Lactoside Acceptor Research Articles

Chemoenzymatic Synthesis of Globo-series Glycosphingolipids and Evaluation of Their Immunosuppressive Activities.

Glycosphingolipids (GSLs) play essential roles in many important biological processes, making them attractive synthetic targets. In this paper, a viable chemoenzymatic method is described for the synthesis of globo-series GSLs, namely, Gb4, Gb5, SSEA-4, and Globo H. The strategy uses a chemically synthesized lactoside acceptor equipped with a partial ceramide structure that is uniquely extended by glycosyltransferases in a highly efficient one-pot multiple enzyme (OPME) procedure. A direct and quantitative conversion of Gb4 sphingosine to Globo H sphingosine is achieved by performing two-sequential OPME glycosylations. A reduction and N-acylation protocol allows facile incorporation of various fatty acids into the lipid portions of the GSLs. The chemically well-defined lipid-modified Globo H-GSLs displayed some differences in their immunosuppressive activities, which may benefit the structural modifications of Globo H ceramides in finding new types of immunosuppressive agents. The strategy outlined in this work should be applicable to the rapid access to other complex GSLs.

Synthesis of an S-Linked α(2→8) GD3 Antigen and Evaluation of the Immunogenicity of Its Glycoconjugate.

Replacing the interglycosidic oxygen atom of oligosaccharides with a nonhydrolyzable sulfur atom has attracted significant interest because it provides opportunities for developing new glycoconjugate vaccines. Herein, a stereocontrolled and highly convergent method to synthesize a non-reducing-end inter-S-glycosidic variant of the GD3 antigen (S-linked α(2→8) GD3) that is resistant to enzymatic hydrolysis is reported. The key steps in the synthesis are a regio- and stereoselective α(2→3) sialylation of a lactoside acceptor with a C8-iodide-derivatized sialyl donor and an anomeric S-alkylation, which enable stereoselective construction of a terminal S-linked α(2→8) disialyl residue. The sulfhydryl-reactive maleimide group was used as the linker for the well-defined conjugation of these antigens to the immunogenic protein keyhole limpet hemocyanin (KLH). Groups of mice were immunized with the GD3-KLH and S-linked GD3-KLH glycoconjugates in the presence of complete Freund's adjuvant. Microarray analysis of the sera showed the promise of the S-linked GD3-KLH vaccine: it stimulated a high immunoglobulin G response against S-linked GD3 and cross-reactivity with the O-linked GD3 antigen was low. The activity of the S-linked GD3-KLH vaccine was comparable to that of the O-linked GD3-KLH vaccine, which highlighted the effectiveness of generating glycoconjugate vaccines and immunotherapies by relatively simple means.

Enzymatic synthesis of human blood group P1 pentasaccharide antigen

The enzymatic synthesis of biologically important and structurally unique human P1PK blood group type P1 pentasaccharide antigen is described. This synthesis features a three-step sequential one-pot multienzyme (OPME) glycosylation for the stepwise enzymatic chain elongation of readily available lactoside acceptor with cheap and commercially available galactose and N-acetylglucosamine as donor precursors. This enzymatic synthesis provides an operationally simple approach to access P1 pentasaccharide and its structurally related Gb3 and P1 trisaccharide epitopes.

Bacterial synthesis of polysialic acid lactosides in recombinant Escherichia coli K-12.

Bacterial polysialyltransferases (PSTs) are processive enzymes involved in the synthesis of polysialic capsular polysaccharides. They can also synthesize polysialic acid in vitro from disialylated and trisialylated lactoside acceptors, which are the carbohydrate moieties of GD3 and GT3 gangliosides, respectively. Here, we engineered a non-pathogenic Escherichia coli strain that overexpresses recombinant sialyltransferases and sialic acid synthesis genes and can convert an exogenous lactoside into polysialyl lactosides. Several PSTs were assayed for their ability to synthesize polysialyl lactosides in the recombinant strains. Fed-batch cultures produced α-2,8 polysialic acid or alternate α-2,8-2,9 polysialic acid in quantities reaching several grams per liter. Bacterial culture in the presence of propargyl-β-lactoside as the exogenous acceptor led to the production of conjugatable polysaccharides by means of copper-assisted click chemistry.

