Synthesis of oligogalacturonates conjugated to BSA

The synthesis of three oligogalacturonates with an aldehyde spacer attached at the reducing end is described. Trigalacturonates alpha-d-GalpA-(1-->4)-alpha-d-GalpA-(1-->4)-alpha-d-GalpA-(1-->O(CH(2))(7)CHO and alpha-d-GalpA(Me)-(1-->4)-alpha-d-GalpA(Me)-(1-->4)-alpha-d-GalpA(Me)-(1-->O(CH(2))(7)CHO as well as hexagalacturonate alpha-d-GalpA-(1-->4)-[alpha-d-GalpA-(1-->4)](4)-alpha-d-GalpA-(1-->O(CH(2))(7)CHO are prepared by stepwise coupling of galactose units followed by oxidation of the 6-positions. The alpha-linkages are formed by employing n-pentenyl galactosides as glycosyl donors and N-iodosuccinimide/triethylsilyl triflate as the promoter. Deprotection furnishes the three target oligogalacturonates, which are subsequently linked to bovine serum albumin by reductive amination. These neoglycoproteins will serve as immunogens for generation of new antibodies that can be used for localization and characterization of pectin in plants.

Synthesis of a Mimic for the Heterogeneous Surface of Core 2 Sialoglycan-Linked Glycoprotein

[structure: see text] An O-glycosylated peptide carrying two core 2 sialopentasaccharides of different glycoform was synthesized as a model of the heterogeneous surface of mucin glycoprotein. Solid-phase synthesis and the subsequent enzymatic sialylation gave two segments, which were coupled by selective thioester activation to produce the title compound.

Glycodendriproteins: a synthetic glycoprotein mimic enzyme with branched sugar-display potently inhibits bacterial aggregation

The continuing ability of bacteria to resist current antibiotic treatments highlights the need for alternative strategies for inhibiting their pathogenicity. Bacterial attachment is a major factor in infectivity and virulence. This key binding phase of bacteria to any potential host is mediated by adhesin proteins and so these present an attractive therapeutic target for antiinfective blocking strategies. However, the natural ligands to adhesins are large, typically complex molecules that are difficult to mimic with small molecules. We describe here a method that creates precise synthetic mimics of glycoproteins that are designed to bind adhesins. By using protein-degrading enzymes as the basis for these mimics we have created large-molecule protein ligands that inhibit aggregation of pathogenic bacteria at levels greater than a million-fold higher than small-molecule inhibitors of adhesins.

Cyanogen Bromide Cleavage Generates Fragments Suitable for Expressed Protein and Glycoprotein Ligation

Herein cyanogen bromide is employed for the efficient production of N-terminal cysteine containing protein fragments for expressed protein ligation (EPL) from polyhistidine-tagged precursors. We provide three examples of efficient CNBr cleavage of fragments of the glycoprotein erythropoietin that can be ligated with peptides or glycopeptide mimetics potentially giving rise to semisynthetic glycoprotein therapeutics.

Site-specific glycosylation of an aglycosylated human IgG1-Fc antibody protein generates neoglycoproteins with enhanced function

A range of well-defined IgG glycoforms was prepared by employing a combination of synthetic carbohydrate chemistry and genetic engineering. The key aspect of this methodology is the coupling of thioaldoses with cysteine-containing proteins to give disulfide-linked neoglycoproteins. This technology was applied to the synthesis of a series of synthetic N-glycan thioaldoses which were coupled to an aglycosylated IgG1-Fc fragment, engineered to have Cys-297 in place of glycan-linked Asn (Deltah-Fc N297C). Analysis of the resulting Fc neoglycoproteins by mass spectrometry and trypsin digestion showed that the saccharides were site-selectively incorporated at Cys-297 to full occupancy without affecting other Fc protein disulfides. The neoglycoproteins were tested for their ability to interact with human FcgammaRI by inhibiting superoxide production by gamma-interferon-stimulated U937 cells. The neoglycoproteins displayed enhanced superoxide inhibition relative to aglycosylated Deltah-Fc N297C, where increased glycan size correlated positively with increased inhibition.

