Synthesis of α-Ketoamide-Based Stereoselective Calpain-1 Inhibitors as Neuroprotective Agents

Calpain inhibitors have been proposed as drug candidates for neurodegenerative disorders, with ABT-957 entering clinical trials for Alzheimer's disease and mild cognitive impairment. The structure of ABT-957 was very recently disclosed, and trials were terminated owing to inadequate CNS concentrations to obtain a pharmacodynamic effect. The multistep synthesis of an α-ketoamide peptidomimetic inhibitor series potentially including ABT-957 was optimized to yield diastereomerically pure compounds that are potent and selective for calpain-1 over papain and cathepsins B and K. As the final oxidation step, with its optimized synthesis protocol, does not alter the configuration of the substrate, the synthesis of the diastereomeric pair (R)-1-benzyl-N-((S)-4-((4-fluorobenzyl)amino)-3,4-dioxo-1-phenylbutan-2-yl)-5-oxopyrrolidine-2-carboxamide (1 c) and (R)-1-benzyl-N-((R)-4-((4-fluorobenzyl)amino)-3,4-dioxo-1-phenylbutan-2-yl)-5-oxopyrrolidine-2-carboxamide (1 g) was feasible. This allowed the exploration of stereoselective inhibition of calpain-1, with 1 c (IC₅₀ =78 nM) being significantly more potent than 1 g. Moreover, inhibitor 1 c restored cognitive function in amnestic mice.

Synthesis of the Thomsen-Friedenreich-antigen (TF-antigen) and binding of Galectin-3 to TF-antigen presenting neo-glycoproteins

The Thomsen-Friedenreich-antigen, Gal(β1-3)GalNAc(α1-O-Ser/Thr (TF-antigen), is presented on the surface of most human cancer cell types. Its interaction with galectin 1 and galectin 3 leads to tumor cell aggregation and promotes cancer metastasis and T-cell apoptosis in epithelial tissue. To further explore multivalent binding between the TF-antigen and galectin-3, the TF-antigen was enzymatically synthesized in high yields with GalNAc(α1-EG3-azide as the acceptor substrate by use of the glycosynthase BgaC/Glu233Gly. Subsequently, it was coupled to alkynyl-functionalized bovine serum albumin via a copper(I)-catalyzed alkyne-azide cycloaddition. This procedure yielded neo-glycoproteins with tunable glycan multivalency for binding studies. Glycan densities between 2 and 53 glycan residues per protein molecule were obtained by regulated alkynyl-modification of the lysine residues of BSA. The number of coupled glycans was quantified by sodium dodecyl sulfate polyacrylamide gel electrophoresis and a trinitrobenzene sulfonic acid assay. The binding efficiency of the neo-glycoproteins with human galectin-3 and the effect of multivalency was investigated and assessed using an enzyme-linked lectin assay. Immobilized neo-glycoproteins of all modification densities showed binding of Gal-3 with increasing glycan density. However, multivalent glycan presentation did not result in a higher binding affinity. In contrast, inhibition of Gal-3 binding to asialofetuin was effective. The relative inhibitory potency was increased by a factor of 142 for neo-glycoproteins displaying 10 glycans/protein in contrast to highly decorated inhibitors with only 2-fold increase. In summary, the functionality of BSA-based neo-glycoproteins presenting the TF-antigen as multivalent inhibitors for Gal-3 was demonstrated.

Purification of Glycosylphosphatidylinositol-Anchored Mucins from Trypanosoma cruzi Trypomastigotes and Synthesis of α-Gal-Containing Neoglycoproteins: Application as Biomarkers for Reliable Diagnosis and Early Assessment of Chemotherapeutic Outcomes of Chagas Disease

Chagas disease (ChD), caused by the protozoan parasite Trypanosoma cruzi, affects millions of people worldwide. Chemotherapy is restricted to two drugs, which are partially effective and may cause severe side effects, leading to cessation of treatment in a significant number of patients. Currently, there are no biomarkers to assess therapeutic efficacy of these drugs in the chronic stage. Moreover, no preventive or therapeutic vaccines are available. In this chapter, we describe the purification of Trypanosoma cruzi trypomastigote-derived glycosylphosphatidylinositol (GPI)-anchored mucins (tGPI-mucins) for their use as antigens for the reliable primary or confirmatory diagnosis and as prognostic biomarkers for early assessment of cure following ChD chemotherapy. We also describe, as an example, the synthesis of a potential tGPI-mucin-derived α-Gal-terminating glycan and its coupling to a carrier protein for use as diagnostic and prognostic biomarker in ChD.

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.

