1-Thio-beta-D-galactofuranosides: synthesis and evaluation as beta-D-galactofuranosidase inhibitors

Galactofuranose-Related Enzymes: Challenges and Hopes

Galactofuranose is a rare form of the well-known galactose sugar, and its occurrence in numerous pathogenic micro-organisms makes the enzymes responsible for its biosynthesis interesting targets. Herein, we review the role of these carbohydrate-related proteins with a special emphasis on the galactofuranosidases we recently characterized as an efficient recombinant biocatalyst.

Towards the preparation of synthetic outer membrane vesicle models with micromolar affinity to wheat germ agglutinin using a dialkyl thioglycoside

A series of alkyl thioglycosides and alkyl thiodiglycosides bearing glucose and N-acetylglucosamine residues were prepared by thiol-ene coupling in moderate to good yields (40-85%). Their binding ability towards wheat germ agglutinin was measured by competitive enzyme-linked lectin assays. One of the synthetic compounds presenting two GlcNAc units showed the highest inhibitory effect of this study with an IC₅₀ of 11 µM corresponding to a 3182-fold improvement compared to GlcNAc. These synthetic molecules were used to produce giant vesicles, alone or in mixture with phospholipids, mimicking bacterial outer membrane vesicles (OMV) with potential antiadhesive properties.

Insulin and novel thioglycosides exert suppressive effect on human breast and colon carcinoma cells

The rationale for the implementation of novel therapies should be based on hallmarks of cancer. Two novel compounds labelled as thioglycoside A and B were designed and evaluated on breast and colon cancer cell lines. We assessed their cytotoxic effect after sensitizing cancer cells with insulin. In order to explore the underlying mechanisms, we performed tests to assess cell migration and motility, apoptosis, expression of glucose transporter 1 and proapoptotic proteins. Both compounds proved to have an antitumor effect which was significantly enhanced in combination with insulin. Linking glucose and anticancer agent presents an approach that exploits the Warburg effect. Targeting dysfunctional glycometabolism and increased glucose absorption is emerging as a promising anticancer strategy.

Regioselective 5-O-Opening of Conformationally Locked 3,5-O-Di-tert-butylsilylene-d-galactofuranosides. Synthesis of (1→5)-β-d-Galactofuranosyl Derivatives

The use of thiogalactofuranoside as donors for the construction of internal Galf containing oligosaccharide is limited, probably due to the difficulty to functionalize thiogalactofuranoside derivatives showing O-2, O-3, and O-5 with similar reactivity. An efficient method for complete regioselective 5-O-opening of conformationally restricted 3,5-O-di-tert-butylsilylene-d-galactofuranoside derivatives was developed. The use of a solution nBu₄NF (1.1 equiv) in CH₂Cl₂ on 6 gave the 5-OH free derivative 10 as the only product (90%). 3-O-Di-tert-butylhydroxysilyl derivative 10 was stable upon purification and glycosylation reaction. Preactivation of conformationally restricted thioglycoside 6 employing p-NO₂-benzensulfenyl chloride/AgOTf followed by condensation over the 5-OH thioglycoside acceptor 10 gave the corresponding disaccharide 12 without autocondensation byproduct. Regioselective 5-O-deprotection was also successfully performed over the (1→5)-β-d-galactofuranosyl di- and trisaccharide derivatives 12 and 13. This methodology allowed the differentiation between the secondary hydroxyl groups OH-3 and OH-5 of 1,2-cis or 1,2-trans d-galactofuranoside derivatives, and it still constitutes an innovative approach to access oligosaccharides of pharmacological importance.

