Utility of the Phenacyl Protecting Group in Traceless Protein Semisynthesis through Ligation-Desulfurization Chemistry

Synthesis of N-Glycosylated Soluble Fas Ligand

Controlled cell death is essential for the regulation of the immune system and plays a role in pathogen defense. It is often altered in pathogenic conditions such as cancer, viral infections and autoimmune diseases. The Fas receptor and its corresponding membrane-bound ligand (FasL) are part of the extrinsic apoptosis pathway activated in these cases. A soluble form of FasL (sFasL), produced by ectodomain shedding, displays a diverse but still elusive set of non-apoptotic functions and sometimes even serves as a pro-survival factor. To gather more knowledge about the characteristics of this protein and the impact N-glycosylations may have, access to homogeneous posttranslationally modified variants of sFasL is needed. Therefore, we developed a flexible strategy to obtain such homogeneously N-glycosylated variants of sFasL by applying chemical protein synthesis. This strategy can be flexibly combined with enzymatic methods to introduce more complex, site selective glycosylations.

HSP27 Inhibitory Activity against Caspase-3 Cleavage and Activation by Caspase-9 Is Enhanced by Chaperone O-GlcNAc Modification in Vitro

One of the O-GlcNAc modifications is the protection of cells against a variety of stressors that result in cell death. Previous experiments have focused on the overall ability of O-GlcNAc to prevent protein aggregation under stress as well as its ability to affect stress-response signaling pathways. Less attention has been paid to the potential role for O-GlcNAc in the direct inhibition of a major cell-death pathway, apoptosis. Apoptosis involves the sequential activation of caspase proteases, including the transfer of cell-stress information from initiator caspase-9 to effector caspase-3. Cells have multiple mechanisms to slow the apoptotic cascade, including heat shock protein HSP27, which can directly inhibit the activation of caspase-3 by caspase-9. We have previously shown that O-GlcNAc modification increases the chaperone activity of HSP27 against amyloid aggregation, raising the question as to whether this modification may play important roles in other facets of HSP27 biology. Here, we use protein chemistry to generate different versions of O-GlcNAc modified HSP27 and demonstrate that the modification enhances this antiapoptotic function of the chaperone, at least in an in vitro context. These results provide additional molecular insight into how O-GlcNAc functions as a mediator of cellular stress with important implications for human diseases like cancer and neurodegeneration.

OaAEP1 Ligase-Assisted Chemoenzymatic Synthesis of Full Cysteine-Rich Metal-Binding Cyanobacterial Metallothionein SmtA

Among all approaches used for the semisynthesis of natural or chemically modified products, enzyme-assisted ligation is among the most promising and dynamically developing approaches. Applying an efficient C247A mutant of Oldenlandia affinis plant ligase OaAEP1 and solid-phase peptide synthesis chemistry, we present the chemoenzymatic synthesis of a complete sequence of the cysteine-rich and metal-binding cyanobacterial metallothionein Synechococcus metallothionein A (SmtA). Zn(II) and Cd(II) binding to the newly synthesized SmtA showed identical properties to the protein expressed in Escherichia coli. The presented approach is the first example of the use of OaAEP1 mutant for total protein synthesis of metallothionein, which occurs in mild conditions preventing cysteine thiol oxidation. The recognition motif of the applied enzyme could naturally occur in the protein structure or be synthetically or genetically incorporated in some loops or secondary structure elements. Therefore, we envision that this strategy can be used for efficiently obtaining SmtA and for a wide range of proteins and their derivatives.

Protein Ligation and Labeling Enabled by a C-Terminal Tetracysteine Tag

The hydrazinolysis of S-cyanylated peptide provides an alternative way to afford protein α-hydrazide, a key reagent used in native chemical ligation (NCL), without the aid of any inteins or enzymes. The currently used non-selective S-cyanylation, however, allows no other cysteine in the protein besides the one at the cleavage site. Herein, we report a regioselective S-cyanylation and hydrazinolysis strategy achieved via the fusion of a tetracysteine tag to the C-terminal of the protein of interest. We term it tetracysteine enabled protein ligation (TCEPL). While highly selective, the strategy is applicable for proteins expressed as inclusion bodies, and this was showcased by the efficient semi-synthesis of an iron-sulfur protein rubredoxin and the catalytic and hinge domains of matrix metalloprotease-14 (MMP-14) containing 207 amino acid residues. Furthermore, the TCEPL strategy was exploited for protein C-terminal labeling with amino reagents bearing a variety of functional groups, demonstrating its versatility and generality.

