Chemical glycobiology

Chemical tools have proven indispensable for studies in glycobiology. Synthetic oligosaccharides and glycoconjugates provide materials for correlating structure with function. Synthetic mimics of the complex assemblies found on cell surfaces can modulate cellular interactions and are under development as therapeutic agents. Small molecule inhibitors of carbohydrate biosynthetic and processing enzymes can block the assembly of specific oligosaccharide structures. Inhibitors of carbohydrate recognition and biosynthesis can reveal the biological functions of the carbohydrate epitope and its cognate receptors. Carbohydrate biosynthetic pathways are often amenable to interception with synthetic unnatural substrates. Such metabolic interference can block the expression of oligosaccharides or alter the structures of the sugars presented on cells. Collectively, these chemical approaches are contributing great insight into the myriad biological functions of oligosaccharides.

Toward automated synthesis of oligosaccharides and glycoproteins

The discovery of previously unknown functions associated with carbohydrates and the study of their structure-function relations are of current interest in carbohydrate chemistry and biology. Progress in this area is, however, hampered by the lack of convenient and effective tools for the synthesis and analysis of oligosaccharides and glycoconjugates. Development of automated synthesis of such materials is necessary to facilitate research in this field. This review describes recent advances in carbohydrate synthesis, with particular focus on developments that have potential application to the automated synthesis of oligosaccharides, glycopeptides, and glycoproteins.

The cellular response to neuregulins is governed by complex interactions of the erbB receptor family

Deregulated signaling by the four members of the epidermal growth factor receptor tyrosine kinase family (erbB family) is implicated in the genesis or progression of human cancers. However, efforts to analyze signaling by these receptors have been hampered by the diversity of ligands and extensive interreceptor cross talk. We have expressed the four human erbB family receptors, singly and in pairwise combinations, in a pro-B-lymphocyte cell line (Ba/F3) and investigated the range of interactions activated by the epidermal growth factor homology domain of the agonist neuregulin beta. The results provide the first comprehensive analysis of the response of this receptor family to a single peptide agonist. This peptide induced complex patterns of receptor tyrosine phosphorylation and regulation of Ba/F3 cell survival and proliferation. These data demonstrate the existence of several previously undocumented receptor interactions driven by neuregulin.

Toward fully synthetic glycoproteins by ultimately convergent routes: a solution to a long-standing problem

A method is disclosed for the convergent synthesis of multiply glycosylated peptides. The approach centers on a convergent technique for generating masked, complex glycopeptide-containing C-terminal acyl donors. Activation of the latent donor in situ and use directly in segment coupling with a second peptide bearing a complex carbohydrate produces a completely unprotected, bifunctional glycopeptide. The system demonstrates a minimum level of hydrolysis and epimerization at the C-terminal amino acid residue of the acyl donor during fully convergent segment coupling and is therefore a powerful tool for the synthesis of glycoproteins.

Sialoside specificity of the siglec family assessed using novel multivalent probes: identification of potent inhibitors of myelin-associated glycoprotein

Ten of the 11 known human siglecs or their murine orthologs have been evaluated for their specificity for over 25 synthetic sialosides representing most of the major sequences terminating carbohydrate groups of glycoproteins and glycolipids. Analysis has been performed using a novel multivalent platform comprising biotinylated sialosides bound to a streptavidin-alkaline phosphatase conjugate. Each siglec was found to have a unique specificity for binding 16 different sialoside-streptavidin-alkaline phosphatase probes. The relative affinities of monovalent sialosides were assessed for each siglec in competitive inhibition studies. The quantitative data obtained allows a detailed analysis of each siglec for the relative importance of sialic acid and the penultimate oligosaccharide sequence on binding affinity and specificity. Most remarkable was the finding that myelin-associated glycoprotein (Siglec-4) binds with 500-10,000-fold higher affinity to a series of mono- and di-sialylated derivatives of the O-linked T-antigen (Galbeta(1-3)-GalNAc(alpha)OThr) as compared with alpha-methyl-NeuAc.

Synthetic glycopeptides and glycoproteins as tools for biology

Investigations into the roles of protein glycosylation have revealed functions such as modulating protein structure and localization, cell-cell recognition, and signaling in multicellular systems. However, detailed studies of these events are hampered by the heterogeneous nature of biosynthetic glycoproteins that typically exist in numerous glycoforms. Research into protein glycosylation, therefore, has benefited from homogeneous, structurally-defined glycoproteins obtained by chemical synthesis. This tutorial review focuses on recent applications of homogeneous synthetic glycopeptides and glycoproteins for studies of structure and function. In addition, the future of synthetic glycopeptides and glycoproteins as therapeutics is discussed.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Chemoselective ligation reactions with proteins, oligosaccharides and cells

Traditional chemical synthesis does not lend itself to the easy, rapid construction of even moderately sized biomolecules, because it requires elaborate protection schemes. Furthermore, many biological studies would be aided by the ability to assemble biomolecules under physiological conditions. These challenges have motivated the development of chemoselective ligation, the selective covalent coupling of mutually and uniquely reactive functional groups under mild, aqueous conditions. This technique has attracted significant attention recently for the synthesis of biological macromolecules of defined homogeneous composition, the design of self-assembling drugs and the chemical remodeling of cell surfaces.

