune system, we have developed an easy chemoenzymatic method to synthesize multivalent neoglycoconjugates carrying defined amounts of LDNF antigens. An acceptor substrate providing a terminal N-acetylglucosamine was prepared by coupling a fluorescent hydrophobic aglycon, 2,6-diaminopyridine (DAP), to N,N'-diacetylchitobiose. By the subsequent action of recombinant Caenorhabditis elegans beta1,4-N-acetylgalactosaminyltransferase and human alpha1,3-fucosyltransferase VI (FucT-VI), this substrate was converted to the LDNF antigen. We showed that human FucT-VI has a relatively high affinity for the unusual substrate GalNAc beta1-4GlcNAc (LDN), and this enzyme was used to produce micromolar amounts of LDNF-DAP. The synthesized LDNF-DAP was coupled to carrier protein via activation of the DAP moiety by diethyl squarate. By varying the molar glycan:protein ratio, neoglycoconjugates were constructed with defined amounts of LDNF, as was determined by MALDI-TOF analysis and ELISA using an anti-LDNF antibody.

Exploring the immunology of parasitism--from surface antigens to the hygiene hypothesis

Helminth immunology is a field which has changed beyond recognition in the past 30 years, transformed not only by new technologies from cDNA cloning to flow cytometry, but also conceptually as our definition of host immune pathways has matured. The molecular revolution defined key nematode surface and secreted antigens, and identified candidate immunomodulators that are likely to underpin parasites' success in eluding immune attack. The immunological advances in defining cytokine networks, lymphocyte subsets and innate cell recognition have also made a huge impact on our understanding of helminth infections. Most recently, the ideas of regulatory immune cells, in particular the regulatory T cell, have again overturned older thinking, but also may explain immune hyporesponsiveness observed in chronic helminth diseases, as well as the link to reduced allergic reactions observed in human and animal infections. The review concludes with a forward look to where we may make future advances towards the final eradication of helminth diseases.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

O-methylated glycans from Toxocara are specific targets for antibody binding in human and animal infections

The parasitic helminth Toxocara canis is a widely distributed nematode of mammals. Larval parasites, which infect a wide range of hosts including mice and humans, export glycosylated macromolecules bearing novel methylated oligosaccharide structures, similar to the mammalian blood group antigen H but bearing one or two O-methylated substitutions on the terminal fucose and subterminal galactose residues. We have studied the reactivity of synthetic forms of these glycans to parasite-specific antibodies and mammalian immune system lectins. Murine antibodies, generated to T. canis infection, predominantly recognise the mono-O-methylated form with the beta-configuration of the GalNAc residue (MoMbeta), and antibodies are entirely IgM isotype. The mAb Tcn-2 reproduces this pattern, and shows little reactivity to either the alpha isomer (MoMalpha) or the di-O-methylated form (DiM). Antibodies generated to helminth infections other than T. canis were unreactive with the glycans, except antibodies to other members of the Toxocara genus. Hence, the carbohydrate structures represent immunogenic, genus-specific antigens. Antibodies from human toxocariasis patients are reactive with the same sugars, although preferentially towards DiM. Sera from unrelated helminth infections do not react, confirming the status of these structures as Toxocara-specific glycans. The human dendritic cell lectin, DC-SIGN, was found to bind both Toxocara excretory/secretory products and mammalian blood group antigen H3. However, DC-SIGN did not bind the synthetic glycans, indicating additional non-methylated carbohydrates may also play a role in the interaction between T. canis and its host.

Biosynthesis of conjugatable saccharidic moieties of GM2 and GM3 gangliosides by engineered E. coli

Oligosaccharidic moieties of GM(2) and GM(3) gangliosides bearing an allyl or a propargyl aglycon, are efficiently biosynthesized on the gram scale by growing metabolically engineered Escherichia coli cells in the presence of the corresponding lactoside acceptors and sialic acid.

The synthesis of phosphorylated disaccharide components of the extracellular phosphomannan of Pichia (Hansenula) holstii NRRL Y-2448

Methods for the stereoselective synthesis of alpha-(1-->2)- and alpha-(1-->3)-linked 6(II)-O-phosphomannobiosides were developed. Two strategies were successfully employed: a D-mannosyl acceptor was coupled with a phosphorylated D-mannosyl trichloroacetimidate donor, or alternatively with a differentially 6-O-protected D-mannosyl trichloroacetimidate donor which, after glycosylation, was selectively deprotected and phosphorylated. Two target phosphomannobiosides intended for use in SAR studies of the antiangiogenic drug candidate PI-88, 2-O-(6-O-phospho-alpha-D-mannopyranosyl)-D-mannopyranose and methyl 3-O-(6-O-phospho-alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, were synthesized. The former is a minor component of the side-chain repeating unit of the extracellular phosphomannan of Pichia (Hansenula) holstii NRRL Y-2448, whilst the latter represents a nonreducing end fragment of the phosphomannan.

A new synthesis of the oligosaccharide domain of acarbose

Synthesis of the oligosaccharide domain of acarbose was reinvestigated and was optimally performed using a maltosidic acceptor, already bearing a alpha-D-Glc-(1-->4)-D-Glc bond, and a new D-fucopyranosyl donor. The crucial glycosylation step was improved by varying three different parameters and notably by focusing on the C-4 protecting group of the fucosyl residue, solvent and promoter. The resulting trisaccharide was further transformed into an electrophilic species in order to open further derivatization perspectives for designing new acarbose analogues. Substitution reactions were efficiently carried out with azide and thiocyanate anions. Two other potentially interesting trisaccharidic compounds were also synthesized, i.e. the C-4III amine and the corresponding isothiocyanate.