Pentafluorophenyl esters for temporary carboxyl group protection in solid phase synthesis of N-linked glycopeptides

Generation of a glycosylated asparagine residue through chemoselective acylation of a glycosylhydrazide

Herein, we report the first selective anomeric N-acylation of a glycosylhydrazide. We show that this transformation can be harnessed to generate amino acid building blocks including FmocAsn(GlcNAc)OH (1), a residue that has been previously shown to be a competent reagent in the solid-phase peptide synthesis of N-linked glycopeptides.

Advances in Fmoc solid-phase peptide synthesis

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.

Facile preparation of the N-acetyl-glucosaminylated asparagine derivative with TFA-sensitive protecting groups useful for solid-phase glycopeptide synthesis

In this study, a novel N-acetyl-glucosaminylated asparagine derivative was developed. This derivative carried TFA-sensitive protecting groups and was derived from commercially available compounds only in three steps. It was applicable to the ordinary 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis (SPPS) method, and the protecting groups on the carbohydrate moiety could be removed by a single step of TFA cocktail treatment generally used for the final deprotection step in Fmoc-SPPS.

Context-dependent effects of asparagine glycosylation on Pin WW folding kinetics and thermodynamics

Asparagine glycosylation is one of the most common and important post-translational modifications of proteins in eukaryotic cells. N-glycosylation occurs when a triantennary glycan precursor is transferred en bloc to a nascent polypeptide (harboring the N-X-T/S sequon) as the peptide is cotranslationally translocated into the endoplasmic reticulum (ER). In addition to facilitating binding interactions with components of the ER proteostasis network, N-glycans can also have intrinsic effects on protein folding by directly altering the folding energy landscape. Previous work from our laboratories (Hanson et al. Proc. Natl. Acad. Sci. U.S.A. 2009, 109, 3131-3136; Shental-Bechor, D.; Levy, Y. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 8256-8261) suggested that the three sugar residues closest to the protein are sufficient for accelerating protein folding and stabilizing the resulting structure in vitro; even a monosaccharide can have a dramatic effect. The highly conserved nature of these three proximal sugars in N-glycans led us to speculate that introducing an N-glycosylation site into a protein that is not normally glycosylated would stabilize the protein and increase its folding rate in a manner that does not depend on the presence of specific stabilizing protein-saccharide interactions. Here, we test this hypothesis experimentally and computationally by incorporating an N-linked GlcNAc residue at various positions within the Pin WW domain, a small β-sheet-rich protein. The results show that an increased folding rate and enhanced thermodynamic stability are not general, context-independent consequences of N-glycosylation. Comparison between computational predictions and experimental observations suggests that generic glycan-based excluded volume effects are responsible for the destabilizing effect of glycosylation at highly structured positions. However, this reasoning does not adequately explain the observed destabilizing effect of glycosylation within flexible loops. Our data are consistent with the hypothesis that specific, evolved protein-glycan contacts must also play an important role in mediating the beneficial energetic effects on protein folding that glycosylation can confer.

Fmoc-protected iminosugar modified asparagine derivatives as building blocks for glycomimetics-containing peptides

CSF114(Glc) is the first synthetic Multiple Sclerosis Antigenic Probe able to identify autoantibodies in a statistically significant number of Multiple Sclerosis patients. The beta-turn conformation of this glucopeptide is fundamental for a correct presentation of the epitope Asn(Glc). To verify the influence of sugar mimics in antibody recognition in Multiple Sclerosis, we synthesized Fmoc-protected Asn derivatives containing alkaloid-type sugar mimics. The corresponding glycomimetics-containing peptide derivatives of the CSF114-type sequence were tested in competitive and solid-phase non-competitive ELISA on Multiple Sclerosis patients' sera.

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Synthesis of tumor associated sialyl-T-glycopeptides and their immunogenicity