Top-Down Chemoenzymatic Approach to Synthesizing Diverse High-Mannose N-Glycans and Related Neoglycoproteins for Carbohydrate Microarray Analysis

High-mannose-type N-glycans are an important component of neutralizing epitopes on HIV-1 envelope glycoprotein gp120. They also serve as signals for protein folding, trafficking, and degradation in protein quality control. A number of lectins and antibodies recognize high-mannose-type N-glycans, and glycan array technology has provided an avenue to probe these oligomannose-specific proteins. We describe in this paper a top-down chemoenzymatic approach to synthesize a library of high-mannose N-glycans and related neoglycoproteins for glycan microarray analysis. The method involves the sequential enzymatic trimming of two readily available natural N-glycans, the Man9GlcNAc2Asn prepared from soybean flour and the sialoglycopeptide (SGP) isolated from chicken egg yolks, coupled with chromatographic separation to obtain a collection of a full range of natural high-mannose N-glycans. The Asn-linked N-glycans were conjugated to bovine serum albumin (BSA) to provide neoglycoproteins containing the oligomannose moieties. The glycoepitopes displayed were characterized using an array of glycan-binding proteins, including the broadly virus-neutralizing agents, glycan-specific antibody 2G12, Galanthus nivalis lectin (GNA), and Narcissus pseudonarcissus lectin (NPA).

Enzymatic Synthesis of N-Acetyllactosamine (LacNAc) Type 1 Oligomers and Characterization as Multivalent Galectin Ligands

Repeats of the disaccharide unit N-acetyllactosamine (LacNAc) occur as type 1 (Galβ1, 3GlcNAc) and type 2 (Galβ1, 4GlcNAc) glycosylation motifs on glycoproteins and glycolipids. The LacNAc motif acts as binding ligand for lectins and is involved in many biological recognition events. To the best of our knowledge, we present, for the first time, the synthesis of LacNAc type 1 oligomers using recombinant β1,3-galactosyltransferase from Escherichia coli and β1,3-N-acetylglucosaminyltranferase from Helicobacter pylori. Tetrasaccharide glycans presenting LacNAc type 1 repeats or LacNAc type 1 at the reducing or non-reducing end, respectively, were conjugated to bovine serum albumin as a protein scaffold by squarate linker chemistry. The resulting multivalent LacNAc type 1 presenting neo-glycoproteins were further studied for specific binding of the tumor-associated human galectin 3 (Gal-3) and its truncated counterpart Gal-3∆ in an enzyme-linked lectin assay (ELLA). We observed a significantly increased affinity of Gal-3∆ towards the multivalent neo-glycoprotein presenting LacNAc type 1 repeating units. This is the first evidence for differences in glycan selectivity of Gal-3∆ and Gal-3 and may be further utilized for tracing Gal-3∆ during tumor progression and therapy.

Glycoproteins functionalized natural and synthetic polymers for prospective biomedical applications: A review

Glycoproteins have multidimensional properties such as biodegradability, biocompatibility, non-toxicity, antimicrobial and adsorption properties; therefore, they have wide range of applications. They are blended with different polymers such as chitosan, carboxymethyl cellulose (CMC), polyvinyl pyrrolidone (PVP), polycaprolactone (PCL), heparin, polystyrene fluorescent nanoparticles (PS-NPs) and carboxyl pullulan (PC) to improve their properties like thermal stability, mechanical properties, resistance to pH, chemical stability and toughness. Considering the versatile charateristics of glycoprotein based polymers, this review sheds light on synthesis and characterization of blends and composites of glycoproteins, with natural and synthetic polymers and their potential applications in biomedical field such as drug delivery system, insulin delivery, antimicrobial wound dressing uses, targeting of cancer cells, development of anticancer vaccines, development of new biopolymers, glycoproteome research, food product and detection of dengue glycoproteins. All the technical scientific issues have been addressed; highlighting the recent advancement.

Galectin Binding to Neo-Glycoproteins: LacDiNAc Conjugated BSA as Ligand for Human Galectin-3

Carbohydrate-lectin interactions are relatively weak. As they play an important role in biological recognition processes, multivalent glycan ligands are designed to enhance binding affinity and inhibitory potency. We here report on novel neo-glycoproteins based on bovine serum albumin as scaffold for multivalent presentation of ligands for galectins. We prepared two kinds of tetrasaccharides (N-acetyllactosamine and N,N-diacetyllactosamine terminated) by multi-step chemo-enzymatic synthesis utilizing recombinant glycosyltransferases. Subsequent conjugation of these glycans to lysine groups of bovine serum albumin via squaric acid diethyl ester yielded a set of 22 different neo-glycoproteins with tuned ligand density. The neo-glycoproteins were analyzed by biochemical and chromatographic methods proving various modification degrees. The neo-glycoproteins were used for binding and inhibition studies with human galectin-3 showing high affinity. Binding strength and inhibition potency are closely related to modification density and show binding enhancement by multivalent ligand presentation. At galectin-3 concentrations comparable to serum levels of cancer patients, we detect the highest avidities. Selectivity of N,N-diacetyllactosamine terminated structures towards galectin-3 in comparison to galectin-1 is demonstrated. Moreover, we also see strong inhibitory potency of our scaffolds towards galectin-3 binding. These novel neo-glycoproteins may therefore serve as selective and strong galectin-3 ligands in cancer related biomedical research.