Synthesis of Enantiomeric Polyhydroxyalkylpyrrolidines from 1,3-Dipolar Cycloadducts. Evaluation as Inhibitors of a β-Galactofuranosidase

Enantiomeric 2,3,4-tris(hydroxyalkyl)-5-phenylpyrrolidines have been synthesized from the major cycloadducts obtained by the 1,3-dipolar cycloaddition of sugar enones with azomethine ylides derived from natural amino acids. Reduction of the ketone carbonyl group of the cycloadducts, which possess a basic structure of bicyclic 6-(menthyloxy)hexahydropyrano[4,3-c]pyrrol-7(6H)one, afforded a number of pyrrolidine-based bicyclic systems. A sequence of reactions, which involved hydrolysis of the menthyloxy substituent, reduction, N-protection, and degradative oxidation, afforded varied pyrrolidine structures having diverse configurations and patterns of substitution; in particular, polyhydroxylated derivatives have been obtained. The unprotected products were isolated as pyrrolidinium trifluoroacetates. Because of the furanose-like nature of the target trihydroxyalkyl pyrrolidines, these molecules have been evaluated as inhibitors of the β-galactofuranosidase from Penicillium fellutanum. The compounds showed practically no inhibitory activity for concentration of pyrrolidines in the range of 0.1-1.6 mM.

Identification and Characterization of a Novel Galactofuranose-Specific β-D-Galactofuranosidase from Streptomyces Species

β-D-galactofuranose (Galf) is a component of polysaccharides and glycoconjugates and its transferase has been well analyzed. However, no β-D-galactofuranosidase (Galf-ase) gene has been identified in any organism. To search for a Galf-ase gene we screened soil samples and discovered a strain, identified as a Streptomyces species by the 16S ribosomal RNA gene analysis, that exhibits Galf-ase activity for 4-nitrophenyl β-D-galactofuranoside (pNP-β-D-Galf) in culture supernatants. By draft genome sequencing of the strain, named JHA19, we found four candidate genes encoding Galf-ases. Using recombinant proteins expressed in Escherichia coli, we found that three out of four candidates displayed the activity of not only Galf-ase but also α-L-arabinofuranosidase (Araf-ase), whereas the other one showed only the Galf-ase activity. This novel Galf-specific hydrolase is encoded by ORF1110 and has an optimum pH of 5.5 and a Km of 4.4 mM for the substrate pNP-β-D-Galf. In addition, this enzyme was able to release galactose residue from galactomannan prepared from the filamentous fungus Aspergillus fumigatus, suggesting that natural polysaccharides could be also substrates. By the BLAST search using the amino acid sequence of ORF1110 Galf-ase, we found that there are homolog genes in both prokaryotes and eukaryotes, indicating that Galf-specific Galf-ases widely exist in microorganisms.

Galactofuranose Biosynthesis: Discovery, Mechanisms and Therapeutic Relevance

Galactofuranose, the atypical and thermodynamically disfavored form of d-galactose, has in reality a very old history in chemistry and biochemistry. The purpose of this book chapter is to give an overview on the fundamental aspects of the galactofuranose biosynthesis, from the biological occurrence to the search of inhibitors.

Synthesis of galactofuranosyl-(1 → 5)-thiodisaccharide glycomimetics as inhibitors of a β-d-galactofuranosidase

Description of the synthesis, molecular modeling and inhibitory properties of furanosyl thiodisaccharides that are mimetics of the motif β-d-Galf-(1 → 5)-d-Galf, found in glycoconjugates of pathogenic microorganisms.

Influence of silyl protections on the anomeric reactivity of galactofuranosyl thioglycosides and application of the silylated thiogalactofuranosides to one-pot synthesis of diverse β-D-oligogalactofuranosides

We describe in this paper the tuning effect of silyl protecting groups on the donor reactivity of galactofuranosyl phenyl thioglycosides. Silyl ethers on the galactofuranose ring are found to have an arming effect on the glycosylation reactivity, but the cyclic 3,5-acetal protecting group decreases the reactivity. The reactive phenyl 2,6-di-O-Bz-3,5-di-O-TBS-1-thio-β-d-galactofuranoside 3 is proved to be a useful glycosyl building block. By taking advantage of this donor, we achieved the highly efficient one-pot solution-phase assembly of a panel of β-d-galactofuranosyl tri- and tetrasaccharides possessing diverse glycosidic linkages.