Sialyl-T-glycopeptides were synthesized by solid-phase techniques, using a PEGA resin as the solid support. An appropriately protected building block containing alpha-Neu5Ac-(2 --> 3)-beta-Gal-(1 --> 3)-alpha-GalN3-(1-->) attached to Fmoc-Thr/Ser-OPfp was employed in a solid phase glycopeptide assembly of a 10-mer glycopeptide, using a general Fmoc/OPfp-ester strategy. Reduction of the azido group of the GalN3 residue was effected on solid-phase, using DTT and DBU. After acidolytic cleavage from the resin, the methyl ester of the sialic acid residue and acetyl groups were removed with 30% NaOMe/MeOH in MeOH and water pH 14, at -30 degrees C for 2 h. At this low temperature, the highly basic conditions did not result in any detectable beta-elimination. However, one O-acetyl group, located at the 2-position of the Gal was resistant to hydrolysis. To remove this remaining acetyl group, reaction with hydrazine hydrate in CHCl3 and MeOH at room temperature for 2.5 h was successful. The two target sequences of sialyl-T-glycopeptides were obtained in good yield. In contrast to the the analogs carrying the T-antigen, the Sial-T-glycopeptides were nonimmunogenic, supporting the idea that the sialylation is a method of circumventing the recognition by the immune system.

Glycopeptide synthesis and the effects of glycosylation on protein structure and activity

Despite the omnipresence of protein glycosylation in nature, little is known about how the attachment of carbohydrates affects peptide and protein activity. One reason is the lack of a straightforward method to access biologically relevant glycopeptides and glycoproteins. The isolation of homogeneous glycopeptides from natural sources is complicated by the heterogeneity of naturally occuring glycoproteins. It is chemical and chemoenzymatic synthesis that is meeting the challenge to solve this availability problem, thus playing a key role for the advancement of glycobiology. The current art of glycopeptide synthesis, albeit far from being routine, has reached a level of maturity that allows for the access to homogeneous and pure material for biological and medicinal research. Even the ambitious goal of the total synthesis of an entire glycoprotein is within reach. It is demonstrated that with the help of synthetic glycopeptides the effects of glycosylation on protein structure and function can be studied in molecular detail. For example, in immunology, synthetic (tumour-specific) glycopeptides can be used as immunogens to elicit a tumour-cell-specific immune response. Again, synthetic glycopeptides are an invaluable tool to determine the fine specificity of the immune response that can be mediated by both carbohydrate-specific B and T cells. Furthermore, selected examples for the use of synthetic glycopeptides as ligands of carbohydrate-binding proteins and as enzyme substrates or inhibitors are presented.

Carbopeptides: carbohydrates as potential templates for de novo design of protein models

De novo design of proteins has evolved into a powerful approach for studying the factors governing protein folding and stability. Among the families of structures frequently studied is the 'four-helix bundle' in which four alpha-helical peptide strands, linked by loops, form a hydrophobic core. Assembly of protein models on a template has been suggested as a way to reduce the entropy of folding. Here we describe the potential use of a carbohydrate as such a template. The monosaccharide D-galactose was per-O-acylated with (Nbeta-Fmoc-betaAla)2O to give a penta-substituted derivative, which was converted to the corresponding glycosyl bromide and used for the glycosylation of 4-hydroxymethylbenzoic acid pentafluorophenyl ester (HMBA-OPfp). The beta-glycosidic carbohydrate template (Nbeta-Fmoc-3Ala)4-beta-D-Galp-(1-O)-MBA-OPfp thus obtained was coupled to a PAL-PEG-PS resin and simultaneously extended at the four arms to yield, after cleavage from the solid support, a carbopeptide with four identical peptide strands. Extension of this concept to, for example, synthesis of novel multiple antigenic peptides (MAPs) and synthesis of carbohydrate clusters can be easily envisioned. The ability to efficiently synthesize such structures sets the stage for further studies to test whether the carbohydrate templates do indeed nucleate folding.

Solid-phase synthesis of an O-linked glycopeptide based on a benzyl-protected glycan approach

The solid-phase synthesis of asialo-[Ala18]-B-chain (2) of human alpha 2HS glycoprotein is described. Disaccharide-linked serine unit 12, carrying a benzyl protecting group, was synthesized via stereoselective glycosylation of 8 with 6. Peptide synthesis was carried out by the Fmoc method utilizing an automated peptide synthesizer. A modified procedure using a mechanical shaker at the coupling step with 12 made easy the recovery of unreacted 12. The benzylated glycopeptide thus synthesized was cleaved from the resin and hydrogenated to give 2.

