Modified Hederagenin Derivatives Demonstrate Ex Vivo Anthelmintic Activity against Fasciola hepatica

Infection with Fasciola hepatica (liver fluke) causes fasciolosis (or fascioliasis) and poses a considerable economic as well as welfare burden to both the agricultural and animal health sectors. Here, we explore the ex vivo anthelmintic potential of synthetic derivatives of hederagenin, isolated in bulk from Hedera helix. Thirty-six compounds were initially screened against F. hepatica newly excysted juveniles (NEJs) of the Italian strain. Eleven of these compounds were active against NEJs and were selected for further study, using adult F. hepatica derived from a local abattoir (provenance unknown). From these eleven compounds, six demonstrated activity and were further assessed against immature liver flukes of the Italian strain. Subsequently, the most active compounds (n = 5) were further evaluated in ex vivo dose response experiments against adult Italian strain liver flukes. Overall, MC042 was identified as the most active molecule and the EC₅₀ obtained from immature and adult liver fluke assays (at 24 h post co-culture) are estimated as 1.07 μM and 13.02 μM, respectively. When compared to the in vitro cytotoxicity of MDBK bovine cell line, MC042 demonstrated the highest anthelmintic selectivity (44.37 for immature and 3.64 for adult flukes). These data indicate that modified hederagenins display properties suitable for further investigations as candidate flukicides.

Synthesis of a B-Antigen Hexasaccharide, a B-Lewis b Heptasaccharide and Glycoconjugates Thereof to Investigate Binding Properties of Helicobacter pylori

Infecting the stomach of almost 50 % of people, Helicobacter pylori is a causative agent of gastritis, peptic ulcers and stomach cancers. Interactions between bacterial membrane-bound lectin, Blood group Antigen Binding Adhesin (BabA), and human blood group antigens are key in the initiation of infection. Herein, the synthesis of a B-antigen hexasaccharide (B6) and a B-Lewis b heptasaccharide (BLeb7) and Bovine Serum Albumin glycoconjugates thereof is reported to assess the binding properties and preferences of BabA from different strains. From a previously reported trisaccharide acceptor a versatile key Lacto-N-tetraose tetrasaccharide intermediate was synthesized, which allowed us to explore various routes to the final targets, either via initial introduction of fucosyl residues followed by introduction of the B-determinant or vice versa. The first approach proved unsuccessful, whereas the second afforded the target structures in good yields. Protein conjugation using isothiocyanate methodology allowed us to reach high glycan loadings (up to 23 per protein) to mimic multivalent displays encountered in Nature. Protein glycoconjugate inhibition binding studies were performed with H. pylori strains displaying high or low affinity for Lewis b hexasaccharide structures showing that the binding to the high affinity strain was reduced due to the presence of the B-determinant in the Bleb7-conjugates and further reduced by the absence of the Lewis fucose residue in the B6-conjugate.

An investigation of the antileishmanial properties of semi-synthetic saponins

Leishmaniasis is a neglected tropical disease caused by insect-vector borne protozoan parasites of the, Leishmania species. Whilst infection threatens and affects millions of the global poor, vaccines are absent and drug therapy limited. Extensive efforts have recently been made to discover new leads from small molecule synthetic compound libraries held by industry; however, the number of new chemical entities identified and entering development as anti-leishmanials has been very low. This has led to increased interest in the possibility of discovering naturally derived compounds with potent antileishmanial activity which may be developed towards clinical applications. Plant-derived triterpenoid and steroidal saponins have long been considered as anti-microbials and here we describe an investigation of a library of 137 natural (9) and semi-synthetic saponins (128) for activity against Leishmania mexicana, a causative agent of cutaneous leishmaniasis. The triterpenoid sapogenin, hederagenin, readily obtained in large quantities from Hedera helix (common ivy), was converted into a range of 128 derivatives. These semi-synthetic compounds, as well as saponins isolated from ivy, were examined with a phenotypic screening approach to identify potent and selective anti-leishmanial hits. This led to the identification of 12 compounds, including the natural saponin gypsogenin, demonstrating high potency (ED50 < 10.5 μM) against axenic L. mexicana amastigotes, the mammalian pathogenic form. One of these, hederagenin disuccinate, was sufficiently non-toxic to the macrophage host cell to facilitate further analyses, selectivity index (SI) > 10. Whilst this was not active in an infected cell model, the anti-leishmanial properties of hederagenin-derivatives have been demonstrated, and the possibility of improving the selectivity of natural hederagenin through chemical modification has been established.

Synthesis of type 1 Lewis b hexasaccharide antigen structures featuring flexible incorporation of l-[U-13C6]-fucose for NMR binding studies

While 13C-labelled proteins are common tools in NMR studies, lack of access to 13C-labelled carbohydrate structures has restricted their use. l-Fucose is involved in a wide range of physiological and pathophysiological processes in mammalian organisms. Here, l-[U-13C6]-Fuc labelled type I Lewis b (Leb) structures have been synthesised for use in NMR binding studies with the Blood-group Antigen Binding Adhesin (BabA), a membrane-bound protein from the bacterium Helicobacter pylori. As part of this work, an efficient synthesis of a benzylated l-[U-13C6]-Fuc thioglycoside donor from l-[U-13C6]-Gal has been developed. The design and synthesis of an orthogonally protected tetrasaccharide precursor enabled controlled introduction of one or two 13C-labelled or non-labelled fucosyl residues prior to global deprotection. NMR analysis showed that it is straightforward to assign the anomeric centres as well as the H-5 positions to the individual fucosyl residues which are relevant for NMR binding studies.

