An approach for a synthesis of asparagine-linked sialylglycopeptides having intact and homogeneous complex-type undecadisialyloligosaccharides

Chemical and Chemoenzymatic Synthesis of Peptide and Protein Therapeutics Conjugated with Human N-Glycans

Glycosylation is one of the most ubiquitous post-translational modifications. It affects the structure and function of peptides/proteins and consequently has a significant impact on various biological events. However, the structural complexity and heterogeneity of glycopeptides/proteins caused by the diversity of glycan structures and glycosylation sites complicates the detailed elucidation of glycan function and hampers their clinical applications. To address these challenges, chemical and/or enzyme-assisted synthesis methods have been developed to realize glycopeptides/proteins with well-defined glycan morphologies. In particular, N-glycans are expected to be useful for improving the solubility, in vivo half-life and aggregation of bioactive peptides/proteins that have had limited clinical applications so far due to their short duration of action in the blood and unsuitable physicochemical properties. Chemical glycosylation performed in a post-synthetic procedure can be used to facilitate the development of glycopeptide/protein analogues or mimetics that are superior to the original molecules in terms of physicochemical and pharmacokinetic properties. N-glycans are used to modify targets because they are highly biodegradable and biocompatible and have structures that already exist in the human body. On the practical side, from a quality control perspective, close attention should be paid to their structural homogeneity when they are to be applied to pharmaceuticals.

Synthesis of Four Pentacyclic Triterpene-Sialylglycopeptide Conjugates and Their Affinity Assays with Hemagglutinin

Influenza outbreaks pose a serious threat to human health. Hemagglutinin (HA) is an important target for influenza virus entry inhibitors. In this study, we synthesized four pentacyclic triterpene conjugates with a sialylglycopeptide scaffold through the Cu(I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC) and prepared affinity assays of these conjugates with two HAs, namely H1N1 (A/WSN/1933) and H5N1 (A/Hong Kong/483/97), respectively. With a dissociation constant (KD) of 6.89 μM, SCT-Asn-betulinic acid exhibited the strongest affinity with the H1N1 protein. Furthermore, with a KD value of 9.10 μM, SCT-Asn-oleanolic acid exhibited the strongest affinity with the H5N1 protein. The conjugates considerably enhanced antiviral activity, which indicates that pentacyclic triterpenes can be used as a ligand to improve the anti-influenza ability of the sialylglycopeptide molecule by acting on the HA protein.

N-Linked Glycosyl Auxiliary-Mediated Native Chemical Ligation on Aspartic Acid: Application towards N-Glycopeptide Synthesis

A practical approach towards N-glycopeptide synthesis using an auxiliary-mediated dual native chemical ligation (NCL) has been developed. The first NCL connects an N-linked glycosyl auxiliary to the thioester side chain of an N-terminal aspartate oligopeptide. This intermediate undergoes a second NCL with a C-terminal thioester oligopeptide. Mild cleavage provides the desired N-glycopeptide.

Total chemical synthesis and biological activities of glycosylated and non-glycosylated forms of the chemokines CCL1 and Ser-CCL1

CCL1 is a naturally glycosylated chemokine protein that is secreted by activated T-cells and acts as a chemoattractant for monocytes. Originally, CCL1 was identified as a 73 amino acid protein having one N-glycosylation site, and a variant 74 residue non-glycosylated form, Ser-CCL1, has also been described. There are no systematic studies of the effect of glycosylation on the biological activities of either CCL1 or Ser-CCL1. Here we report the total chemical syntheses of both N-glycosylated and non-glycosylated forms of (Ser-)CCL1, by convergent native chemical ligation. We used an N-glycan isolated from hen egg yolk together with the Nbz linker for Fmoc chemistry solid phase synthesis of the glycopeptide-(α) thioester building block. Chemotaxis assays of these glycoproteins and the corresponding non-glycosylated proteins were carried out. The results were correlated with the chemical structures of the (glyco)protein molecules. To the best of our knowledge, these are the first investigations of the effect of glycosylation on the chemotactic activity of the chemokine (Ser-)CCL1 using homogeneous N-glycosylated protein molecules of defined covalent structure.