Total synthesis of glycosylated proteins

Glycoproteins are an important class of naturally occurring biomolecules which play a pivotal role in many biological processes. They are biosynthesized as complex mixtures of glycoforms through post-translational protein glycosylation. This fact, together with the challenges associated with producing them in homogeneous form, has hampered detailed structure-function studies of glycoproteins as well as their full exploitation as potential therapeutic agents. By contrast, chemical synthesis offers the unique opportunity to gain access to homogeneous glycoprotein samples for rigorous biological evaluation. Herein, we review recent methods for the assembly of complex glycopeptides and glycoproteins and present several examples from our laboratory towards the total chemical synthesis of clinically relevant glycosylated proteins that have enabled synthetic access to full-length homogeneous glycoproteins.

Protein glycosylation in diverse cell systems: implications for modification and analysis of recombinant proteins

A major challenge for the biotechnology industry is to engineer the glycosylation pathways of expression systems to synthesize recombinant proteins with human glycosylation. Inappropriate glycosylation can result in reduced activity, limited half-life in circulation and unwanted immunogenicity. In this review, the complexities of glycosylation in human cells are explained and compared with glycosylation in bacteria, yeasts, fungi, insects, plants and nonhuman mammalian species. Key advances in the engineering of the glycosylation of expression systems are highlighted. Advances in the challenging and technically complex field of glycan analysis are also described. The emergence of a new generation of expression systems with sophisticated engineering for humanized glycosylation of glycoproteins appears to be on the horizon.

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.

Preparation of mannan–protein conjugates using high-temperature glycation

In the present paper, we describe a procedure for the preparation of mannan-protein conjugates. Mannan from Saccharomyces cerevisiae (baker's yeast) was oxidized by NaIO4 and coupled with BSA by means of high-temperature glycation. Depending on the temperature and degree of mannan oxidation, the procedure enabled different levels of substitution to be obtained. For conjugate prepared under selected conditions, the average substitution level was 1.64 molecules of mannan per albumin molecule. The mannan-albumin conjugate was purified by means of ion-exchange chromatography on CM-cellulose at a pH below the pI of albumin. The influence of coupling conditions on protein activity was examined using urease as an example. Urease linked to mannan retained its activity. The mannan-protein conjugates can be used, for example, as carriers of therapeutic substances to macrophages.

Design of a functionally equivalent nonglycosylated analog of the glycopeptide antibiotic formaecin I

Various nonglycosylated analogs were designed in order to explore the role of glycosylation in formaecin I, an antibacterial glycopeptide of insect origin. The functional behavior of a designed nonglycosylated analog (P(7),endo P(8a),DeltaT(11))formaecin I was found to be similar to that of native glycosylated peptide. Both the peptides showed similar antibacterial activities against Escherichia coli and Salmonella strains. The designed nonglycosylated analog (P(7),endo P(8a),DeltaT(11))formaecin I has low binding affinity to LPS identical to that of native glycopeptide, formaecin I. Both the peptides have similar killing kinetics and are nontoxic to erythrocytes. Formaecin I and designed nonglycosylated (P(7),endo P(8a),DeltaT(11))formaecin I have no definite conformational features associated with them. The glycosylated residue of threonine in formaecin I and proline residues in designed peptide [(P(7),endo P(8a),DeltaT(11))formaecin I], possibly help in stabilizing the correct conformation that facilitates presentation of the peptide to its receptor. It is evident that a functionally equivalent nonglycosylated analog of native glycosylated antibacterial peptide can be designed by strategically modifying the sequence.

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.

Development of Calpain-specific Inactivators by Screening of Positional Scanning Epoxide Libraries

Calpains are calcium-dependent proteases that are required for numerous intracellular processes but also play an important role in the development of pathologies such as ischemic injury and neurodegeneration. Many current small molecule calpain inhibitors also inhibit other cysteine proteases, including cathepsins, and need improved selectivity. The specificity of inhibition of several calpains and papain was profiled using synthetic positional scanning libraries of epoxide-based compounds that target the active-site cysteine. These peptidomimetic libraries probe the P4, P3, and P2 positions, display (S,S)- or (R,R)-epoxide stereochemistries, and incorporate both natural and non-natural amino acids. To facilitate library screening, an SDS-PAGE assay that measures the extent of hydrolysis of an inactive recombinant m-calpain was developed. Individual epoxide inhibitors were synthesized guided by calpain-specific preferences observed from the profiles and tested for inhibition against calpain. The most potent compounds were assayed for specificity against cathepsins B, L, and K. Several compounds demonstrated high inhibition specificity for calpains over cathepsins. The best of these inhibitors, WRH(R,R), irreversibly inactivates m- and mu-calpain rapidly (k(2)/K(i) = 131,000 and 16,500 m(-1) s(-1), respectively) but behaves exclusively as a reversible and less potent inhibitor toward the cathepsins. X-ray crystallography of the proteolytic core of rat mu-calpain inactivated by the epoxide compounds WR gamma-cyano-alpha-aminobutyric acid (S,S) and WR allylglycine (R,R) reveals that the stereochemistry of the epoxide influences positioning and orientation of the P2 residue, facilitating alternate interactions within the S2 pocket. Moreover, the WR gamma-cyano-alpha-aminobutyric acid (S,S)-complexed structure defines a novel hydrogen-bonding site within the S2 pocket of calpains.