Glycosyl phenylthiosulfonates (glyco-PTS): novel reagents for glycoprotein synthesis

Controlled site-selective glycosylation can be achieved by combining site-directed cysteine mutagenesis with chemical modification of the introduced thiol; a new class of more efficient chemoselective reagents, glycosyl phenylthiosulfonates, allow rapid glycosylations of representative simple thiols, peptides and proteins.

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.

Effect of limited solid-state glycation on the conformation of lysozyme by ESI-MSMS peptide mapping and molecular modeling

Although protein glycation has been implicated in the alteration of protein functionality, both in vivo (in biological systems) and in vitro (in food systems), the effect of the protein-bound glycan moiety on the structure/conformation of proteins that result in the modification of functionality is not clear. In this article, we report a study of the conformational changes of glycated lysozyme using LC-ESI-MSMS peptide mapping, and molecular modeling. A comparison of the RP-HPLC of the tryptic digests of unglycated and glycated lysozyme showed markedly different chromatographic profiles. Analysis of the peptide composition of the chromatographic fractions of the tryptic digests revealed that glycation of lysozyme resulted in the modification of its conformation. Glycation-induced changes in the conformation of lysozyme resulted in the exposure of its active site region to increased proteolytic activity of trypsin. Molecular simulation of triglycated lysozyme also showed that limited glycation of lysozyme caused reorientation of the active site residues (Arg 45, Arg 68, Asn 44, and Trp 62) and increased solvent accessibility into the active site region of the protein. The results of the modeling experiment corroborated the results of the RP-HPLC and ESI-MSMS peptide mapping.

A synthetic peptide fragment of human MEPE stimulates new bone formation in vitro and in vivo

Matrix extracellular phosphoglycoprotein (MEPE) was proposed as a candidate for the phosphaturic hormone phosphatonin. We found that a synthetic peptide fragment of MEPE containing the RGD and SGDG sequence stimulated new bone formation in vitro and in vivo.

INTRODUCTION

Matrix extracellular phosphoglycoprotein (MEPE) was recently identified as a candidate for the phosphaturic hormone phosphatonin, which has been implicated in disturbed phosphate metabolism, rickets, and osteomalacia associated with X-linked hypophosphatemic rickets (XLH) and oncogenic hypophosphatemic osteomalacia (OHO). MEPE expression was predominantly found in osteoblasts, and mice deficient in a homolog of MEPE showed increased bone density, suggesting that MEPE produced in osteoblasts negatively regulates bone formation. In this study, we examined the effects of a synthetic 23mer peptide fragment of MEPE (AC-100, region 242-264) containing the RGD (integrin-binding) and SGDG (glycosaminoglycan-attachment) motif on bone formation in vitro and in vivo.

MATERIALS AND METHODS

The osteogenic activity of AC-100 was examined in organ cultures of neonatal mouse calvariae and in vivo by injecting AC-100 onto the calvariae of mice.

RESULTS

Histomorphometric examination showed that AC-100 stimulated new bone formation with increased numbers of osteoblasts in neonatal mouse calvariae in organ culture. In contrast, synthetic MEPE fragment peptides without either the RGD or SGDG motif failed to increase new bone formation. Repeated daily subcutaneous injections of AC-100 onto the calvariae in mice increased bone thickness and stimulated new bone formation as determined by the calcein double-labeling technique. However, peptides in which the RGD or SGDG sequence was scrambled did not stimulate new bone formation in vivo. AC-100 increased cell proliferation and alkaline phosphatase activity and activated focal adhesion kinase (FAK) and extracellular signal-regulated protein kinase (ERK) in human primary osteoblasts.

CONCLUSION

Our results show that a synthetic peptide corresponding with the sequence of human MEPE fragment stimulates new bone formation with increased number of osteoblasts. The results also suggest that the RGD and SGDG motifs are critical to the osteogenic activity of AC-100, presumably through activating integrin signaling pathways in osteoblasts. The anabolic effects of AC-100 may be beneficial for bone diseases associated with decreased bone formation.