A Convenient Synthesis of Partially Benzylated Derivative of β-DGlcpNAc-( 1→3)-β-D-Galp-(1→4)-β-D-Glcp-1-OBn as a Versatile Building Block for Sialyl Lewis X Antigens

Synthesis of the trisaccharide, benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-β-Dglucopyranosyl-(1→3)-2,4,6-tri-O-benzyl-β-D-galactopyranosyl-(1→4)-2,3,6-tri-O-benzyl-β-Dglucopyranoside (9), was achieved from building block derivatives of the component mono- and disaccharide units. Initially the benzyl lactoside acceptor 3, which has a free hydroxyl group at position O-3`, was prepared via selective opening of the 3',4'-cyclic orthoester derivative 2. The glucosaminyl donor, 2-methyl (3,4,6-tri-O-acetyl-l,2-dideoxy-(α-D-glucopyrano)-[2`,1`:4,5]-2-oxazoline (4), was coupled to 3 affording the trisaccharide glycoside 5 in a good yield. Successive de-O-acetylation (5→6), benzylidenation (6→7), benzylation (7→8) and reductive opening of the benzylidene acetal function of 8 gave the target trisaccharide 9, which is a useful building block for the construction of complex oligosaccharides. Keywords: Carbohydrates, Building block, Oxazoline coupling, Cyclic orthoester, Sialyl Lex.

Chemoenzymatic synthesis of lacto-N-tetrasaccharide and sialyl lacto-N-tetrasaccharides

A concise and practical chemoenzymatic synthesis of lacto-N-tetraose, a major component and one of the most common core structures of human milk oligosaccharides (HMOs) was reported. This convergent synthesis relies on the glycosylation of a readily available lactoside acceptor with a lacto-N-biose donor generated from a highly efficient one-pot two-enzyme synthesis.

Synthesis of Lewis X and three Lewis X trisaccharide analogues in which glucose and rhamnose replace N-acetylglucosamine and fucose, respectively

Three analogues of the Le x trisaccharide: α- l-Fuc p-(1→3)-[β- d-Gal p-(1→4)]- d-GlcNAc p as well as the Le x trisaccharide itself were synthesized as methyl glycosides. In the analogues, either only the fucose residue is replaced by rhamnose or both the N-acetylglucosamine and the fucosyl residues are replaced by glucose and rhamnose, respectively. Our synthetic strategy relied on the use of lactoside and 2-azido lactoside derivatives as disaccharide acceptors, which were submitted to either fucosylation or rhamnosylation. Our results confirm that the reactivity of lactose in protection and glycosylation reactions is greatly affected by (1) the structure of the aglycone and (2) the presence of an azido substituent at C-2 of the glucose moiety. Thus, a methyl lactoside acceptor was easily glycosylated at O-3 with perbenzylated β-thiophenyl fucoside and rhamnoside to give anomerically pure α-fucosylated and α-rhamnosylated trisaccharides, respectively. In contrast, the same reactions on a 2-azido methyl lactoside acceptor led to the formation of anomeric mixtures. While the α- and β-fucosylated 2-azido trisaccharides could be separated by RP-HPLC, such separation was not possible for the rhamnosylated anomers. The desired rhamnosylated trisaccharide was finally obtained anomerically pure using an isopropylidene-protected rhamnosyl donor. The deprotection sequences also showed that the presence of a 2-azido substituent at C-2 of the glucose residue conferred stability to the vicinal fucosidic linkage at C-3. To test their relative affinity for anti-Le x Abs the Le x analogues will be used as competitive inhibitors against methyl Le x. In addition, their conformational behavior will be studied by NMR spectroscopy and molecular modeling experiments.