Architecture and Evolution of Blade Assembly in β-propeller Lectins

Lectins with a β-propeller fold bind glycans on the cell surface through multivalent binding sites and appropriate directionality. These proteins are formed by repeats of short domains, raising questions about evolutionary duplication. However, these repeats are difficult to detect in translated genomes and seldom correctly annotated in sequence databases. To address these issues, we defined the blade signature of the five types of β-propellers using 3D-structural data. With these templates, we predicted 3,887 β-propeller lectins in 1,889 species and organized this information in a searchable online database. The data reveal a widespread distribution of β-propeller lectins across species. Prediction also emphasizes multiple architectures and led to the discovery of a β-propeller assembly scenario. This was confirmed by producing and characterizing a predicted protein coded in the genome of Kordia zhangzhouensis. The crystal structure uncovers an intermediate in the evolution of β-propeller assembly and demonstrates the power of our tools.

Improving the antiprotozoal effect of saponins in the rumen by combination with glycosidase inhibiting iminosugars or by modification of their chemical structure

The antiprotozoal effect of saponins is transitory, as when saponins are deglycosylated to sapogenins by rumen microorganisms they become inactive. We hypothesised that the combination of saponins with glycosidase-inhibiting iminosugars might potentially increase the effectiveness of saponins over time by preventing their deglycosylation in the rumen. Alternatively, modifying the structure of the saponins by substituting the sugar moiety with other small polar residues might maintain their activity as the sugar substitute would not be enzymatically cleaved. The aim of this in vitro study was to evaluate the acute antiprotozoal effect and the stability of this effect over a 24 h incubation period using ivy saponins, a stevia extract rich in iminosugars, ivy saponins with stevia extract, and a chemically modified ivy saponin, hederagenin bis-succinate (HBS). The effects on fermentation parameters and rumen bacterial communities were also studied. Ivy saponins with stevia and HBS had a greater antiprotozoal effect than ivy saponins, and this effect was maintained after 24 h of incubation (P<0.001). The combination of ivy and stevia extracts was more effective in shifting the fermentation pattern towards higher propionate (+39%) and lower butyrate (-32%) and lower ammonia concentration (-64%) than the extracts incubated separately. HBS caused a decrease in butyrate (-45%) and an increase in propionate (+43%) molar proportions. However, the decrease in ammonia concentration (-42%) observed in the presence of HBS was less than that caused by ivy saponins, either alone or with stevia. Whereas HBS and stevia impacted on bacterial population in terms of community structure, only HBS had an effect in terms of biodiversity (P<0.05). It was concluded that ivy saponins with stevia and the modified saponin HBS had a strong antiprotozoal effect, although they differed in their effects on fermentation parameters and bacteria communities. Ivy saponins combined with an iminosugar-rich stevia extract and/or HBS should be evaluated to determine their antiprotozoal effect in vivo.

Antiprotozoal Effect of Saponins in the Rumen Can Be Enhanced by Chemical Modifications in Their Structure

The antiprotozoal effect of saponins is transitory, as when saponins are deglycosylated to the sapogenin by rumen microorganisms they become inactive. We postulated that the substitution of the sugar moiety of the saponin with small polar residues would produce sapogen-like analogs which might be resistant to degradation in the rumen as they would not be enzymatically cleaved, allowing the antiprotozoal effect to persist over time. In this study, we used an acute assay based on the ability of protozoa to break down [¹⁴C] leucine-labeled Streptococcus bovis and a longer term assay based on protozoal motility over 24 h to evaluate both the antiprotozoal effect and the stability of this effect with fifteen hederagenin bis-esters esterified with two identical groups, and five cholesterol and cholic acid based derivatives carrying one to three succinate residues. The acute antiprotozoal effect of hederagenin derivatives was more pronounced than that of cholesterol and cholic acid derivatives. Modifications in the structure of hederagenin, cholesterol, and cholic acid derivatives resulted in compounds with different biological activities in terms of acute effect and stability, although those which were highly toxic to protozoa were not always the most stable over time. Most of the hederagenin bis-esters, and in particular hederagenin bis-succinate (TSB24), hederagenin bis-betainate dichloride (TSB37) and hederagenin bis-adipate (TSB47) had a persistent effect against rumen protozoa in vitro, shifting the fermentation pattern toward higher propionate and lower butyrate. These chemically modified triterpenes could potentially be used in ruminant diets as an effective defaunation agent to, ultimately, increase nitrogen utilization, decrease methane emissions, and enhance animal production. Further trials in vivo or in long term rumen simulators are now needed to confirm the in vitro observations presented.

Preparation and immunogenicity of gold glyco-nanoparticles as antipneumococcal vaccine model

AIM

Nanotechnology-based fully synthetic carbohydrate vaccines are promising alternatives to classic polysaccharide/protein conjugate vaccines. We have prepared gold glyco-nanoparticles (GNP) bearing two synthetic carbohydrate antigens related to serotypes 19F and 14 of Streptococcus pneumoniae and evaluated their immunogenicity in vivo.