A new approach for the synthesis of O-glycopeptides through a combination of solid-phase glycosylation and fluorous tagging chemistry (SHGPFT)

A new and efficient hybrid approach has been developed for the synthesis and purification of O-linked glycopeptides with high purity.

Preparation and evaluation at the delta opioid receptor of a series of linear leu-enkephalin analogues obtained by systematic replacement of the amides

Leu-enkephalin analogues, in which the amide bonds were sequentially and systematically replaced either by ester or N-methyl amide bonds, were prepared using classical organic chemistry as well as solid phase peptide synthesis (SPPS). The peptidomimetics were characterized using competition binding, ERK1/2 phosphorylation, receptor internalization, and contractility assays to evaluate their pharmacological profile over the delta opioid receptor (DOPr). The lipophilicity (LogD7.4) and plasma stability of the active analogues were also measured. Our results revealed that the last amide bond can be successfully replaced by either an ester or an N-methyl amide bond without significantly decreasing the biological activity of the corresponding analogues when compared to Leu-enkephalin. The peptidomimetics with an N-methyl amide function between residues Phe and Leu were found to be more lipophilic and more stable than Leu-enkephalin. Findings from the present study further revealed that the hydrogen-bond donor properties of the fourth amide of Leu-enkephalin are not important for its biological activity on DOPr. Our results show that the systematic replacement of amide bonds by isosteric functions represents an efficient way to design and synthesize novel peptide analogues with enhanced stability. Our findings further suggest that such a strategy can also be useful to study the biological roles of amide bonds.

Chemical assembly of N-glycoproteins: a refined toolbox to address a ubiquitous posttranslational modification

Incremental developments in the chemistry of peptides, proteins and carbohydrates have enabled researchers to assemble entire glycoproteins with high precision. Based on sophisticated ligation chemistries pure glycoproteins bearing a single glycosylation pattern have become available. The impact of N-glycosylation on the function of glycoproteins is generally recognized but not well understood. Based on the recent advances in the synthesis of glycoproteins by chemical methods researchers can finally start to elucidate the various roles of carbohydrates in complex biomolecules in detail.

Chemical synthesis of intentionally misfolded homogeneous glycoprotein: a unique approach for the study of glycoprotein quality control

Biosynthesis of glycoproteins in the endoplasmic reticulum employs a quality control system, which discriminates and excludes misfolded malfunctional glycoproteins from a correctly folded one. As chemical tools to study the glycoprotein quality control system, we systematically synthesized misfolded homogeneous glycoproteins bearing a high-mannose type oligosaccharide via oxidative misfolding of a chemically synthesized homogeneous glycopeptide. The endoplasmic reticulum folding sensor enzyme, UDP-glucose:glycoprotein glucosyltransferase (UGGT), recognizes a specific folding intermediate, which exhibits a molten globule-like hydrophobic nature.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Total synthesis of the α-subunit of human glycoprotein hormones: toward fully synthetic homogeneous human follicle-stimulating hormone

Described herein is the first total chemical synthesis of the unique α-subunit of the human glycoprotein hormone (α-hGPH). Unlike the biologically derived glycoprotein hormones, which are isolated as highly complex mixtures of glycoforms, α-hGPH obtained by chemical synthesis contains discrete homogeneous glycoforms. Two such systems have been prepared. One contains the disaccharide chitobiose at the natural N-glycosylation sites. The other contains dodecamer oligosaccharides at these same sites. The dodecamer sugar is a consensus sequence incorporating the key features associated with human glycoproteins.