Kinetic and Mechanistic Analysis of Trypanosoma cruzi Trans-Sialidase Reveals a Classical Ping-Pong Mechanism with Acid/Base Catalysis

The trans-sialidase from Trypanosoma cruzi catalyzes the transfer of a sialic acid moiety from sialylated donor substrates to the terminal galactose moiety of lactose and lactoside acceptors to yield alpha-(2,3)-sialyllactose or its derivatives with net retention of anomeric configuration. Through kinetic analyses in which the concentrations of two different donor aryl alpha-sialoside substrates and the acceptor substrate lactose were independently varied, we have demonstrated that this enzyme follows a ping-pong bi-bi kinetic mechanism. This is supported for both the native enzyme and a mutant (D59A) in which the putative acid/base catalyst has been replaced by the demonstration of the half-reaction in which a sialyl-enzyme intermediate is formed. Mass spectrometric analysis of the protein directly demonstrates the formation of a covalent intermediate, while the observation of release of a full equivalent of p-nitrophenol by the mutant in a pre-steady state burst provides further support. The active site nucleophile is confirmed to be Tyr342 by trapping of the sialyl-enzyme intermediate using the D59A mutant and sequencing of the purified peptic peptide. The role of D59 as the acid/base catalyst is confirmed by chemical rescue studies in which activity is restored to the D59A mutant by azide and a sialyl azide product is formed.

Synthesis of globo- and isoglobotriosides bearing a cinnamoylphenyl tag as novel electrophilic thiol-specific carbohydrate reagents

The galactosyl donor, 4,6-di-O-acetyl-2,3-di-O-benzyl-d-galactopyranosyl trichloroacetimidate, was efficiently coupled with regioselectively benzylated lactoside acceptors under standard conditions to stereoselectively afford the corresponding globotrioside and isoglobotrioside derivatives in very good yields. These glycosides were smoothly functionalized with a 6-(p-cinnamoylphenoxy)-hexyl tether tag as novel electrophilic thiol-specific carbohydrate reagents. Immobilization of the globotrioside conjugate to Thiopropyl Sepharose 6B for purification of B-subunit of Shiga toxin (StxB) and coupling of a model cysteine-containing protein (StxB-Z(n)-Cys) to the isoglobotrioside conjugate were both performed with high efficiency.

Differential Recognition of the Type I and II H Antigen Acceptors by the Human ABO(H) Blood Group A and B Glycosyltransferases

The human ABO(H) blood group A and B antigens are generated by the homologous glycosyltransferases A (GTA) and B (GTB), which add the monosaccharides GalNAc and Gal, respectively, to the cell-surface H antigens. In the first comprehensive structural study of the recognition by a glycosyltransferase of a panel of substrates corresponding to acceptor fragments, 14 high resolution crystal structures of GTA and GTB have been determined in the presence of oligosaccharides corresponding to different segments of the type I (alpha-l-Fucp-(1-->2)-beta-D-Galp-(1-->3)-beta-D-GlcNAcp-OR, where R is a glycoprotein or glycolipid in natural acceptors) and type II (alpha-l-Fucp-(1-->2)-beta-D-Galp-(1-->4)-beta-d-GlcNAcp-OR) H antigen trisaccharides. GTA and GTB differ in only four "critical" amino acid residues (Arg/Gly-176, Gly/Ser-235, Leu/Met-266, and Gly/Ala-268). As these enzymes both utilize the H antigen acceptors, the four critical residues had been thought to be involved strictly in donor recognition; however, we now report that acceptor binding and subsequent transfer are significantly influenced by two of these residues: Gly/Ser-235 and Leu/Met-266. Furthermore, these structures show that acceptor recognition is dominated by the central Gal residue despite the fact that the L-Fuc residue is required for efficient catalysis and give direct insight into the design of model inhibitors for GTA and GTB.

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.

Efficient synthesis of globoside and isogloboside tetrasaccharides by using beta(1-->3) N-acetylgalactosaminyltransferase/UDP-N-acetylglucosamine C4 epimerase fusion protein.

The beta(1-->3) N-acetylgalactosaminyltransferase/UDP-N-acetylglucosamine C4 epimerase fusion protein was constructed and used in coupled enzymatic reactions to synthesize a variety of globotetraose and isoglobotetraose derivatives from the corresponding lactoside acceptors.