RESULTS

A tetrasaccharide fragment of serotype 14 (Tetra-14), a trisaccharide fragment of serotype 19F (Tri-19F), a T-helper peptide and d -glucose were loaded onto GNP in different ratios. Mice immunization showed that the concomitant presence of Tri-19F and Tetra-14 on the same nanoparticle critically enhanced the titers of specific IgG antibodies toward type 14 polysaccharide compared with GNP exclusively displaying Tetra-14, while no IgG antibodies against type 19F polysaccharide were elicited.

CONCLUSION

This work is a step forward toward synthetic nanosystems combining carbohydrate antigens and immunogenic peptides as potential carbohydrate-based vaccines.

Structural Insights into Polymorphic ABO Glycan Binding by Helicobacter pylori

The Helicobacter pylori adhesin BabA binds mucosal ABO/Le(b) blood group (bg) carbohydrates. BabA facilitates bacterial attachment to gastric surfaces, increasing strain virulence and forming a recognized risk factor for peptic ulcers and gastric cancer. High sequence variation causes BabA functional diversity, but the underlying structural-molecular determinants are unknown. We generated X-ray structures of representative BabA isoforms that reveal a polymorphic, three-pronged Le(b) binding site. Two diversity loops, DL1 and DL2, provide adaptive control to binding affinity, notably ABO versus O bg preference. H. pylori strains can switch bg preference with single DL1 amino acid substitutions, and can coexpress functionally divergent BabA isoforms. The anchor point for receptor binding is the embrace of an ABO fucose residue by a disulfide-clasped loop, which is inactivated by reduction. Treatment with the redox-active pharmaceutic N-acetylcysteine lowers gastric mucosal neutrophil infiltration in H. pylori-infected Le(b)-expressing mice, providing perspectives on possible H. pylori eradication therapies.

Isolation and characterisation of 13 pterosins and pterosides from bracken (Pteridium aquilinum (L.) Kuhn) rhizome

Systematic phytochemical investigations of the underground rhizome of Pteridium aquilinum (L.) Kuhn (Dennstaedtiaceae) afforded thirty-five pterosins and pterosides. By detailed analysis of one- and two-dimensional nuclear magnetic resonance spectroscopy, circular dichroism (CD) and high-resolution mass spectrometric data, thirteen previously undescribed pterosins and pterosides have been identified. Interestingly, for the first time 12-O-β-D-glucopyranoside substituted pterosins, rhedynosides C and D, and the sulfate-containing pterosin, rhedynosin H, alongside the two known compounds, histiopterosin A and (2S)-pteroside A2, were isolated from the rhizomes of subsp. aquilinum of bracken. In addition, six-membered cyclic ether pterosins and pterosides, rhedynosin A and rhedynoside A, are the first examples of this type of pterosin-sesquiterpenoid. Additionally, the three previously reported compounds (rhedynosin I, (2S)-2-hydroxymethylpterosin E and (2S)-12-hydroxypterosin A) were obtained for the first time from plants as opposed to mammalian metabolic products. Single crystal X-ray diffraction analysis was applied to the previously undescribed compounds (2R)-rhedynoside B, (2R)-pteroside B and (2S)-pteroside K, yielding the first crystal structures for pterosides, and three known pterosins, (2S)-pterosin A, trans-pterosin C and cis-pterosin C. Rhedynosin C is the only example of the cyclic lactone pterosins with a keto group at position C-14. Six selected pterosins ((2S)-pterosin A, (2R)-pterosin B and trans-pterosin C) and associated glycosides ((2S)-pteroside A, (2R)-pteroside B and pteroside Z) were assessed for their anti-diabetic activity using an intestinal glucose uptake assay; all were found to be inactive at 300 μM.

A soluble fucose-specific lectin from Aspergillus fumigatus conidia--structure, specificity and possible role in fungal pathogenicity

Aspergillus fumigatus is an important allergen and opportunistic pathogen. Similarly to many other pathogens, it is able to produce lectins that may be involved in the host-pathogen interaction. We focused on the lectin AFL, which was prepared in recombinant form and characterized. Its binding properties were studied using hemagglutination and glycan array analysis. We determined the specificity of the lectin towards l-fucose and fucosylated oligosaccharides, including α1-6 linked core-fucose, which is an important marker for cancerogenesis. Other biologically relevant saccharides such as sialic acid, d-mannose or d-galactose were not bound. Blood group epitopes of the ABH and Lewis systems were recognized, Le(Y) being the preferred ligand among others. To provide a correlation between the observed functional characteristics and structural basis, AFL was crystallized in a complex with methyl-α,L-selenofucoside and its structure was solved using the SAD method. Six binding sites, each with different compositions, were identified per monomer and significant differences from the homologous AAL lectin were found. Structure-derived peptides were utilized to prepare anti-AFL polyclonal antibodies, which suggested the presence of AFL on the Aspergillus' conidia, confirming its expression in vivo. Stimulation of human bronchial cells by AFL led to IL-8 production in a dose-dependent manner. AFL thus probably contributes to the inflammatory response observed upon the exposure of a patient to A. fumigatus. The combination of affinity to human epithelial epitopes, production by conidia and pro-inflammatory activity is remarkable and shows that AFL might be an important virulence factor involved in an early stage of A. fumigatus infection.