Chemical synthesis of homogeneous glycopeptides and glycoproteins

Oligosaccharides linked to proteins are known to play important roles in several biological events. However, oligosaccharides are heterogeneous, which has hindered detailed elucidation of oligosaccharide functions. In order to solve this problem, glycoproteins having homogeneous oligosaccharides have long been required. For this purpose, an efficient preparative method of complex-type oligosaccharides has been investigated from a natural source and this method was found to afford over 24 kinds of diverse complex-type oligosaccharides by use of chemical methods and branch-specific sequential glycosidase digestion. The sufficient amount of homogeneous complex type oligosaccharides obtained enabled us to examine the synthesis of homogeneous glycopeptides as well as glycoproteins by use of solid phase glycopeptide synthetic method and native chemical ligation. This review describes recent progress related to the efficient method of oligosaccharide preparation and synthesis of glycoproteins including bioactive erythropoietin.

Study of on-resin convergent synthesis of N-linked glycopeptides containing a large high mannose N-linked oligosaccharide

Here we present a convergent on-resin glycosylamine coupling strategy for solid phase N-linked glycopeptide synthesis, and apply it to the synthesis of high mannose containing glycopeptides. In this strategy, the 2-phenylisopropyl protecting group is used as an orthogonal handle to create glycosylation sites on-resin after synthesis of nonglycosylated peptides. In addition to allowing selective deprotection of aspartic acid residues for creation of glycosylation sites, the 2-phenylisopropyl protecting group also efficiently suppresses aspartimide formation during peptide synthesis. The key step of on-resin glycosylamine coupling to an aspartic acid residue was first optimized for a small sugar, N-acetylglucosamine, and then applied to a much larger high mannose oligosaccharide, Man(8)GlcNAc(2). Satisfying coupling yields were obtained for both small and large sugars. The use of on-resin glycosylamine coupling simplifies purification of N-linked glycopeptides, and also allows convenient recovery of unreacted valuable large oligosaccharides. This approach was applied to the solid phase synthesis of glycosylated forms of the 34 amino acid HIV-1 gp41 C34 glycopeptide, which is an HIV-1 entry inhibitor. The HIV-1 entry inhibition assay of synthesized glycopeptides showed the retention of bioactivity of high mannose Man(8)GlcNAc(2)-C34.

Recent Departures in the Synthesis of Peptides and Glycopeptides

In this account, we describe the results of a research program directed to the proposition that chemical synthesis can play a valuable role in identifying biologic level molecules worthy of pharma level development. We recount our journey towards the chemical synthesis of homogeneous erythropoietin, the challenges we encountered, and our efforts to address deficiencies in the current "state of the art" of glycopeptide synthesis. Here we describe new methods for the synthesis of glycopeptides that have emerged from the erythropoietin adventure, including the development of unique C-terminal acyl donors, novel amide bond forming methods, and new ligation and coupling strategies.

Chemical synthesis of a glycoprotein having an intact human complex-type sialyloligosaccharide under the Boc and Fmoc synthetic strategies

The chemical synthesis of complex glycoproteins is an ongoing challenge in protein chemistry. We have examined the synthesis of a single glycoform of monocyte chemotactic protein-3 (MCP-3), a CC-chemokine that consists of 76 amino acids and one N-glycosylation site. A three-segment native chemical ligation strategy was employed using unprotected peptides and glycopeptide. Importantly, the synthesis required the development of methods for the generation of sialylglycopeptide-alphathioesters. For the sialylglycopeptide-alphathioester segment, we examined and successfully implemented approaches using Fmoc-SPPS and Boc-SPPS. To avoid use of hydrogen fluoride, the Boc approach utilized minimal side chain protection and direct thiolysis of the resin bound peptide. Using these strategies, we successfully synthesized a glycoprotein having an intact and homogeneous complex-type sialyloligosaccharide.

Recent progress in the field of glycopeptide synthesis

Glycosylation is a common post-translational modification of proteins. Although its significance in biological system is well recognized, approaches to analyze carbohydrate function are limited. This is because of difficulty in obtaining homogeneous glycoproteins from natural sources. Due to the progress of the carbohydrate and peptide chemistry, syntheses of various homogeneous glycopeptides and glycoproteins, which are suitable for biological studies, have been achieved by chemical means. In this review, we briefly summarize recent advances in the field of glycopeptide synthesis after 1999.