Oligosaccharides, neoglycoproteins and humanized plastics: their biocatalytic synthesis and possible medical applications

Glycobiology has become one of the fastest growing branches of the biological sciences. Glycomics, which is the study of an organism's entire array of oligosaccharides, is now emerging as the third informatics wave after genomics and proteomics. For example, it is possible to see this progress in the KEGG (Kyoto Encyclopedia of Genes and Genomes) database (http://www.genome.jp/kegg/pathway/map/map01110.html). The interest in this area stems from the realization that carbohydrates, especially oligosaccharides, and their interactions with proteins, play diverse informative roles in all organisms, and that more than half of all proteins are glycosylated. When the biological and pharmaceutical importance of glycoconjugates is considered, it is surprising how little glycobiotechnology has developed. This review reports the latest developments in the biocatalytic synthesis of oligosaccharides and glycoconjugates, with special attention paid to the glycosyltransferase approach. The second part of the review takes the 'conceptual approach' and covers possible medical applications of synthesized glycoconjugates. Various new examples of the conjugation of glyco-informative saccharide sequence to known pharmaceuticals or biomaterials are cited.

Chemoenzymatic approaches to glycoprotein synthesis

The construction of homogeneous glycoproteins presents a formidable challenge to the synthetic chemist. Over the past few years there has been an explosion in the number of methods developed to address this problem. These methods include the development of novel ligation technologies for the synthesis of the protein backbone, as well chemical and enzymatic approaches for introducing complex glycans into the peptide backbone. This tutorial review discusses the application of these techniques to the synthesis of peptides and proteins possessing well defined glycans.

Synthesis of functionalized de novo designed 8–16 kDa model proteins towards metal ion-binding and esterase activity

De novo design and total chemical synthesis of proteins provides a powerful approach for biological and biophysical studies with the ability to prepare artificial proteins with tailored properties, potentially of importance for biophysical studies, material science, nanobioscience, and as molecular probes. In this paper, the previously developed concept of carbohydrates as templates is employed in the de novo design of model proteins (artificial helix bundles) termed 'carboproteins'. The 4-alpha-helix bundle is a macromolecular structure, where four amphiphilic alpha-helical peptide strands form a hydrophobic core. Here this structure is modified towards achieving metal ion-binding and catalytic activity. We report: (i) test of directional effects from different tetravalent carbohydrate templates, (ii) synthesis and evaluation of carboproteins functionalized with phenol, pyridyl or imidazolyl moieties as potential ligands for metal ion-binding as well as for catalysis. Our results include: (i) support of our previous 'controversial' finding that for some carboproteins the degree of alpha-helicity depends on the template, i.e., that there is, to some extent, a controlling effect from the template, (ii) demonstration of binding of Cu(ii) to tetra-functional carboproteins by electrospray ionization-time of flight-mass spectrometry (ESI-TOF-MS), UV-VIS absorption spectroscopy and size exclusion chromatography-inductively coupled plasma-mass spectrometry (SEC-ICP-MS); (iii) a kinetic investigation of the esterase activity.

A trisulfide-linked glycoprotein

The first member of a novel class of chemoselective reagents, glycosyl methanedithiosulfonates, has been synthesized, identified and employed in the first examples of chemical, site-selective construction of a trisulfide-modified protein with complete conversion.

Exploration of oligosaccharide-protein interactions in glycoprotein quality control by synthetic approaches

High-mannose-type oligosaccharides, which are cotranslationally introduced to nascent polypeptides, play important roles in glycoprotein quality control. This process is highly complex, involving a number of lectins, chaperones, and glycan-processing enzymes. For example, calnexin and calreticulin (CRT) are molecular chaperones that recognize monoglucosylated forms of high-mannose-type glycans. UDP-glucose : glycoprotein glucosyltransferase (UGGT) only glucosylates high-mannose-type glycans attached to partially folded proteins. Fbs1 is a component of ubiquitin ligase that recognizes sugar chains. Although recent studies have clarified the properties of these proteins, most of them used oligosaccharides derived from natural sources, which contain structural heterogeneity. In order to gain a more precise understanding, we started our program to comprehensively synthesize high-mannose-type glycans associated with a protein quality control system. Additionally, investigation of artificial glycoproteins led us to the discovery of the first nonpeptidic substrate of UGGT. These synthetic oligosaccharide probes have allowed us to conduct quantitative evaluations of the activity and specificity of CRT, Fbs1, and UGGT.

