Synthese der invarianten pentasaccharid-core-region der kohlenhydrat-ketten der N-glycoproteine

Chemical synthesis of highly congested gp120 V1V2 N-glycopeptide antigens for potential HIV-1-directed vaccines

Critical to the search for an effective HIV-1 vaccine is the development of immunogens capable of inducing broadly neutralizing antibodies (BnAbs). A key first step in this process is to design immunogens that can be recognized by known BnAbs. The monoclonal antibody PG9 is a BnAb that neutralizes diverse strains of HIV-1 by targeting a conserved carbohydrate-protein epitope in the variable 1 and 2 (V1V2) region of the viral envelope. Important for recognition are two closely spaced N-glycans at Asn(160) and Asn(156). Glycopeptides containing this synthetically challenging bis-N-glycosylated motif were prepared by convergent assembly, and were shown to be antigenic for PG9. Synthetic glycopeptides such as these may be useful for the development of HIV-1 vaccines based on the envelope V1V2 BnAb epitope.

Toward fully synthetic homogeneous beta-human follicle-stimulating hormone (beta-hFSH) with a biantennary N-linked dodecasaccharide. synthesis of beta-hFSH with chitobiose units at the natural linkage sites

A highly convergent synthesis of the sialic acid-rich biantennary N-linked glycan found in human glycoprotein hormones and its use in the synthesis of a fragment derived from the beta-domain of human Follicle-Stimulating Hormone (hFSH) are described. The synthesis highlights the use of the Sinay radical glycosidation protocol for the simultaneous installation of both biantennary side-chains of the dodecasaccharide as well as the use of glycal chemistry to construct the tetrasaccharide core in an efficient manner. The synthetic glycan was used to prepare the glycosylated 20-27aa domain of the beta-subunit of hFSH under a Lansbury aspartylation protocol. The proposed strategy for incorporating the prepared N-linked dodecasaccharide-containing 20-27aa domain into beta-hFSH subunit was validated in the context of a model system, providing protected beta-hFSH subunit functionalized with chitobiose at positions 7 and 24.

A convergent strategy for the preparation of N-glycan core di-, tri-, and pentasaccharide thioaldoses for the site-specific glycosylation of peptides and proteins bearing free cysteines

Mammalian glycoprotein biosynthesis produces heterogeneous ranges of proteins that possess the same peptide backbone but differ in the nature and site of glycosylation. This feature has frustrated efforts to develop therapeutic glycoproteins as well as the elucidation of biological functions of individual glycoforms. We have developed an attractive approach to well-defined glycoforms of glycoproteins by oxidative coupling of thioaldoses to cysteine-containing peptides and proteins to give disulfide-linked neoglycoconjugates. To this end, the chemical synthesis di-, tri-, and pentasaccharide N-glycan thioaldoses was undertaken. A convergent approach was used for the preparation of the pentasaccharide containing a 'synthetically difficult' beta-mannoside linkage. This linkage was installed by forming initially the corresponding beta-glucoside-containing pentasaccharide, followed by inversion of configuration at C-2. This approach exploited a levulonyl ester at C-2 of a glucosyl donor, which directed the coupling to give the beta-glucoside exclusively and could be removed selectively using hydrazine acetate without affecting other base-labile functionalities. The resulting alcohol was converted into a triflate, which was displaced by tri-n-butylammonium acetate to give a beta-mannosidic linkage. The trisaccharide N-glycan was prepared in a similar manner. Thioaldoses were prepared by displacing the peracetylated alpha-glycosyl chlorides with thioacetate to give the peracetylated beta-thioacetates, which upon saponification gave the desired compounds. The incubation of molar excesses of chitobiose thioaldose with cysteine-containing glutathione and BSA resulted in the site-specific formation of a disulfide-linked neoglycopeptide and neoglycoprotein, respectively.

Enzymic synthesis of the trisaccharide core region of the carbohydrate chain of N-glycoprotein

Transmannosylation from mannotriose (Man beta 1-4Man beta 1-4Man) to the 4-position at the nonreducing end N-acetylglucosaminyl residue of N,N'-diacetylchitobiose was regioselectively induced through the use of beta-D-mannanase from Aspergillus niger. The enzyme formed the trisaccharide Man beta 1-4GlcNAc beta 1-4GlcNAc (3.7% of the enzyme-catalysed net decrease of N,N'-diacetylchitobiose) from mannotriose as a donor and N,N'-diacetylchitobiose as an acceptor. Mannobiose (Man beta 1-4Man) was also shown to be useful as a donor substrate for the desired trisaccharide synthesis.

