A total synthesis of hematoside, alpha-NeuGc-(2----3)-beta-Gal-(1----4)-beta-Glc-(1----1)-Cer

Gut bacteria responding to dietary change encode sialidases that exhibit preference for red meat-associated carbohydrates

Dietary habits have been associated with alterations of the human gut resident microorganisms contributing to obesity, diabetes and cancer¹. In Western diets, red meat is a frequently eaten food², but long-term consumption has been associated with increased risk of disease^3,4. Red meat is enriched in N-glycolylneuraminic acid (Neu5Gc) that cannot be synthesized by humans⁵. However, consumption can cause Neu5Gc incorporation into cell surface glycans⁶, especially in carcinomas^4,7. As a consequence, an inflammatory response is triggered when Neu5Gc-containing glycans encounter circulating anti-Neu5Gc antibodies^8,9. Although bacteria can use free sialic acids as a nutrient source^10-12, it is currently unknown if gut microorganisms contribute to releasing Neu5Gc from food. We found that a Neu5Gc-rich diet induces changes in the gut microbiota, with Bacteroidales and Clostridiales responding the most. Genome assembling of mouse and human shotgun metagenomic sequencing identified bacterial sialidases with previously unobserved substrate preference for Neu5Gc-containing glycans. X-ray crystallography revealed key amino acids potentially contributing to substrate preference. Additionally, we verified that mouse and human sialidases were able to release Neu5Gc from red meat. The release of Neu5Gc from red meat using bacterial sialidases could reduce the risk of inflammatory diseases associated with red meat consumption, including colorectal cancer⁴ and atherosclerosis¹³.

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

A General Chemoenzymatic Strategy for the Synthesis of Glycosphingolipids

A concise, prototypical, and stereoselective strategy for the synthesis of therapeutically and immunologically significant glycosphingolipids has been developed. This strategy provides a universal platform for glycosphingolipid synthesis by block coupling of enzymatically prepared free oligosaccharideglycans to lipids using glycosyl N-phenyltrifluoroacetimidates as efficient activated intermediates. As demonstrated here, two different types of glycosphingolipids were obtained in excellent yields using the method.

Non-natural glycosphingolipids and structurally simpler analogues bind HIV-1 recombinant Gp120

Interactions of recombinant gp120 (rgp120) with non-natural glycosphingolipids (GSLs) and structurally simpler analogues have been studied using a competitive adhesion assay. Conjugates of cellobiosyl ceramide and melibiosyl ceramide were synthetically prepared as water-soluble GSL analogues. These ligands were screened against a panel of biologically relevant analogues, and the results show that their interactions with rgp120 are comparable to natural cellular receptors. Glycolipid interactions with rgp120 were probed further by the synthesis and testing of structurally simpler analogues that were obtained by reductive amination of lactose, cellobiose, and melibiose with a biotinylated amino ethylene glycol moiety. RGp120 did not recognize conjugates lacking a lipid component. However, palmitoylation of the secondary amino alditols yielded compounds with comparable rgp120 affinity to the natural cellular receptor, galactosyl ceramide (GalCer). Taken together, the SAR showed that both a hydrophobic and a hydrophilic component are required for rgp120 recognition. Moreover, structural variability in the carbohydrate headgroup did not significantly alter rgp120 recognition indicating that this interaction is not highly specific.

Preparative route to N-glycolylneuraminic acid phenyl 2-thioglycoside donor and synthesis of Neu5Gc-alpha-(2-->3')-lactosamine 3-aminopropyl glycoside

The spacer-armed trisaccharide, Neu5Gc-alpha-(2-->3')-lactosamine 3-aminopropyl glycoside, was synthesized by regio- and stereoselective sialylation of the suitably protected triol acceptor, 3-trifluoroacetamidopropyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-4-O-(6-O-benzyl-beta-D-galactopyranosyl)-beta-D-glucopyranoside, with the donor methyl [phenyl 5-acetoxyacetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-alpha,beta-D-galacto-2-nonulopyranosid]onate. The donor was obtained, in turn, from methyl [phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-alpha,beta-D-galacto-2-nonulopyranosid]onate by N-tert-butoxycarbonylation of the acetamido group followed by total N- and O-deacetylation, per-O-acetylation, subsequent Boc group removal, and N-acetoxyacetylation.