Nonequilibrium antifreeze peptides and the recrystallization of ice

Evidence is presented that the nonequilibrium antifreeze peptide (AFP) from winter flounder has a special ability to inhibit recrystallization in ice only when an appreciable amount of liquid is present, as is the case when the system contains salts and the temperature is not too low. In this circumstance the AFP binds to the ice surface at the ice-solution interfaces in grain boundaries, preventing migration of the solution and effectively immobilizing the boundaries. In the absence of liquid, recrystallization inhibition appears to be a common property of many peptides. This is consistent with the view that the special effects of AFPs require a structural fit onto ice, and therefore require the AFP molecules to have the mobility to achieve that fit. Since the concentration of salt required to induce the special recrystallization inhibition effects of AFPs is lower (< 10 mM) than that found normally in physiological fluids, AFPs could play a role in the survival of organisms by preventing damage due to recrystallization. The proposition that mobility is needed for AFP molecules to produce their special influence upon ice growth argues against any special effects of AFPs in devitrification.

Glycopeptides as versatile tools for glycobiology

This review describes the recent advances in the field of glycopeptide and small glycoprotein synthesis. The strategies covered include chemical and chemoenzymatic synthesis, native chemical ligation (NCL), and expressed chemical ligation. The importance of glycopeptide synthesis is exemplified by giving the reader an overview of how versatile and important these well-defined glycopeptides are as tools in glycobiology.

Conformational implications of asparagine-linked glycosylation

The effects of cotranslational protein modification on the process of protein folding are poorly understood. Time-resolved fluorescence energy transfer has been used to assess the impact of glycosylation on the conformational dynamics of flexible oligopeptides. The peptide sequences examined are selected from glycoproteins of known three-dimensional structure. The energy transfer modulation associated with N-linked glycosylation is consistent with the glycopeptides sampling different conformational profiles in water. Results show that glycosylation causes the modified peptides to adopt a different ensemble of conformations, and for some peptides this change may lead to conformations that are more compact and better approximate the conformation of these peptides in the final folded protein. This result further implies that cotranslational glycosylation can trigger the timely formation of structural nucleation elements and thus assist in the complex process of protein folding.

Chemoselective approaches to glycoprotein assembly

Oligosaccharides on proteins and lipids play central roles in human health and disease. The molecular analysis of glycoconjugate function has benefited tremendously from new methods for their chemical synthesis, which provides homogeneous material not attainable from biosynthetic systems. Still, glycoconjugate synthesis requires the manipulation of multiple stereocenters and protecting groups and remains the domain of a few expert laboratories around the world. This Account summarizes chemoselective approaches for assembling homogeneous glycoconjugates that attempt to reduce the barriers to their synthesis. The objective of these methods is to make glycoconjugate synthesis accessible to a broader community, thereby accelerating progress in glycobiology.

Homogeneous glycopeptides and glycoproteins for biological investigation

Protein glycosylation is widely recognized as a modulator of protein structure, localization, and cell-cell recognition in multicellular systems. Glycoproteins are typically expressed as mixtures of glycoforms, their oligosaccharides being generated by a template-independent biosynthetic process. Investigation of their function has been greatly assisted by sources of homogeneous material. This review summarizes current efforts to obtain homogeneous glycopeptide and glycoprotein materials by a variety of methods that draw from the techniques of recombinant expression, chemical synthesis, enzymatic transformation, and chemoselective ligation. Some of these techniques remove obstacles to glycoprotein synthesis by installing nonnative linkages and other modifications for facilitated assembly. The end purpose of the described approaches is the production of glycosylated materials for experiments relevant to the biological investigation of glycoproteins, although the strategies presented apply to other posttranslational modifications as well.

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.