Synthetic Studies on Sialoglycoconjugates. CXXIV. Total Synthesis of O-Glycan on the L-Selectin Ligand GlyCAM-1

The total synthesis of O-glycans containing the sulfated sialyl Lex (sLex) determinant on GlyCAM-1 (1), the counter-receptor glycoprotein for L-selectin, was accomplished by a simultaneous glycosylation procedure. The glycosylation of the lactoside acceptor (4) with the 2-azidogalactopyranosyl trichloroacetimidate donor (3) resulted in the desired trisaccharide (5). This trisaccharide, after transformation to the glycosyl acceptor, was successfully coupled with the phenylthioglycoside of glucosamine (13) to give the core 6 structure (14). Following proper manipulation of the protecting groups for the hydroxyl, as well as the amino function, the resulting free hydroxyl of the tetrasaccharide at C3 of the GlcNAc was then fucosylated to give the desired pentasaccharide (19). This pentasaccharide was converted into the key glycosyl acceptor (21), which was then simultaneously glycosylated with the sialyl α (2→3)-galactosyl trichloroacetimidate (22), forming the desired nonasaccharide (23). Finally, sulfation of the 6-OH of the GlcNAc residue and removal of all the protecting groups furnished the spacer-armed O-glycans on GlyCAM-1 (1).

Synthesis of propyl and 2-aminoethyl glycosides of α- d-galactosyl-(1→3′)-β-lactoside

Propyl and 2-aminoethyl α- d-galactopyranosyl-(1→3′)-β-lactosides ( 1 and 2) were prepared from the corresponding perbenzylated trisaccharide allyl glycoside 6 which, in turn, was obtained by methyl triflate promoted α-galactosylation of benzylated allyl lactoside acceptor 4 with thiogalactoside 3. Transformation of the allyl moiety in compound 6 into 2-azidoethyl one was achieved by cleavage of the double bond followed by reduction into alcohol 9, subsequent mesylation, and mesylate→azide substitution. Alternatively trisaccharide 2 was synthesized using α-galactosylation of selectively benzoylated 2-azidoethyl lactoside 19 with 3 as the key step.

Synthesis of selected aminodeoxy analogues of galabiose and globotriose

Six aminodeoxy 2-(trimethylsilyl)ethyl (Me 3SiEt) glycoside analogues of galabiose (4′- and 6′-aminodeoxy) and globotriose (6″-, 4″-, 2″-, and 6′-aminodeoxy) were synthesized by glycosylation of protected Me 3SiEt galactoside and lactoside acceptors with azido-substituted monosaccharide donors, followed by reduction of the azido groups and removal of the protecting groups.

Active–latent glycosylation strategy toward Lewis X pentasaccharide in a form suitable for neoglycoconjugate syntheses

Glycosylation of 4-nitrophenyl 2-acetamido-6- O-tert-butyldiphenylsilyl-2-deoxy-1-thio-β- d-glucopyranoside with phenyl 2,3,4,6-tetra- O-benzoyl-1-thio-β- d-galactopyranoside in the presence of NIS and TfOH as catalyst gave the lactosamine derivative regiospecifically in high yield. Further 3- O-fucosylation with phenyl 2,3,4-tri- O-benzyl-1-thio-β- l-fucopyranoside using DMTST as promoter afforded the Le x trisaccharide intermediate. The latent glycosyl donor was transformed into its active form ( p-acetamidothiophenyl) by reduction with zinc in acetic acid and N-acetylation. Glycosidation with p-nitrothiophenyl lactoside acceptor in the presence of NIS/TfOH as catalyst gave the Le x pentasaccharide in 71% yield.

Novel Selectivity in Carbohydrate Reactions Ii. Selective 6-O-Glycosylation of a Partially Protected Lactoside1 2

A novel regioselectivity was observed in the silver salt promoted glycosylation of 2-(trimethylsilyl)ethyl 3′-O-benzyl-β-D-lactoside using acetobromogalactose as the glycosyl donor. The resulting trisaccharide, obtained in 67% yield, was shown to have the newly formed β-glycosidic linkage at the O-6 position of the lactoside. This was confirmed by synthesis of the authentic product by an alternate route. The novel regioselectivity observed is attributed to the presence of the axially disposed 4′-OH group in the lactoside acceptor. 1. Dedicated to the memory of Professor Akira Hasegawa. 2. Presented at the VIIIth Carbohydrate Conference of the Association of Carbohydrate Chemists and Technologists India, Trivandrum, India, November, 1992. For Part I, see K.P.R. Kartha, M. Kiso, A. Hasegawa and H.J. Jennings, J.Chem.Soc. Perkin Trans. 1, 3023 (1995). 3. Present address: School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, U.K.