Optimization of Semisynthetic Approach for Glycosyl Interferon-β-polypeptide by Utilizing Bacterial Protein Expression and Chemical Modification

Synthesis of a sufficient amount of homogenous glycoprotein is of great interest because the natural glycoproteins show a considerable heterogeneity in oligosaccharide structures making the studies of glycan structure-function relationship...

Cysteine protecting groups: applications in peptide and protein science

Protecting group chemistry for the cysteine thiol group has enabled a vast array of peptide and protein chemistry over the last several decades. Increasingly sophisticated strategies for the protection, and subsequent deprotection, of cysteine have been developed, facilitating synthesis of complex disulfide-rich peptides, semisynthesis of proteins, and peptide/protein labelling in vitro and in vivo. In this review, we analyse and discuss the 60+ individual protecting groups reported for cysteine, highlighting their applications in peptide synthesis and protein science.

On Demand Attachment and Detachment of rac-2-Br-DMNPA Tailoring to Facilitate Chemical Protein Synthesis

Herein, we developed a bifunctional reagent rac-2-Br-DMNPA 2 for the late-stage protection of peptide cysteine. Through the identification of its t-Bu ester 1 as a more competent form under ligation conditions, facile N-terminal and side-chain caging for the model peptide and protein were accomplished. Building upon this, a one-pot ligation and photolysis strategy was applied in the synthesis of the mini-protein chlorotoxin. More importantly, we extended the utility of 2 as a bifunctional linker for traceless solid-phase chemical ligation.

Glycoprotein Semisynthesis by Chemical Insertion of Glycosyl Asparagine Using a Bifunctional Thioacid-Mediated Strategy

Glycosylation is a major modification of secreted and cell surface proteins, and the resultant glycans show considerable heterogeneity in their structures. To understand the biological processes arising from each glycoform, the preparation of homogeneous glycoproteins is essential for extensive biological experiments. To establish a more robust and rapid synthetic route for the synthesis of homogeneous glycoproteins, we studied several key reactions based on amino thioacids. We found that diacyl disulfide coupling (DDC) formed with glycosyl asparagine thioacid and peptide thioacid yielded glycopeptides. This efficient coupling reaction enabled us to develop a new glycoprotein synthesis method, such as the bifunctional thioacid-mediated strategy, which can couple two peptides with the N- and C-termini of glycosyl asparagine thioacid. Previous glycoprotein synthesis methods required valuable glycosyl asparagine in the early stage and subsequent multiple glycoprotein synthesis routes, whereas the developed concept can generate glycoproteins within a few steps from peptide and glycosyl asparagine thioacid. Herein, we report the characterization of the DDC of amino thioacids and the efficient ability of glycosyl asparagine thioacid to be used for robust glycoprotein semisynthesis.

On-Demand Detachment of Succinimides on Cysteine to Facilitate (Semi)Synthesis of Challenging Proteins

The maleimide group is a widely used reagent for bioconjugation of peptides, proteins, and oligonucleotides employing Michael addition and Diels–Alder cycloaddition reactions. However, the utility of this functionality in chemical synthesis of peptides and proteins remains unexplored. We report, for the first time that Pd^II complexes can mediate the efficient removal of various succinimide derivatives in aqueous conditions. Succinimide removal by Pd^II was applied for the synthesis of two ubiquitin activity-based probes (Ub-ABPs) employing solid phase chemical ligation (SPCL). SPCL was achieved through a sequential three segment ligation on a polymer support via a maleimide anchor. The obtained probes successfully formed the expected covalent complexes with deubiquitinating enzymes (DUBs) USP2 and USP7, highlighting the use of our new method for efficient preparation of unique synthetic proteins. Importantly, we demonstrate the advantages of our newly developed method for the protection and deprotection of native cysteine with a succinimide group in a peptide fragment derived from thioredoxin-1 (Trx-1) obtained via intein based expression to enable ligation/desulfurization and subsequent disulfide bond formation in a one-pot process.