Specific and non-specific enzymes for furanosyl-containing conjugates: biosynthesis, metabolism, and chemo-enzymatic synthesis

There is no doubt now that the synthesis of compounds of varying complexity such as saccharides and derivatives thereof continuously grows with enzymatic methods. This review focuses on recent basic knowledge on enzymes specifically involved in the biosynthesis and degradation of furanosyl-containing polysaccharides and conjugates. Moreover, and when possible, biocatalyzed approaches, alternative to standard synthesis, will be detailed in order to strengthen the high potential of these biocatalysts to go further with the preparation of rare furanosides. Interesting results will be also proposed with chemo-enzymatic processes based on nonfuranosyl-specific enzymes.

Chemistry and biology of galactofuranose-containing polysaccharides

The thermodynamically less stable form of galactose-galactofuranose (Galf)-is essential for the viability of several pathogenic species of bacteria and protozoa but absent in this form in mammals, so the biochemical pathways by which Galf-containing glycans are assembled and catabolysed are attractive sites for drug action. This potential has led to increasing interest in the synthesis of molecules containing Galf residues, their subsequent use in studies directed towards understanding the enzymes that process these residues and the identification of potential inhibitors of these pathways. Major achievements of the past several years have included an in-depth understanding of the mechanism of UDP-galactopyranose mutase (UGM), the enzyme that produces UDP-Galf, which is the donor species for galactofuranosyltransferases. A number of methods for the synthesis of galactofuranosides have also been developed, and practitioners in the field now have many options for the initiation of a synthesis of glycoconjugates containing either alpha- or beta-Galf residues. UDP-Galf has also been prepared by a number of approaches, and it appears that a chemoenzymatic approach is currently the most viable method for producing multi-milligram amounts of this important intermediate. Recent advances both in the understanding of the mechanism of UGM and in the synthesis of galactofuranose and its derivatives are highlighted in this review.

Synthesis and biological evaluation of galactofuranosyl alkyl thioglycosides as inhibitors of mycobacteria

As part of our research interest directed toward the development of antimycobacterial agents, we have investigated compounds based on galactofuranose (Galf), an essential cell wall component of mycobacteria. The objective of this study was to explore structure activity relationships of Galf thioglycosides with straight chain and branched aglycons. Acylated Galf 9-heptadecyl thioglycoside was prepared by Lewis acid-catalyzed thioglycosidation of 1,2,3,5,6-penta-O-acyl-D-galactofuranose with 9-heptadecanethiol, and subsequently converted to the corresponding sulfone using m-CPBA. Both Galf 9-heptadecyl thioglycoside and sulfone displayed in vitro inhibition (MIC) of the growth of Mycobacterium smegmatis below 5 microg/mL, while Galf 1-octyl thioglycoside gave no inhibition at or below 32 microg/mL.

Synthesis and evaluation of galactofuranosyl N,N-dialkyl sulfenamides and sulfonamides as antimycobacterial agents

The recent emergence of clinically oppressive superbugs, some with resistance to nearly all frontline drug therapies, has challenged our ability to combat such infectious organisms as Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). Our medicinal chemistry program targeting this pathogen has identified several potent galactofuranose-based in vitro inhibitors of mycobacterial growth. The most potent compound, the Galf N,N-didecyl sulfenamide 8d, displayed anti-mycobacterial activity (MIC) of 1 microg/mL in a cell based assay against a representative strain of Mycobacterium smegmatis.

Synthesis of α-d-Galf-(1→2)-d-galactitol and α-d-Galf-(1→2)[β-d-Galf-(1→3)]-d-galactitol, oligosaccharide derivatives from Bacteroides cellulosolvens glycoproteins

The synthesis of alpha-D-galactofuranosyl-(1-->2)-D-galactitol, which has been isolated by reductive beta-elimination from glycoproteins of Bacteroides cellulosolvens and Clostridium thermocellum, is described. The approach of selective glycosylation of an aldono-1,4-lactone by the trichloroacetimidate method was employed. The synthesis of alpha-D-Gal f-(1-->2)[beta-D-Gal f-(1-->3)]-D-Galol, that contains Gal f units in both anomeric configurations, is also reported. These are the first synthetic oligosaccharides with alpha-D-Gal f, previously found in natural products.

