Total Chemical Synthesis of Aggregation-Prone Disulfide-Rich Starfish Peptides.

A relaxin-like gonad-stimulating peptide (RGP), Aso-RGP, featuring six cysteine residues, was identified in the Crown-of-Thorns Starfish (COTS, Acanthaster cf. solaris) and initially produced through recombinant yeast expression. This method yielded a single-chain peptide with an uncleaved C-peptide (His Tag) and suboptimal purity. Our objective was to chemically synthesize Aso-RGP in its mature form, comprising two chains (A and B) and three disulfide bridges, omitting the C-peptide. Furthermore, we aimed to synthesize a newly identified relaxin-like peptide, Aso-RLP2, from COTS, which had not been previously synthesized. This paper reports the first total chemical synthesis of Aso-RGP and Aso-RLP2. Aso-RGP synthesis proceeded without major issues, whereas the A-chain of Aso-RLP2, in its reduced and unfolded state with two free thiols, presented considerable challenges. These were initially marked by "messy" RP-HPLC profiles, typically indicative of synthesis failure. Surprisingly, oxidizing the A-chain significantly improved the RP-HPLC profile, revealing the main issue was not synthesis failure but the peptide's aggregation tendency, which initially obscured analysis. This discovery highlights the critical need to account for aggregation in peptide synthesis and analysis. Ultimately, our efforts led to the successful synthesis of both peptides with purities exceeding 95 %.

Engineering of a Biologically Active Glycosylated Glucagon-Like Peptide-1 Analogue.

Glucagon-like peptide receptor (GLP-1R) agonists (e.g., semaglutide, liraglutide, etc.) are efficient treatment options for people with type 2 diabetes and obesity. The manufacturing method to produce semaglutide, a blockbuster GLP-1 drug on the market, involves multistep synthesis. The large peptide has a hydrophobic fatty acid side chain that makes it sparingly soluble, and its handling, purification, and large-scale production difficult. The growing demand for semaglutide that the manufacturer is not capable of addressing immediately triggered a worldwide shortage. Thus, we have developed a potential alternative analogue to semaglutide by replacing the hydrophobic fatty acid with a hydrophilic human complex-type biantennary oligosaccharide. Our novel glycoGLP-1 analogue was isolated in an ∼10-fold higher yield compared with semaglutide. Importantly, our glycoGLP-1 analogue possessed a similar GLP-1R activation potency to semaglutide and was biologically active in vivo in reducing glucose levels to a similar degree as semaglutide.

Optimizing the Semisynthesis towards glycosylated interferon-ß-polypeptide by utilizing bacterial protein expression and chemical modification.

The synthesis of a sufficient amount of homogeneous glycoprotein is of great interest because natural glycoproteins show considerable heterogeneity in oligosaccharide structures, making the studies on glycan structure-function relationship difficult. Herein, we report optimized methods that can accelerate the semisynthesis of homogeneous glycoproteins based on recombinant expression and chemical conversion. Peptide thioesters and peptides with Cys residues at their N-terminals are necessary intermediates to perform native chemical ligation. We successfully performed thioesterification for a peptide prepared in E. coli via Cys-cyanylation at its C-terminal followed by hydrazinolysis and acidic thiolysis. These optimized conditions could tolerate an acid labile Thz protected Cys at the N-terminal of a peptide-hydrazide and specific cyanylation of the C-terminal Cys to yield a peptide thioester. To reduce the amount of precious oligosaccharide that is required in the conventional SPPS method, an improved liquid phase glycopeptide coupling was also optimized in a good yield (46% over four steps). Lastly, chemoselective protection of the internal cysteines and activation of the N-terminal cysteine were optimized toward a long peptide prepared in E. coli. By using these strategies, a full-length interferon-ß glycosyl polypeptide as a model was successfully obtained.

Semisynthesis of a Homogeneous Glycoprotein Using Chemical Transformation of Peptides to Thioester Surrogates.

Semisynthesis using recombinant polypeptides as building blocks is a powerful approach for the preparation of proteins with a variety of modifications such as glycosylation. The activation of the C terminus of recombinant peptides is a key step for coupling peptide building blocks and preparing a full-length polypeptide of a target protein. This article reports two chemical approaches for transformation of the C terminus of recombinant polypeptides to thioester surrogates. The first approach relies on efficient substitution of the C-terminal Cys residue with bis(2-sulfanylethyl)amine (SEA) to yield peptide-thioester surrogates. The second approach employs a native tripeptide, cysteinyl-glycyl-cysteine (CGC), to yield peptide-thioesters via a process mediated by a thioester surrogate. Both chemical transformation methods employ native peptide sequences and were thereby successfully applied to recombinant polypeptides. As a consequence, we succeeded in the semisynthesis of a glycosylated form of inducible T cell costimulator (ICOS) for the first time.

Chemical Synthesis and Characterization of a Nonfibrillating Glycoglucagon.

The current commercially available glucagon formulations for the treatment of severe hypoglycemia must be reconstituted immediately prior to use, owing to the susceptibility of glucagon to fibrillation and aggregation in an aqueous solution. This results in the inconvenience of handling, misuse, and wastage of this drug. To address these issues, we synthesized a glycosylated glucagon analogue in which the 25th residue (Trp) was replaced with a cysteine (Cys) and a Br-disialyloligosaccharide was conjugated at the Cys thiol moiety. The resulting analogue, glycoglucagon, is a highly potent full agonist at the glucagon receptor. Importantly, glycoglucagon exhibits markedly reduced propensity for fibrillation and enhanced thermal and metabolic stability. This novel analogue is thus a valuable lead for producing stable liquid glucagon formulations that will improve patient compliance and minimize wastage.

Chemical Modification of the N Termini of Unprotected Peptides for Semisynthesis of Modified Proteins by Utilizing a Hydrophilic Protecting Group.

A simple and efficient strategy for the selective modification of the peptide N terminus with an unnatural amino acid is described. A peptide having a SUMO-HisTag-TEV sequence (SUMO: small ubiquitin-related modifier, TEV: tobacco etch virus) preceding the N terminus of the target peptide was designed. Recombinant expression in E. coli and subsequent SUMO protease cleavage yielded the HisTag-TEV-target peptide. Partial protection of the lysine side chains of this peptide with d-glucopyranosyloxycarbonyl and removal of the HisTag-TEV sequence by TEV protease yielded the partially protected peptide with a free N-terminal amine. Coupling of selenocysteine selectively at the N terminus and subsequent acidic deprotection of the carbohydrate protecting groups yielded a modified peptide that can be used for native chemical ligation (NCL). As a proof of concept, the modification of a longer recombinant peptide with selenocysteinylserine (GalNAc) at the N terminus was demonstrated.