Strategies for the preparation of homogenous glycoproteins

The majority of human proteins are glycosylated, and many of them are known to be involved in important biological processes. This class of proteins is often expressed as a heterogeneous mixture of glycoforms, rendering the isolation of individual species for various studies a difficult task. Recent advances in the development of glycoprotein synthesis have provided promising strategies, which include enzymatic, chemical and in vivo suppressor tRNA methods to obtain homogenous products. Nevertheless, there are many remaining challenges to overcome in both the chemical and biochemical approaches to efficiently obtain homogenous glycoproteins for glycobiology research and for the production of pharmaceutical products.

Investigation of the route of absorption of lipid and sugar modified leu-enkephalin analogues and their enzymatic stability using the caco-2 cell monolayer system

It has been demonstrated that conjugation of lipoamino acids or glucose units to the endogenous opioid peptide, Leu-enkephalin can significantly improve the peptide's metabolic stability and absorption across biological barriers. The purpose of this study was to investigate the possible involvement of specific carrier proteins in the absorption of these peptide conjugates. A series of lipo- glycol- and liposaccharide peptide conjugates were synthesised and examined using the Caco-2 monolayer assay for evidence of interaction with the human H(+)-coupled oligopeptide transporter (hPepT1), glucose transporters and the multidrug resistance efflux pump, p-glycoprotein. The investigation involved determining the apparent permeability of each compound in the absence of any inhibitors and comparing this to the apparent permeabilities of each compound in the presence of glycylsarcosine, glucose or vinblastine, respective inhibitors of the above mentioned transporters. None of the peptide conjugates were found to be substrates for p-glycoprotein. Of the six peptide conjugates examined, only the C-terminus glucose conjugate of Leu-enkephalin (Enk-glu) showed evidence of transport by both glucose transporters and hPepT1. In contrast, N-terminus conjugation of both lipids and sugars appeared to provide the greatest protection against enzymatic degradation.

Synthesis of asparagine-linked bacillosamine

Various types of protein glycosylation have been identified from prokaryotes. Recent investigations have revealed the presence of N-linked glycoproteins in the pathogenic bacterium, Campylobacter jejuni. The structure of this glycan is unique, consisting of 5 GalNAc and 1 Glc, in addition to 2,4-diacetamido-2,4,6-trideoxy-d-glucopyranose (bacillosamine; Bac), which is N-glycosidically linked to the side chain of asparagine (Asn). We synthesized Bac from a 2-azido-2-deoxy-D-galactose derivative, which was further converted to the Asn-linked form.

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.

Branching mode in complex-type triantennary N-glycans as regulatory element of their ligand properties

We address the question whether the two natural types of branching in complex-type triantennary N-glycans differ in ligand properties. Toward this end, we prepared the set of pergalactosylated undecasaccharides and derivatives with alpha2,3/6-sialylation by chemoenzymatic synthesis. Conjugation resulted in neoglycoproteins which were tested in assays with lectins/antibodies, cultured cells and animals. Solid-phase assays with galactoside-specific proteins (a plant toxin, galectins and an antibody fraction) disclosed that the branching mode did not significantly affect affinity. However, compared to previous studies under identical conditions increase in antennae number and presence of substitutions in biantennary N-glycans altered KD-values with differences between receptors. Neoglycoprotein binding to cells of eight human tumor lines was sensitive to N-glycan branching. Staining intensity revealed pronounced branch-mode-dependent differences in four cases. Biodistribution profiles in mice uncovered dramatic changes in clearance rates with prolonged serum presence associated with type II branching of sialylated N-glycans and markedly increased uptake of neoglycoproteins with type I-branched N-glycans into liver, spleen, heart and lungs. This part of the study is relevant for rational glycoengineering of pharmaproteins. In general, our study supports the concept to view details of N-glycan structure, here branching, as a means to modulate ligand properties.

Synthesis of the tetrasaccharide side chain of the major glycoprotein of the Bacillus anthracis exosporium

An alpha-glycoside of the tetrasaccharide sequence beta-Ant-(1-->3)-alpha-l-Rhap-(1-->3)-alpha-l-Rhap-(1-->2)-alpha-l-Rhap whose aglycon allows conjugation to suitable carriers was synthesized. The NMR characteristics of the compound are virtually identical with those of the alpha-anomer of the tetrasaccharide isolated from the major glycoprotein of the Bacillus anthracis exosporium. Thus, the correct structure of the natural product has been proven by chemical synthesis.

A new homobifunctional p-nitro phenyl ester coupling reagent for the preparation of neoglycoproteins

A new linker system has been designed and applied to neoglycoprotein synthesis. Reaction of oligosaccharide omega-aminoalkyl glycosides with homobifunctional adipic acid p-nitrophenyl diesters in dry DMF gave the corresponding amide half ester in good yields and of sufficient stability to permit chromatographic purification. Subsequent conjugation with bovine serum albumin under very mild conditions afforded the corresponding neoglycoproteins with good efficiency. The method is well suited for the coupling of very small amounts (mg) of oligosaccharide and protein. [structure: see text]

Advances in glycoprotein synthesis

The development of chemical and enzymatic methods for the synthesis of homogeneous glycoproteins is a fascinating challenge at the interface between chemistry and biology. Discussed here are the currently available methods for preparation of homogeneous glycoproteins. These methods include (1) glycopeptide ligation; (2) glycoprotein remodeling; and (3) in vivo suppressor tRNA technology.