Facile synthesis of benzyl β-d-galactofuranoside. A convenient intermediate for the synthesis of d-galactofuranose-containing molecules

Benzyl beta-D-galactofuranoside was efficiently obtained from 1,2,3,5,6-penta-O-benzoyl-alpha,beta-D-galactofuranose, via benzyl 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranoside. Conditions for the O-debenzylation were investigated in order to evaluate the synthetic application of the benzyl group as an anomeric protector of a galactofuranose moiety in synthetic strategies involving galactofuranose.

General one-step synthesis of free hexofuranosyl 1-phosphates using unprotected 1-thioimidoyl hexofuranosides

A general one-step strategy is developed for the synthesis of hexofuranosyl 1-phosphates starting from new unprotected glycofuranosyl donors. It required first the preparation of new 1-thiohexofuranosides bearing a thioimidoyl heterocycle as a leaving group. The presence of sulfur and/or nitrogen atom(s) on the aglycon allowed remote activation of these thioglycofuranosides by anhydrous phosphoric acid and led to the target phosphates 9, 27, 29, and 30 in good to excellent selectivities and, more importantly, with very limited or no ring expansion. Moreover, this one-step phosphorylation reaction could be significantly improved by avoiding any tedious protecting group manipulations on negatively charged compounds and by focusing on a simple but general procedure of purification. This approach was applied to the diastereocontrolled synthesis of d-galacto- and d-glucofuranosyl 1-phosphates and also to the preparation of rare epimer and/or deoxy counterparts, that is, d-manno- and d-fucofuranosyl derivatives.

Photoinduced electron transfer and chemical alpha-deoxygenation of D-galactono-1,4-lactone. Synthesis of 2-deoxy-D-lyxo-hexofuranosides

Two simple procedures for the synthesis of 2-deoxy-D-lyxo-hexono-1,4-lactone are described. Reductive cleavage of a 2-O-tosyl derivative of D-galactono-1,4-lactone in the presence of sodium iodide afforded the 2-deoxy derivative. On the other hand, alpha-deoxygenation of D-galactono-1,4-lactone was easily achieved by photochemical electron transfer deoxygenation of HO-2 as the 3-(trifluoromethyl)benzoate. Methyl 2-deoxy-beta-D-lyxo-hexafuranoside ('methyl 2-deoxy-beta-D-galactofuranoside') was synthesized and tested as substrate for exo beta-D-galactofuranosidase from Penicillium fellutanum. The reaction was followed by HPAEC, showing that methyl 2-deoxy-beta-D-galactofuranoside was not hydrolyzed by incubation with the enzyme. Neither the 2-deoxy lactone, nor the 2-deoxy-beta-D-galactofuranoside acted as inhibitors of the reaction with the 4-nitrophenyl beta-D-galactofuranoside. The present and our previous results show that the hydroxyl groups at C-2, C-3 and C-6 of the galactofuranoside are essential for interaction with the exo beta-D-galactofuranosidase.

Influence of exo beta-D-galactofuranosidase inhibitors in cultures of Penicillium fellutanum and modifications in hyphal cell structure

We have examined beta-D-galactofuranosidase production by Penicillium fellutanum in the presence of D-galactono-1,4-lactone or 4-aminophenyl 1-thio-beta-D-galactofuranoside, two potent in vitro inhibitors of the enzyme. Activity of the enzyme in the culture filtrate was increased by 35% when glucose was replaced by D-galactose as the carbon source, and the activity diminished 80% of the control value when the inhibitors were added. Significant alterations of the culture were observed: (a) the medium became increasingly opalescent due to the secretion of a protein aggregate (PA) which contained 15% neutral sugar, mainly ribose; (b) the peptidophosphogalactomannan (pPGM) containing galactofuranose, normally produced by P. fellutanum, could not be obtained from the culture medium in the presence of the inhibitors; (c) the content of galactofuranose in the cell wall was significantly decreased in the presence of D-galactono-1,4-lactone. The influence on the mycelia growth was investigated by light microscopy (LM) and transmission electron microscopy (TEM) showing important alterations.