A one pot synthesis of mono- and di-lactosyl sphingosines

The importance of analogues of lactosyl ceramides as basic structures of many natural glycosphingolipids provided a rationale for developing an efficient synthetic route to these compounds. We report herein a novel approach to synthesize several members of this family. Glycosylation of N-diphenylmethylene-spingosine, which exists in an imine-oxazolidine tautomeric mixture, with acetobromolactose under a modified Koenigs-Knorr condition yielded lactosyl beta-(1 --> 1) sphingosine, lactosyl beta-(1 --> 3) sphingosine and dilactosyl sphingosine in good yields. A similar glycosylation could be applicable to the synthesis of other glycosphingolipids.

Asymmetric allylboration and ring closing alkene metathesis: a novel strategy for the synthesis of glycosphingolipids

A novel strategy for the synthesis of D,L-glucosylceramide 1, a member of the glycosphingolipid class of natural products is described. Reagent-controlled asymmetric Brown allylboration gave excellent stereochemical control in the construction of adjacent stereocenters in the sphingoid base portion of the molecule. The trans-configured double bond was obtained as a single geometrical isomer by use of silicon-tethered olefin metathesis employing the Schrock carbene [(CF3)2MeCO]2Mo(=CHCMe2Ph)(=NC6H3-2,6-i-Pr2++ +) and in situ PhLi-induced ring-opening of the intermediate 5,6-dihydro-2H-1,2-oxasiline followed by protodesilylation with TBAF in DMSO. The synthesis was completed by long chain amide formation and global deprotection.

The total synthesis of ganglioside GM3

Previous syntheses of ganglioside GM3 (NeuAc alpha3Gal beta4Glc beta1Cer) are reviewed, and both chemoenzymatic and chemical total synthetic approaches were investigated. In a chemoenzymatic approach, (2S,3R,4E)-5'''-acetyl-alpha-neuraminyl-(2''' --> 3'')-beta-galactopyranosyl-(1'' --> 4')-beta-glucopyranosyl-(1' <--> 1)-2-azido-4-octadecene-1,3-diol (azidoGM3) was readily prepared utilizing recombinant beta-Gal-(1'' --> 3'/4')-GlcNAc alpha-(2''' --> 3'')-sialyltransferase enzyme, and was evaluated as a synthetic intermediate to ganglioside GM3. The chemical total synthesis of ganglioside GM3 was performed on one of the largest scales yet reported. The highlights of this synthesis include minimizing the steps necessary to prepare the lactosyl acceptor as a useful anomeric mixture, which was present in excess for the highly regioselective and fairly stereoselective sialylation with a known neuraminyl donor to give the protected GM3 trisaccharide. The synthetic methodology maximized convergence by a subsequent glycosidic coupling of the well-characterized GM3 trisaccharide trichloroacetimidate derivative with protected ceramide. The ganglioside GM3 was nearly homogeneous as the two glycosidic couplings utilized preparative HLPC purifications, and variations in the sphingosine base and fatty acyl group were under 0.1 and 0.2%, respectively.

Synthesis of biotinylated glycoconjugates and their use in a novel ELISA for direct comparison of HIV-1 Gp120 recognition of GalCer and related carbohydrate analogues

As part of our program directed toward the design and synthesis of high-affinity ligands for the GalCer-binding site on the HIV cell surface glycoprotein, gp120, we required a reliable method for qualitatively assessing relative binding affinities for related analogues. Due to the hydrophilic nature of these synthetic conjugates, difficulties were encountered with typical ELISA methods, which rely upon hydrophobic interactions to anchor the ligand to a microtiter plate. Other types of assays were also problematic due to nonspecific binding of gp120. Therefore, we developed a general method for plating water-soluble ligands on microtiter plates using biotin/NeutrAvidin recognition for adhesion. A water-soluble GalCer analogue was prepared by conjugating psychosine to biotin using a novel tetraethylene glycol linker. In a similar manner, LacCer and GlcCer analogues were prepared and these conjugates were plated into microtiter wells containing NeutrAvidin. Unoccupied sites were blocked using biotin functionalized as a primary amide. Gp120 binding to galactosyl sphingosine, GalSph (19), GlcSph (22), and LacSph (23) conjugates was assessed through incubation with recombinant HRP-gp120. It was determined that LacSph has the strongest interaction with gp120. The binding affinities of GalSph and GlcSph were similar to each other and less strong than LacSph. These data contradict earlier studies where HPTLC showed that LacCer and GlcCer do not significantly bind gp120. They also contradict liposome-based assays that reported psychosine is not recognized by gp120. The extent of plating for each biotinylated molecule was quantified using HRP-biotin, allowing direct comparison of ligand plating efficiencies for the first time. Several other synthetic biotin conjugates were prepared and tested, demonstrating the feasibility of performing ELISA on water-soluble ligands.