A new cell-permeable calpain inhibitor

The ubiquitous calpains, mu- and m-calpain, are implicated in a variety of vital (patho)physiological processes and therefore cell-permeable specific inhibitors represent important tools for defining the role of calpains in cells and animal models. A synthetic N-acetylated 27-mer peptide derived from exon B of the human calpastatin inhibitory domain 1 is known to be the most potent and selective reversible inhibitor of calpains. To improve the membrane permeability of this peptidic inhibitor, it was N-terminally extended with or disulfide-linked to the C-terminal 7-mer fragment of penetratin, a well-established vector for cell membrane translocation of bioactive compounds. Despite the shorter penetratin sequence, both constructs showed increased cell permeability and retained their full calpain inhibitory potency.

Chemical synthesis of analogs of the glycopeptide contulakin-G, an analgetically active conopeptide from Conus geographus

Cone snails are marine predators that use immobilizing venoms for catching prey. Chemical analysis of the venoms has revealed a variety of biologically active small and intermediate size peptides rich in post-translational modifications (modified amino acids, glycosylation). The glycopeptide contulakin-G (pGlu-Ser-Glu-Glu-Gly-Gly-Ser-Asn-Ala-[beta-D-Galp-(1-->3)-alpha-D-GalpNAc-(1-->]Thr-Lys-Lys-Pro-Tyr-Ile-Leu-OH) is a potent analgesic from Conus geographus venom. The in vivo activity of synthetic contulakin-G was previously found to be significantly higher compared to that of a peptide lacking the glycan. In order to further investigate the importance of the glycan, we have now synthesized analogs of contulakin-G where the glycan chain O-linked to threonine has been altered either to beta-D-Galp-(1-->3)-beta-D-GalpNAc-, alpha-D-Galp-(1-->3)-alpha-D-GalpNAc-, or beta-D-Galp-(1-->6)-alpha-D-GalpNAc-. The glycopeptides were assembled on a Wang resin using commercially available Fmoc amino acids and synthetically prepared Fmoc-protected threonine derivatives carrying O-acetyl protected sugar chains. The final products were thoroughly characterized by NMR and mass spectroscopy.

Adjuvant effect of synthetic oligodeoxyribonucleotides (CpG-ODN) from the Paracoccidioides brasiliensis gp43 gene on the Th2-Th1 immunomodulation of experimental paracoccidioidomycosis

Paracoccidioidomycosis (PCM) is caused by the dimorphic fungus Paracoccidioides brasiliensis. Immunostimulatory effects of P. brasiliensis DNA and CpG-oligodeoxyribonucleotides (CpG-ODN) have shown a Th2-Th1 immunomodulation of the isogenic murine model of susceptibility, which develops a progressive and disseminating disease. In this study, we investigated the optimum time interval and doses of CpG-ODN which are able to induce Th2-Th1 immunomodulation. The optimum concentrations for the induction of a decrease in antibody production were 0.5 and 1 microg. Mice immunized twice with CpG-ODN and gp43 (5 and 7 days before the challenge) showed a 60% higher chance of survival compared with the control group (nonimmunized), and an increase in Th1 isotype (IgG2a) was also observed. In vitro assays of naive and preimmunized mice showed discrete cellular proliferation when stimulated by suitable concentrations of CpG-ODN. Type 1 cytokines interleukin-12 (IL-12) and interferon-gamma were increased in cell culture supernatants, but no significant difference was found in Th2 IL-4 cytokines in stimulated or nonstimulated cell cultures. Concerning the Th2-Th1 kinetics in experimental PCM models by adjuvant effect of CpG-ODN, there are still many questions to be answered and clarified. However, the gathering of data obtained in this investigation has led us to suggest that the modulation of Th2-Th1 in experimental PCM depends on time and CpG-ODN concentration.

A new strategy for the synthesis of dinucleotides loaded with glycosylated amino acids--investigations on in vitro non-natural amino acid mutagenesis for glycoprotein synthesis

The in vitro non-natural amino acid mutagenesis method provides the opportunity to introduce non-natural amino acids site-specifically into proteins. To this end, a chemically synthesised aminoacylated dinucleotide is enzymatically ligated to a truncated suppressor transfer RNA. The loaded suppressor tRNA is then used in translation reactions to read an internal stop codon. Here we report an advanced and general strategy for the synthesis of the aminoacyl dinucleotide. The protecting group pattern developed for the dinucleotide facilitates highly efficient aminoacylation, followed by one-step global deprotection. The strategy was applied to the synthesis of dinucleotides loaded with 2-acetamido-2-deoxy-glycosylated amino acids, including N- and O-beta-glycosides and O- and C-alpha-glycosides of amino acids, thus enabling the extension of in vitro non-natural amino acid mutagenesis towards the synthesis of natural glycoproteins of high biological interest. We demonstrate the incorporation of the glycosylamino acids--although with low suppression efficiency--into the human interleukin granulocyte-colony stimulating factor (hG-CSF), as verified by the ELISA technique.