Multivalent glycoconjugates as anti-pathogenic agents

Multivalency plays a major role in biological processes and particularly in the relationship between pathogenic microorganisms and their host that involves protein-glycan recognition. These interactions occur during the first steps of infection, for specific recognition between host and bacteria, but also at different stages of the immune response. The search for high-affinity ligands for studying such interactions involves the combination of carbohydrate head groups with different scaffolds and linkers generating multivalent glycocompounds with controlled spatial and topology parameters. By interfering with pathogen adhesion, such glycocompounds including glycopolymers, glycoclusters, glycodendrimers and glyconanoparticles have the potential to improve or replace antibiotic treatments that are now subverted by resistance. Multivalent glycoconjugates have also been used for stimulating the innate and adaptive immune systems, for example with carbohydrate-based vaccines. Bacteria present on their surfaces natural multivalent glycoconjugates such as lipopolysaccharides and S-layers that can also be exploited or targeted in anti-infectious strategies.

Gold nanoparticles as carriers for a synthetic Streptococcus pneumoniae type 14 conjugate vaccine

Aims: Coupling of capsular polysaccharides of pathogens to immunogenic protein carriers (conjugate vaccines) improves carbohydrate immune response. Our idea is to explore gold nanoclusters as carriers to prepare fully synthetic carbohydrate vaccines. Materials & methods: Gold glyconanoparticles bearing a synthetic tetrasaccharide epitope related to the Streptococcus pneumoniae type 14 capsular polysaccharide (Pn14PS), the T-helper ovalbumin 323–339 peptide (OVA323–339), and D-glucose were prepared by a one-pot method. Their immunogenicity was tested in mice. Cytokine levels after spleen cell stimulation with OVA323–339 were analyzed using a luminex-multiplex cytokine assay. The capacity of the evoked antibodies to promote the uptake of S. pneumoniae type 14 by leukocytes was assessed. Results & discussion: Glyconanoparticles containing 45% of tetrasaccharide and 5% OVA323–339 triggered specific anti-Pn14PS IgG antibodies. Cytokine levels confirmed that glyconanoparticles led to T-helper cell activation. The anti-saccharide antibodies promoted the phagocytosis of type 14 bacteria by human leukocytes, indicating the functionality of the antibodies. Conclusion: Gold nanoparticles have great potential as carriers for the development of a great diversity of fully synthetic carbohydrate-based vaccines.

Original submitted 17 May 2011; Revised submitted 27 July 2011

Glycocluster design for improved avidity and selectivity in blocking human lectin/plant toxin binding to glycoproteins and cells

Blocking lectin/toxin binding to human cells by suitable inhibitors can therapeutically protect them from harmful effects. Clustered design of ligand presentation holds the promise of affinity increase relative to the free sugar and inherent selectivity among lectin targets. Using first a solid-phase assay with a glycoprotein presenting N-glycans as lectin-reactive probe, we assessed the inhibitory potency of bi- to tetravalent clusters on a plant toxin and three human adhesion/growth-regulatory lectins. Enhanced avidity relative to the free sugar was detected together with lectin-type selectivity. These effects were confirmed on the level of cells in vitro, also for two leguminous lectins. The lack of toxicity in cell proliferation assays excluded concerns to further work on these compounds. The given cluster design and the strategic combination of the two assay systems of increasing biorelevance will thus be helpful to take the next steps in drug development, e.g. tailoring the sugar headgroup.

Synthesis of 6-PEtN-α-D-GalpNAc-(1->6)-β-D-Galp-(1->4)-β-D-GlcpNAc-(1->3)-β-D-Galp-(1->4)-β-D-Glcp, a Haemophilus influenzae lipopolysacharide structure, and biotin and protein conjugates thereof

Background: In bacteria with truncated lipopolysaccharide structures, i.e., lacking the O-antigen polysaccharide part, core structures are exposed to the immune system upon infection and thus their use as carbohydrate surface antigens in glycoconjugate vaccines can be considered and investigated. One such suggested structure from Haemophilus influenzae LPS is the phosphorylated pentasaccharide 6-PEtN-α-D-GalpNAc-(1→6)-β-D-Galp-(1→4)-β-D-GlcpNAc-(1→3)-β-D-Galp-(1→4)-β-D-Glcp.

Results: Starting from a spacer-containing lactose derivative a suitably protected lacto-N-neotetraose tetrasaccharide structure was constructed through subsequential couplings with two thioglycoside donors, a glucosamine residue followed by a galactose derivative, using NIS/AgOTf as promoter. Removal of a silyl protecting group at the primary position of the non-reducing end residue afforded an acceptor to which the terminal α-galactosamine moiety was introduced using a 2-azido bromo sugar and halide assisted coupling conditions. Global deprotection afforded the non-phosphorylated target pentasaccharide, whereas removal of a silyl group from the primary position of the non-reducing end residue produced a free hydroxy group which was phosphorylated using H-phosphonate chemistry to yield the phosphoethanolamine-containing protected pentasaccharide. Partial deprotection afforded the phosphorylated target pentasaccharide with a free spacer amino group but with a protected phosphoethanolamino group. Conjugation of the spacer amino group to biotin or dimethyl squarate followed by deprotection of the phosphoethanolamino group and, in the case of the squarate derivative, further reaction with a protein then afforded the title conjugates.

Conclusion: An effective synthesis of a biologically interesting pentasaccharide structure has been accomplished. The target pentasaccharide, an α-GalNAc substituted lacto-N-neotetraose structure, comprises a phosphoethanolamine motif and a spacer aglycon. Through the spacer, biotin and protein conjugates of the title compound have been constructed to allow further use in biological experiments.