Determination of modulation of ligand properties of synthetic complex-type biantennary N-glycans by introduction of bisecting GlcNAcin silico,in vitroandin vivo

We have investigated the consequences of introducing a bisecting GlcNAc moiety into biantennary N-glycans. Computational analysis of glycan conformation with prolonged simulation periods in vacuo and in a solvent box revealed two main effects: backfolding of the alpha1-6 arm and stacking of the bisecting GlcNAc and the neighboring Man/GlcNAc residues of both antennae. Chemoenzymatic synthesis produced the bisecting biantennary decasaccharide N-glycan and its alpha2-3(6)-sialylated variants. They were conjugated to BSA to probe the ligand properties of N-glycans with bisecting GlcNAc. To assess affinity alterations in glycan binding to receptors, testing was performed with purified lectins, cultured cells, tissue sections and animals. The panel of lectins, including an adhesion/growth-regulatory galectin, revealed up to a sixfold difference in affinity constants for these neoglycoproteins relative to data on the unsubstituted glycans reported previously [André, S., Unverzagt, C., Kojima, S., Dong, X., Fink, C., Kayser, K. & Gabius, H.-J. (1997) Bioconjugate Chem. 8, 845-855]. The enhanced affinity for galectin-1 is in accord with the increased percentage of cell positivity in cytofluorimetric and histochemical analysis of carbohydrate-dependent binding of labeled neoglycoproteins to cultured tumor cells and routinely processed lung cancer sections. Intravenous injection of iodinated neoglycoproteins carrying galactose-terminated N-glycans into mice revealed the highest uptake in liver and spleen for the bisecting compound compared with the unsubstituted or core-fucosylated N-glycans. Thus, this substitution modulates ligand properties in interactions with lectins, a key finding of this report. Synthetic glycan tailoring provides a versatile approach to the preparation of newly substituted glycans with favorable ligand properties for medical applications.

Extending chemoselective ligation to sugar chemistry: convergent assembly of bioactive neoglycoconjugates

Glycoproteins and glycolipids play central roles in human health and disease, and their mimetics are primary candidates for drug development. Our understanding of the molecular level of phenomena based on molecular recognition of oligosaccharides by specific receptors has taken tremendous advantage from their chemical synthesis, which provides homogeneous material not attainable from biosynthetic systems. This review summarizes chemoselective approaches for the assembly of neoglycoconjugates. The objective of these methods is to make glycoconjugate synthesis accessible to a broader community, thus accelerating progress in biotechnology.

Automated Synthesis of a Protected N-Linked Glycoprotein Core Pentasaccharide

[structure: see text] Described is the first automated solid-phase synthesis of the core N-linked pentasaccharide, common to all N-linked glycoproteins via stepwise assembly from mono- and disaccharide building blocks. The challenging beta-mannosidic linkage was incorporated by the inclusion of a disaccharide trichloroacetimidate. This automated synthesis provides rapid access to an oligosaccharide common to an entire class of glycoconjugates.

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.

Regulation of polyclonal T cell responses by an MHC anchor-substituted variant of myelin oligodendrocyte glycoprotein 35-55

Analogs of immunogenic peptides containing substitutions at TCR contact residues (altered peptide ligands (APLs)) have been used to manipulate Ag-specific T cell responses in models of autoimmunity, including experimental autoimmune encephalomyelitis. However, recent clinical trials with APL of a myelin basic protein epitope revealed limitations of this therapy. In this study, we demonstrate that individual myelin oligodendrocyte glycoprotein (MOG) 35-55-specific T cell clones responded differentially to a MOG 35-55 APL, raising questions about the ability of peptide analogs containing amino acid substitutions at TCR contact residues to control polyclonal populations of T cells. In contrast, we found that a variant peptide containing a substitution at an MHC anchor residue uniformly affected multiple MOG 35-55-specific clones and polyclonal lines. Stimulation of polyclonal MOG 35-55-specific T cells with an MHC variant peptide resulted in the induction of anergy, as defined by a dramatic reduction in proliferation and IL-2 production upon challenge with wild-type peptide. Furthermore, treatment of T cell lines with this peptide in vitro resulted in a significant reduction in their encephalitogenicity upon adoptive transfer. These results indicate that the use of MHC anchor-substituted peptides may be efficacious in the regulation of polyclonal T cell responses such as those found in EAE.