Monodisperse Protein-Based Glycopolymers via a Combined Biosynthetic and Chemical Approach

Helical protein polymers with sequences comprising primarily alanine and glutamine have been designed to contain glutamic acid residues at distances that are targeted to match the receptor spacing of certain toxins and lectins. These proteins are readily expressed and purified from E. coli and are highly helical under a variety of solution conditions. The helical artificial proteins are also competent for chemical modification with saccharides for inhibition of select bacterial toxins and lectins. In the investigations reported here, multivalent artificial glycoproteins bearing galactose moieties as pendant groups have been prepared via the coupling reaction of amine-functionalized galactose with the glutamic acid functional groups of the protein polymer. Glycosylation of proteins was confirmed via mass spectrometry, NMR spectroscopy, SDS-PAGE, and photometric methods. CD spectroscopy shows that the resulting glycosylated proteins maintain a highly helical structure, and competitive ELISA assays suggest the efficient binding of these glycoproteins to cholera toxin. These results demonstrate that the integration of biological and chemical approaches in the synthesis of well-defined polymeric structures offers significant opportunities in the purposeful design of glycopolymers for applications in biology.

Structural approaches to the study of oligosaccharides in glycoprotein quality control

High-mannose-type oligosaccharides have been shown to play important roles in protein quality control. Several intracellular proteins, such as lectins, chaperones and glycan-processing enzymes, are involved in this process. These include calnexin/calreticulin, UDP-glucose:glycoprotein glucosyltransferase (UGGT), cargo receptors (such as VIP36 and ERGIC-53), mannosidase-like proteins (e.g. EDEM and Htm1p) and ubiquitin ligase (Fbs). They are thought to recognize high-mannose-type glycans with subtly different structures, although the precise specificities are yet to be clarified. In order to gain a clear understanding of these protein-carbohydrate interactions, comprehensive synthesis of high-mannose-type glycans was conducted. In addition, two approaches to the synthesis of artificial glycoproteins with homogeneous oligosaccharides were investigated. Furthermore, a novel substrate of UGGT was discovered.

Stereoselective Synthesis of α-C-Glucosyl Serine and Alanine via a Cross-Metathesis/Cyclization Strategy

C-Glycosyl amino acids represent stable mimics of monomeric units within natural O-linked glycoproteins. Olefin cross-metathesis has been used to provide alkene precursors for a mercury(II)-mediated cyclization, yielding alpha-C-glucosyl serine and alanine.

Carbohydrate diversity: synthesis of glycoconjugates and complex carbohydrates

The fundamental role of glycoconjugates in many biological processes is now well appreciated and has intensified the development of innovative and improved synthetic strategies. All areas of synthetic methodology have seen major advances and many complex, highly branched carbohydrates and glycoproteins have been prepared using solution- and/or solid-phase approaches. The development of an automated oligosaccharide synthesizer provides rapid access to biologically relevant compounds. These chemical approaches help to produce sufficient quantities of defined oligosaccharides for biological studies. Synthetic chemistry also supports an improved understanding of glycobiology and will eventually result in the discovery of new therapeutics.

Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan

A series of enzymatic substitutions modifies the basic structure of complex-type biantennary N-glycans. Among them, a beta1,2-linked N-acetylglucosamine residue is introduced to the central mannose moiety of the core-fucosylated oligosaccharide by N-acetylglucosaminyltransferase VII. This so-called LEC14 epitope can undergo galactosylation at the beta1,2-linked N-acetylglucosamine residue. Guided by the hypothesis that structural modifications in the N-glycan alter its capacity to serve as ligand for lectins, we prepared a neoglycoprotein with the extended LEC14 N-glycan and tested its properties in three different assays. In order to allow comparison to previous results on other types of biantennary N-glycans the functionalization of the glycans for coupling and assay conditions were deliberately kept constant. Compared to the core-fucosylated N-glycan no significant change in affinity was seen when testing three galactoside-specific proteins. However, cell positivity in flow cytofluorimetry was enhanced in six of eight human tumor lines. Analysis of biodistribution in tumor-bearing mice revealed an increase of blood clearance by about 40%, yielding a favorable tumor/blood ratio. Thus, the extended LEC14 motif affects binding properties to cellular lectins on cell surfaces and organs when compared to the core-fucosylated biantennary N-glycan. The results argue in favor of the concept of viewing substitutions as molecular switches for lectin-binding affinity. Moreover, they have potential relevance for glycoengineering of reagents in tumor imaging.

Chemoenzymatic synthesis of CD52 glycoproteins carrying native N-glycans

A facile synthesis of homogeneous CD52 glycoproteins carrying native N-glycans was achieved using an endolycosidase-catalyzed oligosaccharide transfer as the key step. The synthesis consists of two steps: the solid phase synthesis of GlcNAc-CD52 and the transfer of a high-mannose type or complex type N-glycan from Man(9)GlcNAc(2) Asn or a sialglycopeptide to the GlcNAc-CD52, under the catalysis of the endo-beta-N-acetylglucosaminidases from Arthrobacter (Endo-A) and Mucor hiemalis (Endo-M), respectively.