Control of glycoprotein synthesis: substrate specificity of rat liver UDP-GlcNAc:Man alpha 3R beta 2-N-acetylglucosaminyltransferase I using synthetic substrate analogues

UDP-GlcNAc: Man alpha 3R beta 2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) is the key enzyme in the synthesis of complex and hybrid N-glycans. Rat liver GlcNAc-T I has been purified more than 25,000-fold (M(r) 42,000). The Vmax for the pure enzyme with [Man alpha 6(Man alpha 3)Man alpha 6](Man alpha 3)Man beta 4GlcNAc beta 4GlcNAc beta-Asn as substrate was 4.6 mumol min-1 mg-1. Structural analysis of the enzyme product by proton nuclear magnetic resonance spectroscopy proved that the enzyme adds an N-acetylglucosamine (GlcNAc) residue in beta 1-2 linkage to the Man alpha 3Man beta-terminus of the substrate. Several derivatives of Man alpha 6(Man alpha 3)Man beta-R, a substrate for the enzyme, were synthesized and tested as substrates and inhibitors. An unsubstituted equatorial 4-hydroxyl and an axial 2-hydroxyl on the beta-linked mannose of Man alpha 6(Man alpha 3)Man beta-R are essential for GlcNAc-T I activity. Elimination of the 4-hydroxyl of the alpha 3-linked mannose (Man) of the substrate increases the KM 20-fold. Modifications on the alpha 6-linked mannose or on the core structure affect mainly the KM and to a lesser degree the Vmax, e.g., substitutions of the Man alpha 6 residue at the 2-position by GlcNAc or at the 3- and 6-positions by mannose lower the KM, whereas various other substitutions at the 3-position increase the KM slightly. Man alpha 6(Man alpha 3)4-O-methyl-Man beta 4GlcNAc was found to be a weak inhibitor of GlcNAc-T I.

A regio- and stereo-controlled synthesis of beta-D-Glcp NAc6SO3-(1----3)-beta-D-Galp6SO3-(1----4)-beta-D-GlcpNAc 6SO3- (1----3)-D-Galp, a linear acidic glycan fragment of keratan sulfate I

A stereocontrolled synthesis of beta-D-GlcpNAc6SO3-(1----3)-beta-D-Galp6SO3-(1----4)-beta-D- GlcpNAc6SO3- (1----3)-D-Galp, was achieved by use of benzyl O-(2-acetamido-3,4 di-O-benzyl-2-deoxy-6-O-p-methoxyphenyl-beta-D- glucopyranosyl)-(1----3)-O-(2,4-di-O-tert-butyldiphenylsilyl-beta- D- galactopyranosyl-(1----4)-O-(2-acetamido-3-O-benzyl-2-deoxy-6-O-p-methox yphenyl - beta-D-glucopyranosyl)-(1----3)-2,4,6-tri-O-benzyl-beta-D-galactopyranos ide as a key intermediate, which was in turn prepared by employing two glycosyl donors, 3,4-di-O-benzyl-2-deoxy-6-O-p-methoxyphenyl-2-phthalimido-beta-D- glucopyranosyl trichloroacetimidate and O-(3,6-di-O-acetyl-2,4-di-O-benzyl-beta-D-galactopyranosyl)-(1----4)-3-O - benzyl-2-deoxy-6-O-p-methoxyphenyl-2-phthalimido-beta-D-glucopyranosyl trichloroacetimidate, and a glycosyl acceptor, benzyl 2,4,6-tri-O-benzyl-beta-D-galactopyranoside.

Synthesis of the tetrasaccharide lipid intermediate P1-dolichyl P2-[O-alpha-D-mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4) -O-(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2- deoxy-alpha-D-glucopyranosyl] diphosphate

O-alpha-D-Mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4)-O-(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2-deoxy- D-glucopyranose was isolated from bovine or ovine mannosidosis urine. After peracetylation, treatment with trimethylsilyl trifluoromethanesulfonate gave a high yield of a peracetyl oxazoline, which was phosphorylated with dibenzyl phosphate to give a dibenzyl glycosyl phosphate that was converted into a peracetyl tetrasaccharide phosphate by catalytic hydrogenolysis. A coupling reaction with P1-dolichyl P2-diphenyl diphosphate, prepared in two stages from pig-liver dolichol, yielded a peracetyl diphosphoric diester, which on O-deacetylation gave P1-dolichyl P2-[O-alpha-D-mannopyranosyl-(1----6)-O-beta-D-mannopyranosyl-(1----4)-O -(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----4)-2-acetamido-2-deoxy- alpha-D- glucopyranosyl] diphosphate. This tetrasaccharide lipid intermediate was active as an acceptor of D-mannose residues from GDP-D-mannose in the presence of calf pancreas microsomes.

Synthesis of 1,4-dideoxy-1,4-imino-D-glucitol, a glucosidase inhibitor

1,2:5,6-Di-O-isopropylidene-D-glucitol was converted via its 1,4-dimethanesulfonate into the 1-azido-4-methanesulfonate which, after deprotection and treatment with barium hydroxide, afforded a 9:1 mixture of the corresponding 3,4- and 4,5-anhydro derivatives. Reduction of this mixture by transfer hydrogenation using ammonium formate in methanol and Pd/C as catalyst afforded 1,4-dideoxy-1,4-imino-D-glucitol (4), the structure of which was proved after acetylation by 1H-n.m.r. spectroscopy. Compound 4 is a potent alpha-D-glucosidase inhibitor (Ki 7 X 10(-4)M) and a less potent beta-D-glucosidase inhibitor (Ki 1.25 X 10(-4)M), and inhibits beta-D-galactosidase non-competitively.