A novel neoglycopeptide building block

The synthesis of a neoglycopeptide building block is described. The key step is a cycloaddition where the chemistry is orthogonal to standard glycosyl transfer methodology. Also described is some exploratory chemistry of the building block.

Chemoselective ligation applied to the synthesis of a biantennary N-linked glycoform of CD52

We report here a strategy for the synthesis of N-linked glycopeptide analogues that replace the glycosidic linkages extending from the core pentasaccharide with thioethers amenable to construction by chemoselective ligation. The key building block, a pentasaccharide-Asn analogue containing two thiol residues, was incorporated into CD52 by 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis. An undecasaccharide mimetic was then readily generated by alkylation of this glycopeptide with an N-bromoacetamido trisaccharide. The rapid assembly of a complex type N-linked glycopeptide mimetic was accomplished using this technique.

Total synthesis of the ramoplanin A2 and ramoplanose aglycon

Full details of a convergent total synthesis of the ramoplanin A2 and ramoplanose aglycon are disclosed. Three key subunits composed of residues 3-9 (heptapeptide 15), pentadepsipeptide 26 (residues 1, 2 and 15-17), and pentapeptide 34 (residues 10-14) were prepared, sequentially coupled, and cyclized to provide the 49-membered depsipeptide core of the aglycon. Key to the preparation of the pentadepsipeptide 26 incorporating the backbone ester was the asymmetric synthesis of an orthogonally protected l-threo-beta-hydroxyasparagine and the development of effective and near-racemization free conditions for esterification of its hindered alcohol (EDCI, DMAP, 0 degrees C). The coupling sites were chosen to maximize the convergency of the synthesis including that of the three subunits, to prevent late stage racemization of carboxylate-activated phenylglycine-derived residues, and to enlist beta-sheet preorganization of an acyclic macrocyclization substrate for 49-membered ring closure. By altering the order of final couplings, two macrocyclization sites, Phe(9)-d-Orn(10) and Gly(14)-Leu(15), were examined. Macrocyclization at the highly successful Phe(9)-d-Orn(10) site (89%) may benefit from both beta-sheet preorganization as well as closure at a d-amine terminus within the confines of a beta-turn at the end of the H-bonded antiparallel beta-strands. A more modest, but acceptable macrocyclization reaction at the Gly(14)-Leu(15) site (40-50%) found at the other end of the H-bonded antiparallel beta-strands within a small flexible loop may also benefit from preorganization of the cyclization substrate, is conducted on a substrate incapable of competitive racemization, and accommodates the convergent preparation of analogues bearing depsipeptide modifications. Deliberate late-stage incorporation of the subunit bearing the labile depsipeptide ester and a final stage Asn(1) side-chain introduction provides future access to analogues of the aglycons which themselves are equally potent or more potent than the natural products in antimicrobial assays.

A novel strategy for the synthesis of neoglycoconjugates from deacylated deep rough lipopolysaccharides

We report a novel strategy for the preparation of neoglycoconjugates of oligosaccharides which are obtained after complete deacylation of bacterial deep rough lipopolysaccharides (LPS) isolated from recombinant Escherichia coli bacteria synthesizing a Kdo di-[alpha-Kdo-(2-->4)-alpha-Kdo-(2-->] and a Kdo trisaccharide [alpha-Kdo-(2-->8)-alpha-Kdo-(2-->4)-alpha-Kdo-(2-->] of Re-type and chlamydial LPS, respectively. Unlike acylated LPS, such oligosaccharides can be obtained in pure form and thus lead to well-defined neoglycoconjugates. Cleavage of the 1-phosphate of the lipid A moiety by alkaline phosphatase treatment leads to a free reducing glucosamine which can be further reacted with allylamine. After reductive amination, spacer elongation of the allyl group with cysteamine and activation with thiophosgene, the ligands were reacted with BSA. We have compared the immunological reactivity of such defined neoglycoconjugates obtained from natural sources with those obtained by chemical synthesis and report that such neoglycoconjugates are immunogenic and well suited as antigens for the study of epitope specificities of monoclonal antibodies. In addition, we have compared these conjugates with those in which ligands were coupled by glutardialdehyde to BSA. Our approach proved to be superior since the latter led upon immunization of mice to a relatively high percentage of antibodies that reacted with glutardialdehyde derivatized BSA without the carbohydrate ligand. This was not the case for cysteamine-spacered ligands coupled via their isothiocyanate-derivatives.