Investigations of glycosylation reactions with 2-N-acetyl-2N,3O-oxazolidinone-protected glucosamine donors

NIS/AgOTf-promoted glycosylations with ethyl 2,3-N,O-carbonyl-2-deoxy-1-thio-beta-D-glucopyranoside donors can be performed with either alpha- or beta-selectivity by tuning the reaction conditions. Small amounts of AgOTf (0.1 equiv) and short reaction times give beta-selectivity, whereas 0.4 equiv of AgOTf and prolonged reaction times afford alpha-linked products. NMR-monitored glycosylation and anomerization experiments show initial formation of exclusively the beta-linkage, which anomerizes, through an intramolecular mechanism involving an endocyclic C-O bond cleavage, to the alpha-linkage.

NMR study of hydroxy protons of di- and trimannosides, substructures of Man-9

The chemical shifts, temperature coefficients and inter-residual rotating-frame Overhauser effect (ROE)s for the hydroxy protons of some alpha-(1,2)-, alpha-(1,3)- and alpha-(1,6)-linked di- and trimannosides have been measured for samples in 85% H2O/15% acetone-d6 solution. These mannosides, Manalpha(1-->2)ManalphaOMe (1) Manalpha(1-->3)ManalphaOMe (2), Manalpha(1-->6)ManalphaOMe (3), Manalpha(1-->2)Manalpha(1-->2)ManalphaOMe (4), Manalpha(1-->2)Manalpha(1-->3)ManalphaOMe (5), Manalpha(1-->2)Manalpha(1-->6)ManalphaOMe (6) and Manalpha(1-->3)[Manalpha1-->6]ManalphaOMe (7), are substructures of the N-glycan Man-9. The NMR data show that the hydration of each individual hydroxyl group in the di- and trisaccharides is very similar to the hydration of the corresponding hydroxyl in the monomeric methyl alpha-D-mannoside. No hydrogen-bond interactions were found to stabilize the conformations of the alpha-(1,2)- and alpha-(1,6)-linkages and the chemical shifts for the hydroxy proton resonances of the alpha-(1,6)-linkage indicated high-conformational flexibility. For the alpha-(1,3)-linkage, however, the downfield shift for the signal of O(2)H of the 3-substituted residue together with the ROE between this proton and H5' on the next residue suggest some weak inter-residue interactions.

Synthesis of urine drug metabolites: glucuronic acid glycosides of phenol intermediates

The investigation of drug metabolism requires substantial amount of metabolites. Isolation from urine is tedious, therefore, the material obtained by synthesis is preferred. Substantial amounts of three tentative drug metabolites, phenolic glucuronides, have been prepared using easily available glycosyl donors. The final products [3(2-N-methyl-N-isopropylaminoethoxy)phenyl] beta-D-glucopyranosiduronic acid, 4-amino-3,5-dimethylphenyl beta-D-glucopyranosiduronic acid and [2(S)-propanoyl-6-O-naphthyl] beta-D-glucopyranuronic acid are useful as, for example, reference material in metabolite investigations.

Synthesis of monodeoxy analogues of the trisaccharide α-d-Glcp-(1→3)-α-d-Manp-(1→2)-α-d-ManpOMe recognised by Calreticulin/Calnexin

Six (3,4,4',6',3'' or 6'')-monodeoxy analogues of the title trisaccharide (1-6) have been prepared utilising monodeoxy monosaccharide precursors. The reducing end deoxy derivatives were synthesised by N-iodosuccinimide/silver trifluoromethanesulfonate (NIS/AgOTf)-promoted couplings of a common disaccharide thioglycoside donor 10 to suitably protected monodeoxy acceptors 9 and 12, affording trisaccharides, which after deprotection yielded target structures 1 and 2. The non-reducing end deoxy derivatives could similarly be produced by halide-assisted glycosylations of a common disaccharide acceptor 17 with monodeoxy glycosyl bromide donors (obtained from thioglycosides 18 and 20) to yield, after removal of protecting groups, target trisaccharides 3 and 4. The analogues with the deoxy function in the middle mannose residue, were obtained through orthogonal halide-assisted coupling of tetrabenzyl-glucopyranosyl bromide to monodeoxy thioglycoside acceptors to give thioglycoside disaccharides, which subsequently were used as donors in NIS/AgOTf-promoted couplings to a common 2-hydroxy-mannose acceptor 15 to afford trisaccharides; deprotection yielded the final target compounds 5 and 6.

β-Propeller Crystal Structure of Psathyrella velutina Lectin: An Integrin-like Fungal Protein Interacting with Monosaccharides and Calcium

The lectin from the mushroom Psathyrella velutina recognises specifically N-acetylglucosamine and N-acetylneuraminic acid containing glycans. The crystal structure of the 401 amino acid residue lectin shows that it adopts a very regular seven-bladed beta-propeller fold with the N-terminal region tucked into the central cavity around the pseudo 7-fold axis. In the complex with N-acetylglucosamine, six monosaccharides are bound in pockets located between two consecutive propeller blades. Due to the repeats shown by the sequence the binding sites are very similar. Five hydrogen bonds between the protein and the sugar hydroxyl and N-acetyl groups stabilize the complex, together with the hydrophobic interactions with a conserved tyrosine and histidine. The complex with N-acetylneuraminic acid shows molecular mimicry with the same hydrogen bond network, but with different orientations of the carbohydrate ring in the binding site. The beta-hairpin loops connecting the two inner beta-strands of each blade are metal binding sites and two to three calcium ions were located in the structure. The multispecificity and high multivalency of this mushroom lectin, combined with its similarity to the extracellular domain of an important class of cell adhesion molecules, integrins, are another example of the outstanding success of beta-propeller structures as molecular binding machines in nature.

