Ligation Embedding Aggregation Disruptor Strategy Enables the Chemical Synthesis of PD-1 Immunoglobulin and Extracellular Domains

Selective Peptide Cysteine Manipulation on Demand and Difficult Protein Chemical Synthesis Enabled by Controllable Acidolysis of N,S-Benzylidene Thioacetals

Although solid-phase peptide synthesis combining with chemical ligation provides a way to build up customized polypeptides in general, many targets are still presenting challenges for the conventional synthetic process, such as hydrophobic proteins. New methods and strategies are still required to overcome these obstacles. In this study, kinetic studies of Cys/Pen ligation and its acidolysis were performed, from which the fast acidolysis of substituted N,S-benzylidene thioacetals (NBTs) was discovered. The study demonstrates the potential of NBTs as a promising Cys switchable protection, facilitating the chemical synthesis of peptides and proteins by efficiently disrupting peptide aggregation. The compatibility of NBTs with other commonly adopted Cys protecting groups and their applications in sequential disulfide bond formation were also investigated. The first chemical synthesis of the native human programmed death ligand 1 immunoglobulin V-like (PD-L1 IgV) domain was achieved using the NBT strategy, showcasing its potential in difficult protein synthesis.

An Enzyme-Cleavable Solubilizing-Tag Facilitates the Chemical Synthesis of Mirror-Image Proteins

Mirror-image proteins (D-proteins) are useful in biomedical research for purposes such as mirror-image screening for D-peptide drug discovery, but the chemical synthesis of many D-proteins is often low yielding due to the poor solubility or aggregation of their constituent peptide segments. Here, we report a Lys-C protease-cleavable solubilizing tag and its use to synthesize difficult-to-obtain D-proteins. Our tag is easily installed onto multiple amino acids such as DLys, DSer, DThr, and/or the N-terminal amino acid of hydrophobic D-peptides, is impervious to various reaction conditions, such as peptide synthesis, ligation, desulfurization, and transition metal-mediated deprotection, and yet can be completely removed by Lys-C protease under denaturing conditions to give the desired D-protein. The efficacy and practicality of the new method were exemplified in the synthesis of two challenging D-proteins: D-enantiomers of programmed cell death protein 1 IgV domain and SARS-CoV-2 envelope protein, in high yield. This work demonstrates that the enzymatic cleavage of solubilizing tags under denaturing conditions is feasible, thus paving the way for the production of more D-proteins.

Mirror-image ligand discovery enabled by single-shot fast-flow synthesis of D-proteins

Widespread adoption of mirror-image biological systems presents difficulties in accessing the requisite D-protein substrates. In particular, mirror-image phage display has the potential for high-throughput generation of biologically stable macrocyclic D-peptide binders with potentially unique recognition modes but is hindered by the individualized optimization required for D-protein chemical synthesis. We demonstrate a general mirror-image phage display pipeline that utilizes automated flow peptide synthesis to prepare D-proteins in a single run. With this approach, we prepare and characterize 12 D-proteins - almost one third of all reported D-proteins to date. With access to mirror-image protein targets, we describe the successful discovery of six macrocyclic D-peptide binders: three to the oncoprotein MDM2, and three to the E3 ubiquitin ligase CHIP. Reliable production of mirror-image proteins can unlock the full potential of D-peptide drug discovery and streamline the study of mirror-image biology more broadly.

L-Glycosidase-Cleavable Natural Glycans Facilitate the Chemical Synthesis of Correctly Folded Disulfide-Bonded D-Proteins

D-peptide ligands can be screened for therapeutic potency and enzymatic stability using synthetic mirror-image proteins (D-proteins), but efficient acquisition of these D-proteins can be hampered by the need to accomplish their in vitro folding, which often requires the formation of correctly linked disulfide bonds. Here, we report the finding that temporary installation of natural O-linked-β-N-acetyl-D-glucosamine (O-GlcNAc) groups onto selected D-serine or D-threonine residues of the synthetic disulfide-bonded D-proteins can facilitate their folding in vitro, and that the natural glycosyl groups can be completely removed from the folded D-proteins to afford the desired chirally inverted D-protein targets using naturally occurring O-GlcNAcase. This approach enabled the efficient chemical syntheses of several important but difficult-to-fold D-proteins incorporating disulfide bonds including the mirror-image tumor necrosis factor alpha (D-TNFα) homotrimer and the mirror-image receptor-binding domain of the Omicron spike protein (D-RBD). Our work establishes the use of O-GlcNAc to facilitate D-protein synthesis and folding and proves that D-proteins bearing O-GlcNAc can be good substrates for naturally occurring O-GlcNAcase.

N,O-Benzylidene Acetal Dipeptides (NBDs) Enable the Synthesis of Difficult Peptides via a Kinked Backbone Strategy

Proteins with highly hydrophobic regions or aggregation-prone sequences are typically difficult targets for chemical synthesis at the current stage, as obtaining such type of peptides via solid-phase peptide synthesis requires sophisticated operations. Herein, we report N,O-benzylidene acetal dipeptides (NBDs) as robust and effective building blocks to allow the direct synthesis of difficult peptides and proteins via a kinked backbone strategy. The effectiveness and easy accessibility of NBDs have been well demonstrated in our chemical syntheses of various challenging peptides and proteins, including chemokine, therapeutic hormones, histone, and glycosylated erythropoietin.