Synthesis of N-Fmoc-O- (N'-Boc-N'-methyl)-aminohomoserine, an amino acid for the facile preparation of neoglycopeptides

The synthesis of N-Fmoc-O-(N'-Boc-N'-methyl)-aminohomoserine in 35% overall yield from l-homoserine is described. This amino acid can be efficiently incorporated into peptides using Fmoc-chemistry-based solid-phase peptide synthesis, and the resulting peptides can be chemoselectively glycosylated at the aminooxy side chains to generate neoglycopeptides. The synthesis of this derivative greatly expands the availability of a previously developed neoglycopeptide synthesis strategy.

Synthesis of neoglycoproteins containing O-methylated trisaccharides related to excretory/secretory antigens of Toxocara larvae

The disaccharides allyl beta-D-galactopyranosyl-(1-->3)-2-acetamido-2-deoxy-beta- and alpha-D-galactopyranoside 10a and 10b and the trisaccharides allyl 2-O-methyl-alpha-L-fucopyranosyl-(1-->2)-beta-D-galactopyranosyl-(1-->3)-2-acetamido-2-deoxy-beta- and alpha-D-galactopyranoside 18a and 18b have been prepared using stepwise assembly of the sugar units. The glycosidic linkages were formed employing the trichloroacetimidate procedure for the attachment of the galactopyranosyl residue and N-iodosuccinimide/triflic acid activation of an ethyl 1-thiofucopyranoside donor for fucosylation. Deprotection furnished the allyl glycosides which were converted into cysteamine-spacered ligands, activated with thiophosgene and subsequently linked to bovine serum albumin. The neoglycoproteins serve as immunoreagents to determine epitope specificities of monoclonal antibodies directed against highly immunogenic O-glycans located at the surface of Toxocara larvae.

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.

Oligolysine-based saccharide clusters: synthesis and specificity

In search of specific and highly selective sugar clusters for cell receptors, such as membrane lectins, various disaccharides were coupled to small peptide cores through an amide bond. In a first step, the reducing disaccharides, i.e. lactose and three different dimannoses, were converted into glycosyl-pyroglutamyl-beta-alanine derivatives. The free carboxylic group of these conjugates was then coupled to the alpha and epsilon amino groups of the core peptide (Lys( n )-Ala-Cys-NH2) with n =1 to 5, with complete substitution leading to homogeneous glycoclusters. The thiol group of the cysteine residue was used to tag the glycosylated oligolysines upon reaction with fluorescein iodoacetamide. The affinity of these glycoclusters towards two plant lectins was assessed by surface plasmon resonance. The selectivity of their cell uptake was investigated by flow cytometry using two types of cells: a human hepatoma cell line (HepG2 cells) expressing the plasma membrane galactose-specific lectin, and monocyte-derived dendritic cells expressing the plasma membrane mannose-specific lectin. The glycoclusters containing four or five disaccharides were shown to bind plant lectins and cell surface membrane lectins with a narrow selectivity and with a high affinity.

Convergent synthesis of an inner core GPI of sperm CD52

An inner core of the GPI anchor of sperm CD52 antigens was synthesized by a highly convergent process using specially modified inositol, glucosamine and phospholipid as key building blocks. This paper also presents a new and efficient procedure to prepare 1,2,6-differentiated derivatives of inositol for GPI syntheses.