Shifting paradigms: biopharmaceuticals versus low molecular weight drugs

Biopharmaceuticals are pharmaceutical products consisting of (glyco)proteins. Nowadays a substantial part of the FDA-approved drugs belong to this class of drugs. Biopharmaceuticals deserve special attention as they have a number of characteristics that set them aside from low molecular weight drugs. Their activity depends on their complicated shape based on secondary, tertiary and (sometimes) quaternary structures. These structures cannot be fully defined with our present set of analytical techniques and approaches for potency testing. They often are the same as (or closely resemble) endogenous proteins. This means that in safety testing and clinical test programs questions have to be addressed regarding species specific responses, selection of dosing schedules and route of administration, and the possible occurrence of immunogenicity. As the conformational structure of a protein is easily disturbed, formulation and handling of biopharmaceuticals needs special attention in order to optimize the therapeutic effect and minimize adverse reaction, among which immune responses. The issue of biogenerics is gaining more and more interest and different critical elements in the development of biogenerics are touched upon. In conclusion, biopharmaceuticals cannot be characterized fully in terms of their structure like low molecular weight drugs. The performance of biopharmaceuticals relies on strict production protocols and close monitoring of their activity in the clinical situation.

Sugar-assisted ligation in glycoprotein synthesis

Sugar-assisted ligation (SAL) presents an attractive strategy for the synthesis of glycopeptides, including the synthesis of cysteine-free beta-O-linked and N-linked glycopeptides. Here we extended the utility of SAL for the synthesis of alpha-O-linked glycopeptides and glycoproteins. In order to explore SAL in the context of glycoprotein synthesis, we developed a new chemical synthetic route for the alpha-O-linked glycoprotein diptericin epsilon. In the first stage of our synthesis, diptericin segment Cys(Acm)37-Gly(52) and segment Val(53)-Phe(82) were assembled by SAL through a Gly-Val ligation junction. Subsequently, after Acm deprotection, diptericin segment Cys(37)-Phe(82) was ligated to segment Asp(1)-Asn(36) by means of native chemical ligation (NCL) to give the full sequence of diptericin epsilon. In the final synthetic step, hydrogenolysis was applied to remove the thiol handle from the sugar moiety with the concomitant conversion of mutated Cys(37) into the native alanine residue. In addition, we extended the applicability of SAL to the synthesis of glycopeptides containing cysteine residues by carrying out selective desulfurization of the sulfhydryl-modified sugar moiety in the presence of acetamidomethyl (Acm) protected cysteine residues. The results presented here demonstrated for the first time that SAL could be a general and useful tool in the chemical synthesis of glycoproteins.

Conjugation of p-aminophenyl glycosides with squaric acid diester to a carrier protein and the use of neoglycoprotein in the histochemical detection of lectins

The coupling of p-aminophenyl 2-acetamido-2-deoxy-3-O-beta-D- galactopyranosyl-beta-D-galactopyranoside (gal-beta 1,3-galNAc) to bovine serum albumin (BSA) was achieved by using 1,2-diethoxycyclobutene-3,4-dione (squaric acid diester) as a new coupling reagent. Two selective consequential steps afforded the desired neoglycoprotein: reaction of the p-aminophenyl group of gal-beta 1,3-galNAc with squaric acid diester gave the corresponding squaric acid amide ester, which was transformed into the BSA conjugate by coupling with the lysyl epsilon-amino group of BSA through formation of a squaric acid 1,2-bisamide. The experimental conditions for the reactions and the optimization of average were performed by using p-anisidine as model substance, the methyl group substituting for the carbohydrate part of a p-aminophenylglycoside. Neoglycoproteins have proven to be valuable tools for lectin detection. To evaluate the properties of this type of probe, the obtained neoglycoprotein with the histochemically crucial T-antigen structure was used for glycocytological and glycohistochemical studies. Three cultured human tumor cell lines and tissue sections from human breast carcinomas were chosen. Its efficiency was similar in comparison to measurements with a probe, derived by diazotization with the p-aminophenyl glycosides of gal-beta 1,3-galNAc and already shown to be a reliable marker for lectin localization in tissue sections and cultured cells.

Use of diethyl squarate for the coupling of oligosaccharide amines to carrier proteins and characterization of the resulting neoglycoproteins by MALDI-TOF mass spectrometry

The 8-methoxycarbonyloctyl glycosides of GlcNAc beta, Gal beta 1-4Glc beta, Fuc alpha 1-2Fuc alpha 1-3GalNAc beta and Fuc alpha 1-2Gal beta 1-3[Fuc alpha 1-4]GlcNac beta were converted to primary amines by reaction with neat ethylenediamine and then coupled to bovine serum albumin (BSA) using diethyl squarate as the connector. The average degree of incorporation of the sugar onto the protein, as well as the molecular weight distribution, could be conveniently determined using matrix assisted laser desorption ionization/time of flight (MALDI-TOF) mass spectrometry thus avoiding cumbersome structure-dependent colour-tests or analysis of cleaved ligand. The present coupling method has the advantages of proceeding under very mild conditions, yielding controlled incorporation values and can reliably be used for the coupling of very small amounts (mg) of oligosaccharide.