Chemical Synthesis of Peptides and Proteins Bearing Base-Labile Post-Translational Modifications: Evolution of the Methods in Four Decades

The S-palmitoylation on Cys residue and O-acetylation on Ser/Thr residues are two types of base-labile post-translational modifications (PTMs) in cells. The lability of these PTMs to bases and nucleophiles makes the peptides/proteins bearing S-palmitoyl or O-acetyl groups challenging synthetic targets, which cannot be prepared via the standard Fmoc-SPPS and native chemical ligation. In this review, we summarized the efforts towards their preparation in the past 40 years, with the focus on the evolution of synthetic methods.

Backbone-Installed Split Intein-Assisted Ligation for the Chemical Synthesis of Mirror-Image Proteins

Membrane-associated D-proteins are an important class of synthetic molecules needed for D-peptide drug discovery, but their chemical synthesis using canonical ligation methods such as native chemical ligation is often hampered by the poor solubility of their constituent peptide segments. Here, we describe a Backbone-Installed Split Intein-Assisted Ligation (BISIAL) method for the synthesis of these proteins, wherein the native L-forms of the N- and C-intein fragments of the unique consensus-fast (Cfa) (i.e. L-CfaN and L-CfaC ) are separately installed onto the two D-peptide segments to be ligated via a removable backbone modification. The ligation proceeds smoothly at micromolar (μM) concentrations under strongly chaotropic conditions (8.0 M urea), and the subsequent removal of the backbone modification groups affords the desired D-proteins without leaving any "ligation scar" on the products. The effectiveness and practicality of the BISIAL method are exemplified by the synthesis of the D-enantiomers of the extracellular domains of T cell immunoglobulin and ITIM domain (TIGIT) and tropomyosin receptor kinase C (TrkC). The BISIAL method further expands the chemical protein synthesis ligation toolkit and provides practical access to challenging D-protein targets.

Oxidation and Phenolysis of Peptide/Protein C-Terminal Hydrazides Afford Salicylaldehyde Ester Surrogates for Chemical Protein Synthesis

With the growing popularity of serine/threonine ligation (STL) and cysteine/penicillamine ligation (CPL) in chemical protein synthesis, facile and general approaches for the preparation of peptide salicylaldehyde (SAL) esters are urgently needed, especially those viable for obtaining expressed protein SAL esters. Herein, we report the access of SAL ester surrogates from peptide hydrazides (obtained either synthetically or recombinantly) via nitrite oxidation and phenolysis by 3-(1,3-dithian-2-yl)-4-hydroxybenzoic acid (SAL(-COOH)^PDT). The resulting peptide SAL(-COOH)^PDT esters can be activated to afford the reactive peptide SAL(-COOH) esters for subsequent STL/CPL. While being operationally simple for both synthetic peptides and expressed proteins, the current strategy facilitates convergent protein synthesis and combined application of STL with NCL. The generality of the strategy is showcased by the N-terminal ubiquitination of the growth arrest and DNA damage-inducible protein (Gadd45a), the efficient synthesis of ubiquitin-like protein 5 (UBL-5) via a combined N-to-C NCL-STL strategy, and the C-to-N semisynthesis of a myoglobin (Mb) variant.

Chemical Synthesis of Proteins Using an o-Nitrobenzyl Group as a Robust Temporary Protective Group for N-Terminal Cysteine Protection

We report here a robust and practical strategy for chemical protein synthesis using an o-nitrobenzyl group as a temporary protective group for an N-terminal cysteine residue of intermediate hydrazide fragments. By reinvestigating the photoremoval of an o-nitrobenzyl group, we establish a robust and reliable strategy for its quantitative photodeprotection. The o-nitrobenzyl group is completely stable to oxidative NaNO₂ treatment and has been applied to the convergent chemical synthesis of programmed death ligand 1 fragment, providing a practical avenue for hydrazide-based native chemical ligation.

Total synthesis of interleukin-2 via a tunable backbone modification strategy

Chemical synthesis of hydrophobic proteins presents a formidable task as they are often difficultly achieved via peptide synthesis, purification, and peptide ligation. Thus, peptide solubilizing strategies are needed to integrate with peptide ligation to achieve protein total synthesis. Herein, we report a tunable backbone modification strategy, taking advantage of the tunable stability of the Cys/Pen ligation intermediate, which allows for readily introducing a solubilizing tag for both peptide purification and ligation processes. The effectiveness of this strategy was demonstrated by the chemical synthesis of interleukin-2.

Chemical Synthesis of Proteins with Base-Labile Posttranslational Modifications Enabled by a Boc-SPPS Based General Strategy Towards Peptide C-Terminal Salicylaldehyde Esters

Chemical synthesis of proteins bearing base-labile post-translational modifications (PTMs) is a challenging task. For instance, O-acetylation and S-palmitoylation PTMs cannot survive Fmoc removal conditions during Fmoc-solid phase peptide synthesis (SPPS). In this work, we developed a new Boc-SPPS-based strategy for the synthesis of peptide C-terminal salicylaldehyde (SAL) esters, which are the key reaction partner in Ser/Thr ligation and Cys/Pen ligation. The strategy utilized the semicarbazone-modified aminomethyl (AM) resin, which could support the Boc-SPPS and release the peptide SAL ester upon treatment with TFA/H₂ O and pyruvic acid. The non-oxidative aldehyde regeneration was fully compatible with all the canonical amino acids. Armed with this strategy, we finished the syntheses of the O-acetylated protein histone H3(S10ac, T22ac) and the hydrophobic S-palmitoylated peptide derived from caveolin-1.