Beta-galactofuranoside glycoconjugates on conidia and conidiophores of Aspergillus niger

Galactose in the furanoic conformation appears to be limited to bacteria and lower eukaryotes. Galactofuranoic (Galf)-containing glycoconjugates that occur in organisms pathogenic or allergenic to man are frequently antigenic and immunodominant. We have used an immunochemical approach, employing a monoclonal antibody that recognises Galf epitopes, to investigate the presence of Galf-containing glycoconjugates within conidia and conidiophores of Aspergillus niger. ELISA and immunofluorescence microscopy indicated that specific and saturable binding sites were found on both. Inhibition studies confirmed that this binding was to Galf-containing glycoconjugates. Interestingly, the conidiophore heads were particularly rich in these glycoconjugates. Western blotting identified a Galf glycoprotein of 150-200 kDa from disrupted conidia.

Synthesis of beta-D-galactofuranosyl nucleoside analogues. A new type of beta-D-galactofuranosidase inhibitor

The development of beta-D-galactofuranosidase inhibitors provides a good chemotherapeutic target for treatment of major human diseases, because beta-D-galactofuranose is a constituent of important pathogen microorganisms but is absent in mammals. With this purpose we have prepared beta-D-galactofuranosyl nucleoside analogues, derived by the addition of nucleophiles to perbenzoylated beta-D-galactofuranosyl isothiocyanate, a compound previously prepared in this laboratory. N-beta-D-Galactofuranosyl-O-ethylthiourethane, N-beta-D-galactofuranosyl-4-oxoimidazolidine-2-thione, N-beta-D-galactofuranosyl-4-imidazoline-2-thione, and N-beta-D-galactofuranosyl-4-methoxyimidazolidine-2-thione, were prepared. The biological assays showed that imidazoline and imidazolidine-2-thione derivatives act as a new type of exo beta-D-galactofuranosidase inhibitor.

An extracellular beta-galactofuranosidase from Aspergillus niger and its use as a tool for glycoconjugate analysis

Aspergillus niger produces an extracellular beta-galactofuranosidase, which can specifically hydrolyse beta-D-galactofuranose (Galf) from glycoconjugates. The production of this enzyme can be induced by the addition of a Galf-containing A. niger mycelial wall extract. However, on other carbon sources accumulation occurred only during the starvation conditions of the late stationary phase. Extracellular glucoamylases from this stage of cultivation possessed significantly lower levels of Galf than those from the earlier exponential growth phase when beta-galactofuranosidase is absent, suggesting in situ beta-galactofuranosidic hydrolysis. The beta-galactofuranosidase responsible was subsequently purified to homogeneity and characterised. It is a glycoprotein of 90 kDa (determined by SDS-PAGE) with activity against beta-linked Galf residues, with a Km of 4 mM against p-nitrophenyl-beta-D-galactofuranoside and a pH optimum of 3-4. The preparation did not contain other contaminating glycosidase activities; p-nitrophenyl-beta-D- and -alpha-D-galactopyranose, and alpha-D-methyl-Galf were not hydrolysed. Results are presented to show that this enzyme could be employed as a useful tool for the analysis of glycoconjugates containing biologically important Galf components.

A one-pot synthesis of novel N,N-dialkyl-S-glycosylsulfenamides

An efficient new entry into N,N-dialkyl-S-glycosylsulfenamides is reported. The reaction of bis-activated alkyl halides in the presence of a secondary amine base with glycosylic S-acetyl derivatives (1-S-acetyl-1-thioaldoses or 2-S-acetyl-2-thioketoses) results in the formation of novel carbohydrate sulfonamides. These new carbohydrate-based sulfonamides may provide useful derivatives with biological activity, as well as provide reactive carbohydrate sulfonylating agents.