Synthesis of oligosaccharides corresponding to Vibrio cholerae O139 polysaccharide structures containing dideoxy sugars and a cyclic phosphate

A spacer-equipped tetrasaccharide, p-aminocyclohexylethyl alpha-l-Colp-(1-->2)-beta-d-Galp-(1-->3)-[alpha-l-Colp-(1-->4)]-beta-D-GlcpNAc, containing a 4,6-cyclic phosphate in the galactose residue, has been synthesised. The structure corresponds to a part of the repeating unit of the capsular (and lipo-) polysaccharide of the endemic bacteria Vibrio cholerae type O139 synonym Bengal. The synthetic strategy allows continuous syntheses of the complete O139 hexasaccharide repeating unit as well as of the structurally related repeating unit of serotype O22. Starting from ethyl 2-azido-4,6-O-benzylidene-2-deoxy-1-thio-beta-D-glucopyranoside, a thioglycoside tetrasaccharide donor block was constructed through two orthogonal glycosylations with glycosyl bromide donors. First, a properly protected galactose moiety was introduced using silver triflate as promoter and subsequently the two colitose residues, carrying electron-withdrawing protecting groups for stability reasons, under halide-assisted conditions. The tetrasaccharide block was then linked to the spacer in a NIS-TMSOTf-promoted coupling. Transformation of the azido group into an acetamido group using H2S followed by removal of temporary protecting acetyl groups gave a 4',6'-diol, which was next phosphorylated with methyl dichlorophosphate and deprotected to yield the 4,6-cyclic phosphate tetrasaccharide target structure.

Synthesis of the tetrasaccharide alpha-D-Glcp-(1-->3)-alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->2)-alpha-D-Manp recognized by calreticulin/calnexin

The title compound as its methyl glycoside was efficiently synthesized using a block synthesis approach. Halide-assisted glycosidations between 6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-glucopyranosyl iodide and ethyl 2-O-acetyl-4,6-di-O-benzyl-1-thio-alpha-D-mannopyranoside using triphenylphosphine oxide as promoter yielded, with complete alpha-selectivity, a disaccharide building block in high yield. The perbenzylated derivative of this proved to be an excellent donor affording 88% of the protected target tetrasaccharide in an NIS/AgOTf-promoted coupling to a known methyl dimannoside acceptor. Deprotection through catalytic hydrogenolysis then gave the target compound in 47% overall yield.

Impact of natural variation in bacterial F17G adhesins on crystallization behaviour

Since the introduction of structural genomics, the protein has been recognized as the most important variable in crystallization. Recent strategies to modify a protein to improve crystal quality have included rationally engineered point mutations, truncations, deletions and fusions. Five naturally occurring variants, differing in 1-18 amino acids, of the 177-residue lectin domain of the F17G fimbrial adhesin were expressed and purified in identical ways. For four out of the five variants crystals were obtained, mostly in non-isomorphous space groups, with diffraction limits ranging between 2.4 and 1.1 A resolution. A comparative analysis of the crystal-packing contacts revealed that the variable amino acids are often involved in lattice contacts and a single amino-acid substitution can suffice to radically change crystal packing. A statistical approach proved reliable to estimate the compatibilities of the variant sequences with the observed crystal forms. In conclusion, natural variation, universally present within prokaryotic species, is a valuable genetic resource that can be favourably employed to enhance the crystallization success rate with considerably less effort than other strategies.

Ligands of the asialoglycoprotein receptor for targeted gene delivery, part 1: Synthesis of and binding studies with biotinylated cluster glycosides containing N-acetylgalactosamine

In order to develop the non-viral Bioplex vector system for targeted delivery of genes to hepatocytes, we have evaluated the structure-function relationship for a number of synthetic ligands designed for specific interaction with the hepatic lectin ASGPr. Biotinylated ligand derivatives containing two, three or six beta-linked N-acetylgalactosamine (GalNAc) residues were synthesized, bound to fluorescent-labeled streptavidin and tested for binding and uptake to HepG2 cells using flow cytometry analysis (FACS). Uptake efficiency increased with number of displayed GalNAc units per ligand, in a receptor dependent manner. Thus, a derivative displaying six GalNAc units showed the highest uptake efficacy both in terms of number of internalizing cells and increased amount of material taken up per each cell. However, this higher efficiency was shown to be due not so much to higher number of sugar units, but to higher accessibility of the sugar units for interaction with the receptor (longer spacer). Improving the flexibility and accessibility of a trimeric GalNAc ligand through use of a longer spacer markedly influenced the uptake efficiency, while increasing the number of GalNAc units per ligand above three only provided a minor contribution to the overall affinity. We hereby report the details of the chemical synthesis of the ligands and the structure-function studies in vitro.