Enzymic glycosphingolipid synthesis on polymer supports. III. Synthesis of GM3, its analog [NeuNAcalpha(2-3)Galbeta(1-4)Glcbeta(1-3)Cer] and their lyso-derivatives

Two water-soluble polymers, carrying 0.24 meq g(-1) of lactosyl-beta(1-1)-sphingosine (7) and 0.13 meq g(-1) of lactosyl-beta(1-3)-sphingosine (8) were prepared. The polymers served as acceptors in the alpha-(2-3)-sialyltransferase reaction (up to 55.3 and 38.5% transfer yields, respectively). Subsequent photolysis, released compounds 11 (lyso-GM3) and 12 (lyso-GM3 analog), respectively; acylation and chromatography afforded (5-acetamido-3,5-dideoxy-D-glycero-alpha-galacto-2-nonulopyranosyloni c acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyl-(1-1)-(2 S, 3R, 4E)-2-octadecanoylamino-4-octadecene-1,3-diol (13, GM3) and (5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylo nic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyl-(1-3)-(2 S, 3R, 4E)-2-octadecanoylamino-4-octadecene-1,3-diol (14, GM3 analogue), respectively, thus presenting a route to glycosphingolipids possessing the unusual glycosyl-beta(1-3)-spingosine linkage.

Chemoenzymic synthesis of potentially caged glycosphingolipids [correction of glycoshingolipids] (GSLs): potentially caged lyso-G(M3) and its analogue

In a previous work, a number of potentially caged sphingolipids and glycosphingolipids were chemically synthesized (Zehavi, 1997. Chem. Phys. Lipids 90, 55-61). The effects of GM3 and to a lesser extent, of lyso-GM3, are being studied. Considering that biologically inert, caged lyso-GM3 could be photolysed in the cell to release lyso-GM3, thus creating an attractive opportunity to study the subsequent sequence of events in the cell, the chemoenzymic synthesis of the potentially caged lyso-GM3, (5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylo nic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy (1-1)- (2S,3R,4E)-2-(4-carboxymethyl-2-nitrobenzyloxycarbonyl-amino)-3-hy droxy-4- octadecene and of a potentially caged GM3 analogue, (5-acetamido-3,5-dideoxy-D- glycero-alpha-D-galacto-2-nonulopyranosylonic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy -(1-3)- (2S, 3R, 4E)-2-(4-carboxymethyl-2-nitro-benzyloxycarbonylamino)-1- hydroxy-4-octadecene was undertaken. Both compounds, being 2-nitrobenzyloxycarbonyl derivatives, are light-sensitive and could be efficiently photolysed to the biologically active, corresponding lyso-GSLs.

High performance polymer supports for enzyme-assisted synthesis of glycoconjugates

Efficient and practical methodology for the construction of carbohydrates, including oligosaccharide derivatives and sphingoglycolipids, was established on the basis of a water-soluble polymer supports having unique linkers that can be cleaved by specific conditions. Novel glycomonomers for the construction of polymer supports were synthesized and copolymerized with acrylamide to give three types of water-soluble glycopolymers having primer sugars through the specific linkers containing (i) p-substituted benzyl group, (ii) L-phenylalanine residue, and (iii) ceramide-mimetic L-serine derivative, respectively. These glycopolymers were employed for sugar elongation reactions with glycosyl transferases such as GlcNAc beta 1,4-galactosyl transferase, beta Gall-->3/4GlcNAc alpha-2,6-sialyl transferase, and beta Gall-->3/4GlcNAc alpha-2,3-sialyl transferase in the presence of each sugar nucleotide as glycosyl donor to afford polymers having N-acetyllactosamine, sialyl alpha-(2-->6) N-acetyllactosamine, and sialyl alpha-(2-->3) lactose residues in excellent yield. Subsequent hydrogenolysis, hydrolysis with alpha-chymotrypsin, or transglycosylation to ceramide with ceramide glycanase proceeds smoothly to give N-acetyllactosamine, a versatile sialyl alpha-(2-->6) N-acetyllactosamine derivative having a terminal amino group, and ganglioside GM3 in high yield.