Various stages of schistosoma express Lewis(x), LacdiNAc, GalNAcbeta1-4 (Fucalpha1-3)GlcNAc and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc carbohydrate epitopes: detection with monoclonal antibodies that are characterized by enzymatically synthesized neoglycoproteins

We report here that fucosylated epitopes such as Lewis(x), LacdiNAc, fucosylated LacdiNAc (LDN-F) and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc (LDN-DF) are expressed by schistosomes throughout their life cycle. These four epitopes were enzymatically synthesized and coupled to bovine serum albumin to yield neoglycoproteins. Subsequently these neoglycoproteins were used to probe a panel of 188 monoclonal antibodies obtained from infected or immunized mice, in ELISA and surface plasmon resonance analysis. Of these antibodies, 25 recognized one of the fucosylated structures synthesized, indicating that these structures are immunogenic during infection. The MAbs identified could be subdivided in four different groups based on the recognition of either the Lewis(x)-, the LacdiNAc-, the LDN-DF-, or both the LDN-F- and LDN-DF epitope. These monoclonal antibodies were then used to investigate the localization of the fucosylated epitopes in various stages of Schistosoma mansoni using indirect immunofluorescence. Lewis(x)epitopes were mainly found in the gut and on the tegument of adult worms, on egg shells, and on the oral sucker of cercariae. The LacdiNAc epitope was expressed on the tegument of adult worms, on miracidia, and on the oral sucker of cercariae. In contrast, LDN-DF epitopes were mainly present in the excretory system of adult worms, on miracidia and on whole cercariae. These also stained positive with the LDN-F/LDN-DF epitope antibodies, while whole parenchyma reacted characteristically only with the latter antibodies. The identification of different carbohydrate structures in various stages of schistosomes may lead to a better understanding of the function of glycans in the immune response during infection.

Chemoenzymatic synthesis of glycopeptides and glycoproteins through endoglycosidase-catalyzed transglycosylation

Homogeneous glycopeptides and glycoproteins are indispensable for detailed structural and functional studies of glycoproteins. It is also fundamentally important to correct glycosylation patterns for developing effective glycoprotein-based therapeutics. This review discusses a useful chemoenzymatic method that takes advantage of the endoglycosidase-catalyzed transglycosylation to attach an intact oligosaccharide to a polypeptide in a single step, without the need for any protecting groups. The exploration of sugar oxazolines (enzymatic reaction intermediates) as donor substrates has not only expanded substrate availability, but also has significantly enhanced the enzymatic transglycosylation efficiency. Moreover, the discovery of a novel mutant with glycosynthase-like activity has made it possible to synthesize homogeneous glycoproteins with full-size natural N-glycans. Recent advances in this highly convergent chemoenzymatic approach and its application for glycopeptide and glycoprotein synthesis are highlighted.

Structure-activity profiles of complex biantennary glycans with core fucosylation and with/without additional alpha 2,3/alpha 2,6 sialylation: synthesis of neoglycoproteins and their properties in lectin assays, cell binding, and organ uptake

The consideration of oligosaccharides and glycoconjugates as biopharmaceuticals is an emerging topic in drug design. Chemoenzymatic synthesis of N-glycans was performed to examine the influence of N-glycan core fucosylation on lectin-binding properties and biodistribution. As a first step in a systematic comparison of N-glycans, the core fucose moiety was chemically introduced into a complex-type biantennary heptasaccharide azide. After deprotection and attachment of a spacer, the terminal sections of the N-glycan were elongated enzymatically. Conversion of the amino group in the spacer to an isothiocyanate gave derivatives allowing convenient ligand attachment to bovine serum albumin (BSA). The resulting neoglycoproteins contained an average of 2.9-4.6 chains per carrier molecule. Relative to unsubstituted biantennary complex-type N-glycans, the core fucosylation appears to favor the extended orientation of the alpha 1,6-arm. This was deduced from an up to 5-fold alteration of affinity for lectins in solid-phase assays. Marked differences were also found for cell surface binding of cultured tumor cells, for staining of tumor cells in lung sections, and in organ distribution. In vivo, the alpha 2,6-sialylated neoglycoproteins showed a reduced serum half-life in mice relative to the alpha 2,3-sialylated isomer and the non-fucosylated congeners. These results support the notion that changing the shape of a glycan provides a promising strategy to optimize the affinity of protein-carbohydrate interactions. Overall, our study underscores the importance of chemoenzymatic synthesis to define the effect of chain orientation on the ligand properties of N-glycans.