Synthesis of 4-nitrophenyl beta-D-fucofuranoside and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose: modified substrates for studies on catalytic requirements of beta-D-galactofuranosidase

Syntheses of 4-nitrophenyl beta-D-fucofuranoside (6) and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose (10) are reported. These compounds, as analogues of galactofuranosides, were used for studying the influence of the hydroxyl group at C-6 in the interaction of the substrate with beta-D-galactofuranosidase. For the synthesis of the fucofuranosides, 2,3,5-tri-O-benzoyl-6-bromo-6-deoxy-D-galactono-1,4-lactone (1) was the key intermediate, which upon reduction of the lactone group with diisoamylborane, acetylation of the anomeric hydroxyl group, and catalytic hydrogenolysis of the bromine at C-6, led to 1-O-acetyl-2,3,5-tri-O-benzoyl-alpha,beta-D-fucofuranose (4), a convenient derivative for the preparation of fucofuranosides. Compound 4 was glycosylated in the presence of SnCl4, either with 4-nitrophenol for the preparation of 6, or with 2,5,6-tri-O-benzoyl-D-mannono-1,4-lactone (7), for the synthesis of disaccharide 10, via the glycosyl-aldonolactone approach. The synthetic route developed for the beta-D-fucofuranosides is simple and efficient. Compound 6 was not hydrolyzed by incubation with the exo beta-D-galactofuranosidase from Penicillium fellutanum, showing that HO-6 is essential for interaction of the substrate with the enzyme.

Immobilized 4-aminophenyl 1-thio-beta-D-galactofuranoside as a matrix for affinity purification of an exo-beta-D-galactofuranosidase

An alternative and fast method for the purification of an exo-beta-D-galactofuranosidase has been developed using a 4-aminophenyl 1-thio-beta-D-galactofuranoside affinity chromatography system and specific elution with 10 mM D-galactono-1,4-lactone in a salt gradient. A concentrated culture medium from Penicillium fellutanum was chromatographed on DEAE-Sepharose CL 6B followed by chromatography on the affinity column, yielding two separate peaks of enzyme activity when elution was performed with 10 mM D-galactono-1,4-lactone in a 100-500 mM NaCl salt gradient. Both peaks behaved as a single 70 kDa protein, as detected by SDS-PAGE. Antibodies elicited against a mixture of the single bands excised from the gel were capable of immunoprecipitating 0.2 units out of 0.26 total units of the enzyme from a crude extract. The glycoprotein nature of the exo-beta-D-galactofuranosidase was ascertained through binding to Concanavalin A-Sepharose as well as by specific reaction with Schiff reagent in Western blots. The purified enzyme has an optimum acidic pH (between 3 and 6), and Km and Vmax values of 0.311 mM and 17 mumol h-1 microgram-1 respectively, when 4-nitrophenyl beta-D-galactofuranoside was employed as the substrate.

The glycosyl-aldonolactone approach for the synthesis of beta-D-Galf-(1-->3)-D-Manp and 3-deoxy-beta-D-xylo-hexofuranosyl-(1-->3)-D-Manp

A convenient synthesis of free beta-D-Galf-(1-->3)-D-Manp (8a) is reported. The disaccharide is present as external unit in the lipopeptidophosphoglycan (LPPG) of Trypanosoma cruzi and internally in the lipophosphoglycan (LPG) of Leishmania. Condensation of 2,5,6-tri-O-benzoyl-D-mannono-1,4-lactone (1) with 1,2,3,5,6-penta-O-benzoyl-D-galactofuranose, promoted by SnCl4, led to the beta-glycosyl-lactone, a key intermediate for disaccharide 8a, readily obtained by successive reduction of the lactone with diisoamylborane and debenzoylation. As in the LPG of Leishmania the HO-3 group of the galactofuranose is glycosylated by alpha-D-Galp, we also synthesized 3-deoxy-beta-D-xylo-hexofuranosyl-(1-->3)-D-Manp (8b) and p-nitrophenyl 3-deoxy-beta-D-xylo-hexofuranoside for studying the influence of HO-3 in the interaction with specific glycosidases. The disaccharide 8a, and its corresponding alditol, were good substrates for the beta-D-galactofuranosidase from Penicillium fellutanum, whereas the 3-deoxyglycosides were not hydrolyzed by the enzyme.