Chemoenzymatic Semisynthesis of Proteins

Protein semisynthesis-defined herein as the assembly of a protein from a combination of synthetic and recombinant fragments-is a burgeoning field of chemical biology that has impacted many areas in the life sciences. In this review, we provide a comprehensive survey of this area. We begin by discussing the various chemical and enzymatic methods now available for the manufacture of custom proteins containing noncoded elements. This section begins with a discussion of methods that are more chemical in origin and ends with those that employ biocatalysts. We also illustrate the commonalities that exist between these seemingly disparate methods and show how this is allowing for the development of integrated chemoenzymatic methods. This methodology discussion provides the technical foundation for the second part of the review where we cover the great many biological problems that have now been addressed using these tools. Finally, we end the piece with a short discussion on the frontiers of the field and the opportunities available for the future.

Highly Precise Protein Semisynthesis through Ligation-Desulfurization Chemistry in Combination with Phenacyl Protection of Native Cysteines

Semisynthesis of proteins via expressed protein ligation is a powerful tool to furnish full-length proteins carrying site-specific (posttranslational) modifications. The development of various β-mercapto amino acid building blocks coupled with ligation-desulfurization chemistry enabled further advances in this methodology by alleviating the need for cysteine residues at the desired ligation sites. However, this expansion in the availability of viable ligation sites is sometimes counterbalanced by the inadvertent desulfurization of unprotected native cysteines, which might be of structural and/or functional importance. Here, we provide a detailed protocol for using the cysteine-selective protecting group phenacyl (PAc) to achieve precise protein semisynthesis preserving native cysteine residues. The PAc group can be easily installed on cysteine(s) within recombinantly produced protein thioesters, withstands standard ligation, desulfurization and reversed phase HPLC conditions, and can be smoothly removed. We have previously demonstrated the utility of this protecting group through the semisynthesis of two model proteins, human small heat shock protein Hsp27 and Prion protein, in which one or two native cysteines, respectively, were maintained through the ligation-desulfurization sequence.

Prion protein-Semisynthetic prion protein (PrP) variants with posttranslational modifications

Deciphering the pathophysiologic events in prion diseases is challenging, and the role of posttranslational modifications (PTMs) such as glypidation and glycosylation remains elusive due to the lack of homogeneous protein preparations. So far, experimental studies have been limited in directly analyzing the earliest events of the conformational change of cellular prion protein (PrP^C ) into scrapie prion protein (PrP^Sc ) that further propagates PrP^C misfolding and aggregation at the cellular membrane, the initial site of prion infection, and PrP misfolding, by a lack of suitably modified PrP variants. PTMs of PrP, especially attachment of the glycosylphosphatidylinositol (GPI) anchor, have been shown to be crucially involved in the PrP^Sc formation. To this end, semisynthesis offers a unique possibility to understand PrP behavior invitro and invivo as it provides access to defined site-selectively modified PrP variants. This approach relies on the production and chemoselective linkage of peptide segments, amenable to chemical modifications, with recombinantly produced protein segments. In this article, advances in understanding PrP conversion using semisynthesis as a tool to obtain homogeneous posttranslationally modified PrP will be discussed.

The versatile use of solubilizing trityl tags for difficult peptide/protein synthesis

Solubilizing trityl tags (Trt-oligoLys/Arg) proved applicable to metal-free radical-triggered desulfurization and an Ag-mediated thioester method. Additionally, using the solubilizing trityl tag strategy, synthesis of the influenza BM2 proton channel, which previously required organic solvent-aided native chemical ligation (NCL) and desulfurization due to its low solubility, was achieved without using organic solvents.

Utility of the Phenacyl Protecting Group in Traceless Protein Semisynthesis through Ligation-Desulfurization Chemistry

Semisynthesis of proteins via expressed protein ligation is a widely applicable method, even more so because of the possibility of ligation at non-cysteine sites using β-mercapto amino acids that can be converted to the corresponding native amino acids by desulfurization. A drawback of this ligation- desulfurization approach is the removal of any unprotected native cysteine residues within the ligated protein segments. Here, we show that the phenacyl (PAc) moiety can be successfully used to protect cysteines within recombinantly generated protein segments. As such, this group was selectively appended onto cysteine side chains within bacterially expressed polypeptides following intein cleavage, which reveals a rather sensitive thioester at the C-terminus. The PAc group proved to be compatible with native chemical ligation, radical desulfurization, and reverse-phase HPLC conditions, and was smoothly removed at the end. The utility of the PAc protecting group was then demonstrated by the 'traceless' semisynthesis of two proteins containing one or two native cysteines: human small heat shock protein Hsp27 and murine prion protein.