Multiple column synthesis of a library of T-cell stimulating Tn-antigenic glycopeptide analogues for the molecular characterization of T-cell-glycan specificity

A series of peptides and glycopeptides derived by amino acid and glycosyl amino acid scans through the self peptide from CBA/J mouse haemoglobin Hb (67-76). VITAFNEGLK, was synthesized by multiple column peptide synthesis (MCPS). Investigation of glycopeptide binding to the mouse major histocompatibility class II molecule Ek showed that glycans in position 72 did not interfere with the binding to Ek. Immunization experiments revealed that glycopeptides with the glycan in position 72 were immunogenic. Therefore a series of N-linked and O-linked glycopeptides with the glycan attached in the position 72 either to serine, threonine or asparagine was synthesized by MCPS. The glycan structure was furthermore varied with respect to monosaccharide component, size of oligosaccharide, anomer configuration and stereochemistry of essential hydroxyl groups in order to investigate the specificity of the interaction with the T-cell receptor. Easy synthesis of ready to use Ser and Thr building blocks corresponding to mucin core 1, the Tn-antigen and its beta-anomer were developed using trichloroacetimidates as glycosyl donors and reduction with in situ acetylation of the azide containing glycosylation products. Synthesis of an alpha-linked GlcNAc-Thr building block was achieved by glycosylation of Fmoc-Thr-OPip with 2-azido-2-deoxy-3,4,6-tri-O-acetyl-D-glycopyranosyl trichloroacetimidate as a glycosyl donor. Other building blocks were obtained by previously described procedures.

Solid-phase synthesis of a glycosylated hexapeptide of human sialophorin, using the trichloroacetimidate method

A hexapeptide containing a beta-D-Gal p-(1-->3)-alpha-D-Gal pNAc-(1-->O)-L-threonine unit was synthesized using glycosylated pentafluorophenyl esters in an Fmoc-based strategy. In all of the glycosylation reactions, trichloroacetimidates were successfully employed. The disaccharide moiety was prepared from tetra-O-acetyl-alpha-D-galactopyranosyl trichloroacetimidate and tert-butyldimethylsilyl 2-azido-6-O-benzoyl-2-deoxy-beta-E-galactopyranoside with boron trifluoride etherate as a catalyst. The glycosylated active esters were obtained in the reaction of alpha and beta 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl-(1-->3)-4-O-acetyl-2-azid o-6- O-benzoyl-2-deoxy-D-galactopyranosyl trichloroacetimidates with Fmoc-protected pentafluorophenyl esters of L-serine and L-threonine in the presence of trimethylsilyl trifluoromethanesulfonate as Lewis acid. The glycosylated pentafluorophenyl ester of L-threonine was transformed into glycopeptides via a solid-phase synthesis. Azide reduction and N-acetylation were performed on the solid phase with a thioacetic acid-pyridine mixture. The glycopeptide was then cleaved from the resin with strong acid, also removing the acid-labile protecting groups of the peptide chain. Finally, the acyl groups used for sugar protection were cleaved with sodium methoxide, affording the completely deprotected N-acetyl-L-leucyl-L-glutamyl-O-[beta-D-galactopyranosyl-(1-->3)-2- acetamido-2-deoxy-alpha-D-galactopyranosyl]-L-threonyl-L-seryl-L-threony l- glycinamide (1) in high purity.

Synthesis of glycopeptides and neoglycoproteins containing the fucosylated linkage region of N-glycoproteins

N-Glycoproteins fucosylated in the core region occur in tumor membranes and virus envelopes. Partial structures of such N-glycoproteins containing fucosylated chitobiosyl asparagine conjugates were synthesized using the allyloxycarbonyl (Aloc) and the tert-butyl ester protecting groups in the peptide portion. As the alpha-fucosidic bond of the conjugates revealed to be very sensitive to acids when carrying ether-type protecting groups, a method for exchanging the protecting groups of the fucose portion of saccharides was developed. Conjugates containing O-acetyl protected fucose proved to be stable against acids used in glycopeptide syntheses. These methods were applied in the synthesis of a fucosyl chitobiose hexapeptide with the partial sequence of a leukemia virus envelope glycoprotein. The glycopeptide was coupled to bovine serum albumin yielding a neoglycoprotein which contains a glycoconjugate of exactly specified structure.

Silyl protection in the solid-phase synthesis of N-linked glycopeptides. Preparation of glycosylated fluorogenic substrates for subtilisins

The trimethylsilyl (TMS) group was used for protection of the hydroxy groups of three disaccharide 1-amino-alditols and of the glycosylamines of glucose, maltotriose and maltoheptose. The per-O-trimethylsilylated derivatives were coupled with N alpha-Fmoc-Asp(Cl)-OPfp 7 to give six glycosylated building blocks for the solid-phase synthesis of N-linked glycopeptides. Building block 8 was used in the synthesis of five internally quenched fluorescent substrates which were studied by enzymatic hydrolysis with savinase, a subtilisin-type enzyme.