The fucose-binding lectin from Ralstonia solanacearum. A new type of beta-propeller architecture formed by oligomerization and interacting with fucoside, fucosyllactose, and plant xyloglucan

Plant pathogens, like animal ones, use protein-carbohydrate interactions in their strategy for host recognition, attachment, and invasion. The bacterium Ralstonia solanacearum, which is distributed worldwide and causes lethal wilt in many agricultural crops, was shown to produce a potent L-fucose-binding lectin, R. solanacearum lectin, a small protein of 90 amino acids with a tandem repeat in its amino acid sequence. In the present study, surface plasmon resonance experiments conducted on a series of oligosaccharides show a preference for binding to alphaFuc1-2Gal and alphaFuc1-6Gal epitopes. Titration microcalorimetry demonstrates the presence of two binding sites per monomer and an unusually high affinity of the lectin for alphaFuc1-2Gal-containing oligosaccharides (KD = 2.5 x 10(-7) M for 2-fucosyllactose). R. solanacearum lectin has been crystallized with a methyl derivative of fucose and with the highest affinity ligand, 2-fucosyllactose. X-ray crystal structures, the one with alpha-methyl-fucoside being at ultrahigh resolution, reveal that each monomer consists of two small four-stranded anti-parallel beta-sheets. Trimerization through a 3-fold or pseudo-3-fold axis generates a six-bladed beta-propeller architecture, very similar to that previously described for the fungal lectin of Aleuria aurantia. This is the first report of a beta-propeller formed by oligomerization and not by sequential domains. Each monomer presents two fucose binding sites, resulting in six symmetrically arranged sugar binding sites for the beta-propeller. Crystals were also obtained for a mutated lectin complexed with a fragment of xyloglucan, a fucosylated polysaccharide from the primary cell wall of plants, which may be the biological target of the lectin.

Ethyl 2-acetamido-4,6-di-O-benzyl-2,3-N,O-carbonyl-2-deoxy-1-thio-β-d-glycopyranoside as a versatile GlcNAc donor

The title donor, ethyl 2-acetamido-4,6-di-O-benzyl-2,3-N,O-carbonyl-2-deoxy-1-thio-beta-D-glycopyranoside, is shown to be an excellent glycosyl donor giving immediate and efficient access to variant GlcNAc-containing oligosaccharides.

Functional adaptation of BabA, the H. pylori ABO blood group antigen binding adhesin

Adherence by Helicobacter pylori increases the risk of gastric disease. Here, we report that more than 95% of strains that bind fucosylated blood group antigen bind A, B, and O antigens (generalists), whereas 60% of adherent South American Amerindian strains bind blood group O antigens best (specialists). This specialization coincides with the unique predominance of blood group O in these Amerindians. Strains differed about 1500-fold in binding affinities, and diversifying selection was evident in babA sequences. We propose that cycles of selection for increased and decreased bacterial adherence contribute to babA diversity and that these cycles have led to gradual replacement of generalist binding by specialist binding in blood group O-dominant human populations.

Synthesis of the Lewis b hexasaccharide and HSA-conjugates thereof

An efficient and short route has been elaborated for the aminopropyl spacer equipped Leb hexasaccharide. For the preparation of HSA-conjugates of this oligosaccharide, the use of disuccinimidyl suberate (DSS) and disuccinimidyl glutarate (DSG) as cross-linker reagents has been evaluated. This conjugation method emerged as being faster and easier to monitor by standard MALDI-TOF spectrometry than squarate ester based conjugations of similar efficiency if DSS is used as cross-linker.

SmI2/Water/Amine Mediates Cleavage of Allyl Ether Protected Alcohols: Application in Carbohydrate Synthesis and Mechanistic Considerations

[reaction: see text]. SmI2/H2O/amine provides selective cleavage of unsubstituted allyl ethers in good to excellent yields. This method is therefore useful in deprotection of alcohols and carbohydrates.

The fimbrial adhesin F17-G of enterotoxigenic Escherichia coli has an immunoglobulin-like lectin domain that binds N-acetylglucosamine

The F17-G adhesin at the tip of flexible F17 fimbriae of enterotoxigenic Escherichia coli mediates binding to N-acetyl-beta-D-glucosamine-presenting receptors on the microvilli of the intestinal epithelium of ruminants. We report the 1.7 A resolution crystal structure of the lectin domain of F17-G, both free and in complex with N-acetylglucosamine. The monosaccharide is bound on the side of the ellipsoid-shaped protein in a conserved site around which all natural variations of F17-G are clustered. A model is proposed for the interaction between F17-fimbriated E. coli and microvilli with enhanced affinity compared with the binding constant we determined for F17-G binding to N-acetylglucosamine (0.85 mM-1). Unexpectedly, the F17-G structure reveals that the lectin domains of the F17-G, PapGII and FimH fimbrial adhesins all share the immunoglobulin-like fold of the structural components (pilins) of their fimbriae, despite lack of any sequence identity. Fold comparisons with pilin and chaperone structures of the chaperone/usher pathway highlight the central role of the C-terminal beta-strand G of the immunoglobulin-like fold and provides new insights into pilus assembly, function and adhesion.

Solving the phase problem for carbohydrate-binding proteins using selenium derivatives of their ligands: a case study involving the bacterial F17-G adhesin

The Escherichia coli adhesin F17-G is a carbohydrate-binding protein that allows the bacterium to attach to the intestinal epithelium of young ruminants. The structure of the 17 kDa lectin domain of F17-G was determined using the anomalous dispersion signal of a selenium-containing analogue of the monosaccharide ligand N-acetyl-d-glucosamine in which the anomeric oxygen was replaced by an Se atom. A three-wavelength MAD data set yielded good experimental phases to 2.6 A resolution. The structure was refined to 1.75 A resolution and was used to solve the structures of the ligand-free protein and the F17-G-N-acetyl-d-glucosamine complex. This selenium-carbohydrate phasing method could be of general use for determining the structures of carbohydrate-binding proteins.