Formation of high-axial-ratio-microstructures from natural and synthetic sphingolipids

Amphiphiles that form high-axial-ratio-microstructures (HARMs) are being considered as novel materials for controlled release of drugs and other biologically functional molecules. HARMs consisting of tubules, ribbons, solid rods and helices are formed from sphingolipids by addition of water to a solution of amphiphile in DMF. Single molecular species of galactocerebroside (GalCer) containing long unsaturated fatty acid chains or natural GalCer containing mixed-length, non-hydroxy fatty acids (NFA-GalCer) or alpha-hydroxy fatty acids (HFA-GalCer) form cylindrical structures. In contrast, single molecular species of GalCer containing long saturated fatty acids form ribbons and helices. GalCer HARMs are typically under 100 nm in diameter and have lengths of several microns. The importance of the amide of GalCer for HARM formation was evaluated using psychosine, which forms solid fibers, whereas sphingosine and an analog of GalCer in which the amide is reduced to a secondary amine form amorphous aggregates. Single molecular species of ceramide containing long unsaturated fatty acid chains form cylindrical structures, whereas those with long saturated fatty acids form ribbons and helices. Short chain saturated ceramide also forms cylindrical structures. GalCer analogs with N-acetyl-glycine in place of the galactose form fibers whereas those with N-acetyl-proline yield amorphous material. The N-acetyl-proline-containing amphiphile can de doped into pure GalCer or NFA-GalCer without perturbing tubule formation.

An efficient short-step total synthesis of ganglioside GM3: effective usage of the neighbouring group participation strategy

We have developed an efficient methodology for highly stereoselective sialylation using 3-position substituted sialic acids and have prepared 2a having a 3 beta-phenylthio group as a sialic donor. Glycosylation of suitably protected lactoside 3 with 2a gave only the alpha-sialyl trisaccharide 16 in good yield. Condensation of the azidosphingosine 4 with the acetate 17 using promotors, DMTST or NIS-TfOH, afforded the glycolipid 18, which was directly transformed to 20 by reduction with Bu3P and subsequent acylation with octadecanoic acid in the presence of WSC. Removal of the protecting groups generated ganglioside GM3 (1).

Synthesis of phosphites and phosphates of neuraminic acid and their glycosyl donor properties--convenient synthesis of GM3

The importance and requirements for catalytic activation of sialyl donors are discussed, leading to the acid sensitive phosphite and phosphate moiety, respectively, as leaving group and nitriles as solvent. Therefore, from readily available N-acetylneuraminic acid, derivative 1 with phosphochloridites 2a-f and Huenigs' base sialyl phosphites 3a-f were prepared and isolated in high yields. Oxidation of 3a,c with tert-butyl-hydroperoxide afforded the corresponding phosphates 4a,c. As expected, phosphites 3 could be activated in acetonitrile by catalytic amounts of TMSOTf; thus, from 3a-e as donors and lactose derivatives 8A,B as acceptors the ganglioside building blocks 9A and 9B, respectively, were obtained in good yields. The best results were obtained with diethyl phosphite derivative 3a as sialyl donor, which exceeded by far the results obtained with the corresponding phosphate derivative 4a. Trisaccharide 9B was transformed into known 9A and into the fully O-acetylated GM3-trisaccharide 10.

An efficient synthesis of ganglioside GM3: highly stereocontrolled glycosylations by use of auxiliaries

An efficiently stereocontrolled total synthesis of GM3 alpha-D-Neup5Ac-(2----3)-beta-D-Galp-(1----4)-beta-D-Glcp-(1----1) -Cer was achieved by employing both methyl 5-acetamido-4,7,8,9-tetra-O-benzyl-2-bromo-2,3,5-trideoxy-3- phenylthio-D-erythro-beta-L-gluco-2-nonulopyranosonate for the key sialylation step, and O-[methyl(5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha -D-galacto-2-nonulopyranosyl)onate]-(2----3)-O-(2,4,6-tri-O- acetyl-beta-D-galactopyranosyl-(1----4)-3,6-di-O-acetyl-2-O-pivaloyl- alpha-D-glucopyranosyl trichloroacetimidate and fluoride for the key coupling step with a ceramide derivative. These two steps were significantly altered and improved in comparison with our previous synthesis that had been executed without use of stereocontrolling auxiliaries. GM3 was obtained in 4.5% overall yield in 19 steps starting from allyl O-(2,6-di-O-acetyl-3,4-O-isopropylidene-beta-D-galactopyranosyl)-(1----4 )-2,3,6-tri-O-acetyl-beta-D-glucopyranoside.