Glycosylation of synthetic T helper cell epitopic peptides influences their antigenic potency and conformation in a sugar location-specific manner

The immunodominant T helper cell epitopes 31D and VF13N of rabies virus nucleoprotein and glycoprotein, respectively, correspond to peptide sequences AVYTRIMMNGGRLKR and VVEDEGCTNLSGF, and are expressed between amino acids 404-418 and 29-41, of the appropriate proteins. We investigated how internal or external glycosylation affects the biological activity and conformation of the peptides 31D and VF13N. Mid-chain incorporation of maltobiose or N-acetylglucosamine moieties into the asparagine residues greatly diminished the T-cell stimulatory activity in vitro (due to the diminished ability of the glycopeptides to bind to major histocompatibility complex determinants) and reduced the characteristic alpha-helicity of the peptides in aqueous trifluoroethanol solutions. In contrast, addition of maltobiose- or N-acetylglucosamine-coupled asparagines to the N-termini of peptides 31D and VF13N resulted in unchanged T-cell activity. Furthermore, N-terminal glycosylation of peptide 31D, as indicated by the functional assay, decreased the sensitivity of the peptide to degradation in human serum and did not affect the alpha-helical conformation. These data indicate that glycosylation of T-cell epitopes is not a preferable method for the preparation of antagonists, but incorporation of the sugars to appropriate positions may be advantageous in the design of T-cell agonists and peptide-based vaccines.

Attachment of oligosaccharides to peptide antigen profoundly affects binding to major histocompatibility complex class II molecules and peptide immunogenicity

To investigate the immunogenicity of glycopeptides, a peptide fragment from hen egg lysozyme, HEL(81-96)-Y (here named 1) which is immunogenic in H-2k mice and known to bind to the murine major histocompatibility complex (MHC) class II molecule Ek, was synthesized in five different glycosylated forms. The N-terminal serine of HEL(81-96)-Y was derivatized with D-glucose (2), maltotriose (3), and a branched D-glucose pentasaccharide (4). Furthermore, 1 was prepared with a central serine or asparagine derivatized with the branched D-glucose pentasaccharide (5) and GlcNAc (6), respectively. The ability of the five glycopeptides and the non-glycosylated peptide, labeled with 125I, to bind to the two MHC class II molecules, Ak and Ek, was studied using a gel filtration assay. None of them could bind to Ak. Neither 5 nor 6 were able to bind to Ek. Surprisingly 2, 3 and 4 bound better to Ek than did the non-glycosylated peptide 1. The increased binding varied depending on the type of oligosaccharide attached to the N terminus of the peptide. The better binding to Ek of glycopeptide 4 was found to be due to an increased association rate. The binding of 1 as well as 4 was optimal at pH 5.0. Functional studies showed that 4 was able to elicit a heteroclitic proliferative response from T cells of mice immunized with the native non-glycosylated peptide. Circular dichroism studies of 1 and 4 indicated a more unordered structure of 4 and a predominant alpha-helical conformation of 1, suggesting that the MHC class II molecule may bind to peptides which are in a non-alpha-helical conformation. These results demonstrate that glycosylation has considerable influence on peptide immunogenicity for T lymphocytes.

Chemical and enzymatic synthesis of multivalent sialoglycopeptides

Linear and branched glycopeptides containing multiple sialyl-N-acetyllactosamine side chains have been synthesized using a combined chemical and enzymatic approach. Peptide backbones in which beta-GlcNAc-Asn residues were incorporated were obtained in good yields by optimized solid-phase synthesis following the Boc strategy. The resulting multivalent glycopeptides were galactosylated in near-quantitative yields using bovine galactosyltransferase, UDP-galactose, and calf alkaline phosphatase that destroys the inhibiting side product UDP. Subsequent enzymatic sialylation yielded the desired glycopeptides containing asparagine-linked sialyl-N-acetyllactosamine side chains. The compounds were characterized by 1H NMR and FABMS. Recombinant sialyltransferase and CMP-sialate synthetase were used for the enzymatic synthesis of sialosides on a preparative scale. The synthetic glycopeptides were tested as inhibitors of influenza virus to cells, revealing that most of the multivalent sialoglycopeptides exhibit increased binding that depends on the spacing when compared to monovalent compounds. A possible mechanism for increased binding is proposed.