Design and synthesis of HIV-1 protease inhibitors. Novel tetrahydrofuran P2/P2'-groups interacting with Asp29/30 of the HIV-1 protease. Determination of binding from X-ray crystal structure of inhibitor protease complex

A series of HIV-1 protease inhibitors having new tetrahydrofuran P2/P2' groups have been synthesised and tested for protease inhibition and antiviral activity. Six novel 4-aminotetrahydrofuran derivatives were prepared starting from commercially available isopropylidene-alpha-D-xylofuranose yielding six symmetrical and six unsymmetrical inhibitors. Promising sub nanomolar HIV-1 protease inhibitory activities were obtained. The X-ray crystal structure of the most potent inhibitor (23, K(i) 0.25 nM) co-crystallised with HIV-1 protease is discussed and the binding compared with inhibitors 1a and 1b.

A facile approach to diosgenin and furostan type saponins bearing a 3beta-chacotriose moiety

Combination of a one-pot coupling technique and the use of benzyl ethers as permanent protecting groups offered a short and simple route to dioscin-type saponins. This strategy in combination with a mild reductive opening procedure of the spiroketal function in diosgenin also offered a convenient approach to bidesmosidic furostan type saponins. Me(3)N.BH(3)/AlCl(3) promoted acetal opening of 3-O-TBDMS-protected diosgenin gave the 26-OH acceptor 9 into which a benzylated beta-glucose moiety was introduced by a S(N)2-type imidate coupling. After cleavage of the silyl ether, the 3beta-O-glucose and the 4-O-linked rhamnose of the chacotriose unit were introduced by a NIS/AgOTf-promoted one-pot coupling sequence utilising thioglycoside donors and their different reactivity in different solvents. After removal of a benzoyl group, the same coupling conditions were also used for the coupling of the second 2-O-linked rhamnose unit. The target substance was obtained after cleavage of the protecting benzyl ethers under Birch-type conditions, which did not affect the double bond in the steroid skeleton.

Investigation of the reactivity difference between thioglycoside donors with variant aglycon parts

The reactivity of perbenzoylated thioglycosides with various thiol aglycons has been compared and quantified using competitive glycosylation experiments. Methyl 2,3,4-tri-O-benzyl-α-D-glucopyranoside was employed as acceptor and DMTST as a promoter. The reactivity was found, as expected, to depend on the electron donating properties of the aglycon. Hence, the most reactive donor, the cyclohexyl thioglycoside, was found to be about three times as reactive as the thioethyl glycoside, which in turn was twice as reactive as the thiomethyl donor. The thiophenyl donor was even less reactive, whereas p-halophenyl donors were inert under the glycosylation conditions used — but could be activated using NIS–TfOH as promoter. Furthermore, it was found that galactosyl donors were three to four times more reactive than the corresponding glucosyl derivative. These results allowed the design of an orthogonal coupling between thioglycosides with the same protecting groups (benzoyls) but with different thiol aglycons. Key words: thioglycosides, orthogonal glycosylations, competititive glycosylations.

Synthesis of a polyphosphorylated GPI-anchor core structure

Using a linear assembly approach a highly differentially protected derivative of the common GPI-anchor core structure (α-D-Man-(1[Formula: see text]6)-α-D-Man-(1[Formula: see text]2)-α-D-Man-(1[Formula: see text]4)-α-D-GlcNH2-(1[Formula: see text]6)-D-myo-inositol) has been synthesized. All mannose donors were prepared from a common thioglycoside precursor (1), and coupled to GlcN3-myo-inositol acceptor 5 in a linear five-step glycosylation–deprotection sequence in 49% overall yield, to give the key intermediate 10, with orthogonal temporary protecting groups at the 6'', 2'', 6', and 2 positions of the trimannoside motif and at the 1 and 2 positions of the inositol part. Consecutive removal of the temporary protecting groups in the trimannoside moiety followed by phosphorylation, gave a tetraphosphosphate derivative in 60% overall yield. Removal of a camphor acetal afforded a 1,2-inositol diol, which was converted to a 1,2-cyclic phosphate using commercial methyl dichlorophosphate ([Formula: see text]17, 95%). One-step deprotection using sodium in liquid ammonia afforded the target polyphosphorylated core structure 18 (60%), which will be tested for metabolic insulin action.Key words: glycophosphatidylinositols, linear synthesis, glycosylations, inositolphosphoglycans, IPG.

One-pot oligosaccharide synthesis exploiting solvent reactivity effects

[reaction: see text] One-pot syntheses of trisaccharides have been accomplished simply by changing the solvent system between the two subsequent glycosylation reactions and utilizing the difference in glycosylation rate between different solvents. By tuning the reactivity of acceptors and donors and performing the first glycosylation in Et(2)O (low glycosylation rate) and the second in CH(2)Cl(2)/Et(2)O (higher glycosylation rate), trisaccharides were synthesized in high yields (76-84%).

Synthesis of alpha-tocopheryl oligosaccharides

As effective natural antioxidants, tocopherols and also their esters are frequently added to foodstuffs, pharmaceuticals and cosmetics. For increase of polarity, hence solubility in water, a series of alpha-tocopheryl oligosaccharides was synthesised using BF3-etherate. The pure alpha-tocopheryl beta-maltotetraoside as well as the higher homologues proved to be water-soluble.