Glycopeptide and glycoprotein synthesis involving unprotected carbohydrate building blocks

This review summarizes the chemical and chemoenzymatic synthesis of glycopeptides and glycoproteins using unprotected carbohydrates as key intermediates. The synthetic methods covered herein include the convergent synthesis of glycopeptides by chemoselective ligation of peptides and free glycans, solution- and solid-phase synthesis of glycopeptides by sequential peptide elongation with unprotected glycosyl amino acids or short glycopeptides as building blocks, and the synthesis of glycopeptides by enzymatic and/or chemical elongation of the free glycans. The use of unprotected carbohydrates in these syntheses can circumvent the final-stage carbohydrate deprotection, lead to highly convergent synthetic designs, and more significantly, take advantage of the commercially available free glycans isolated from nature, which could considerably facilitate the synthesis of complex glycopeptides and glycoproteins.

Synthesis of homogenous site-selectively glycosylated proteins

Apparently homogenous glycoproteins can be synthesised in good yield by a combination of site directed mutagenesis, a highly flexible but selective chemical derivatisation and efficient purification through the use of glycosyl thiosulfonates such as 2-((biotinoyl)-amino)-ethyl methanethiosulfonate.

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.

Novel forms of chemical protein diversity — in nature and in the laboratory

Novel chemical variants of proteins have been found in nature, including potent 'microprotein' natural products and folded protein molecules that contain a cyclic polypeptide chain. Researchers have used chemical synthesis and genetic methods to make these proteins and more: protein catenanes, neoglycoproteins, and artificial protein molecules with novel architectures or made from novel building blocks. De novo design has taken a big step forward with the accurate design and construction of proteins with complex molecular structure. A variety of non-coded amino acids and other building blocks has been used to make increasingly sophisticated protein molecular devices for use as biosensors and for the study of signal transduction inside living cells.

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.

Glycoinsulins: Dendritic Sialyloligosaccharide-Displaying Insulins Showing a Prolonged Blood-Sugar-Lowering Activity

Mono-, di-, and trisialyloligosaccharides were introduced to mutant insulins through enzymatic reactions. Sugar chains were sialylated by alpha2,6-sialyltransferase (alpha2,6-SiaT) via an accessible glutamine residue at the N-terminus of the B-chain attached by transglutaminase (TGase). Sia2,6-di-LacNAc-Ins(B-F1Q) and Sia2,6-tri-LacNAc-Ins(B-F1Q), displaying two and three sialyl-N-acetyllactosamines, respectively, were administered to hyperglycemic mice. Both branched glycoinsulins showed prolonged glucose-lowering effects compared to native or lactose-carrying insulins, showing that sialic acid is important in obtaining a prolonged effect. Sia2,6-tri-LacNAc-Ins(B-F1Q), in particular, induced a significant delay in the recovery of glucose levels.

Biomimetic Synthesis of the Tumor-Associated (2,3)-Sialyl-T Antigen and Its Incorporation into Glycopeptide Antigens from the Mucins MUC1 and MUC4

Glycoproteins on epithelial tumor cells often exhibit aberrant glycosylation profiles. The incomplete formation of the glycan side chains resulting from a down-regulated glucosamine transfer and a premature sialylation results in additional peptide epitopes, which become accessible to the immune system in mucin-type glycoproteins. These cancer-specific structure alterations are considered to be a promising basis for selective immunological attack on tumor cells. Among the tumor-associated saccharide antigens, the (2,3)-sialyl-T antigen has been identified as the most abundant glycan, found in several different carcinoma cell lines. According to a linear biomimetic strategy, the (2,3)-sialyl-T antigen was synthesized by a stepwise glycan chain extension of a protected galactosamine-threonine precursor. For the construction of immunostimulating antigens combining both peptide and saccharide motifs, this antigen was incorporated into glycopeptide partial structures from the mucins MUC1 and MUC4 by sequential solid-phase synthesis.

Glycodendritic structures: promising new antiviral drugs

DC-SIGN, a C-type lectin expressed by dendritic cells, is able to recognize high mannosylated glycoproteins at the surface of a broad range of pathogens including viruses, bacteria, fungi and parasites. For at least some of these agents this interaction appears to be an important part of the infection process. Therefore, this lectin might be considered in the design of new antiviral drugs. In this manner, multivalent carbohydrate systems based on dendrimers and dendritic polymers are promising candidates as antiviral drugs. Boltorn hyperbranched dendritic polymers functionalized with mannose have been used to inhibit DC-SIGN-mediated infection in an Ebola-pseudotyped viral model. Their physiological solubility, lack of toxicity and especially their low price suggest the application of these glycodendritic polymers for possible formulation as microbicides.

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.