Novel protein science enabled by total chemical synthesis

Late-Stage Desulfurization Enables Rapid and Efficient Solid-Phase Synthesis of Cathepsin-Cleavable Linkers for Antibody-Drug Conjugates

The synthesis of linker-payloads is a critical step in developing antibody-drug conjugates (ADCs), a rapidly advancing therapeutic approach in oncology. The conventional method for synthesizing cathepsin B-labile dipeptide linkers, which are commonly used in ADC development, involves the solution-phase assembly of cathepsin B-sensitive dipeptides, followed by the installation of self-immolative para-aminobenzyl carbonate to facilitate the attachment of potent cytotoxic payloads. However, this approach is often low yield and laborious, especially when extending the peptide chain with components like glutamic acid to improve mouse serum stability or charged amino acids or poly(ethylene glycol) moieties to enhance linker hydrophilicity. Here, we introduce a novel approach utilizing late-stage desulfurization chemistry, enabling safe, facile, and cost-effective access to the cathepsin B-cleavable linker, Val-Ala-PABC-MMAE, on resin for the first time.

Mirror-image trypsin digestion and sequencing of D-proteins

The development of mirror-image biology systems and related applications is hindered by the lack of effective methods to sequence mirror-image (D-) proteins. Although natural-chirality (L-) proteins can be sequenced by bottom-up liquid chromatography-tandem mass spectrometry (LC-MS/MS), the sequencing of long D-peptides and D-proteins with the same strategy requires digestion by a site-specific D-protease before mass analysis. Here we apply solid-phase peptide synthesis and native chemical ligation to chemically synthesize a mirror-image version of trypsin, a widely used protease for site-specific protein digestion. Using mirror-image trypsin digestion and LC-MS/MS, we sequence a mirror-image large subunit ribosomal protein (L25) and a mirror-image Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4), and distinguish between different mutants of D-Dpo4. We also perform writing and reading of digital information in a long D-peptide of 50 amino acids. Thus, mirror-image trypsin digestion in conjunction with LC-MS/MS may facilitate practical applications of D-peptides and D-proteins as potential therapeutic and informational tools.

Fabrication strategies for chiral self-assembly surface

Chiral self-assembly is the spontaneous organization of individual building blocks from chiral (bio)molecules to macroscopic objects into ordered superstructures. Chiral self-assembly is ubiquitous in nature, such as DNA and proteins, which formed the foundation of biological structures. In addition to chiral (bio) molecules, chiral ordered superstructures constructed by self-assembly have also attracted much attention. Chiral self-assembly usually refers to the process of forming chiral aggregates in an ordered arrangement under various non-covalent bonding such as H-bond, π-π interactions, van der Waals forces (dipole-dipole, electrostatic effects, etc.), and hydrophobic interactions. Chiral assembly involves the spontaneous process, which followed the minimum energy rule. It is essentially an intermolecular interaction force. Self-assembled chiral materials based on chiral recognition in electrochemistry, chiral catalysis, optical sensing, chiral separation, etc. have a broad application potential with the research development of chiral materials in recent years.

Synthetic studies on the extracellular domain of the T cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain (TIGIT) using Trt-K10 solubilizing tags

The T cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain (TIGIT) is an inhibitory immunoreceptor expressed on lymphocytes that serves as a promising target for cancer immunotherapy. In this study, facile synthetic protocols to produce the extracellular domain of TIGIT were investigated for applications of TIGIT in mirror-image screening. During the synthesis via sequential native chemical ligations, we encountered problems with significantly poor solubility of the ligated products. Introducing trityl-type solubilizing auxiliaries, which also functioned as temporary protecting groups for cysteine residues, facilitated a flexible order of ligations and efficient purification protocols. After refolding under appropriate conditions, the synthetic TIGIT showed a sufficient affinity toward its target ligand CD155.

Plant Protein-Derived Active Peptides: A Comprehensive Review

Active peptides are a class of physiologically active protein fragments, which can be prepared from different sources. In the past few decades, the production of peptides with various effects from different plant proteins continues to receive academic attention. With advances in extraction, purification, and characterization techniques, plant protein-derived active peptides continue to be discovered. They have been proven to have various functional activities such as antioxidant, antihypertensive, immunomodulatory, antimicrobial, anti-inflammatory, antidiabetic, antithrombotic, and so on. In this review, we searched Web of Science and China National Knowledge Infrastructure for relevant articles published in recent years. There are 184 articles included in this manuscript. The current status of plant protein-derived active peptides is systematically introduced, including their sources, preparation, purification and identification methods, physiological activities, and applications in the food industry. Special emphasis has been placed on the problems of active peptide exploration and the future trend. Based on these, it is expected to provide theoretical reference for the further exploitation of plant protein-derived active peptides, and promote the healthy and rapid development of active peptide industry.

Synthesis of Peptidyl-tRNA Mimics for Structural Biology Applications

Protein biosynthesis is a central process in all living cells that is catalyzed by a complex molecular machine─the ribosome. This process is termed translation because the language of nucleotides in mRNAs is translated into the language of amino acids in proteins. Transfer RNA (tRNA) molecules charged with amino acids serve as adaptors and recognize codons of mRNA in the decoding center while simultaneously the individual amino acids are assembled into a peptide chain in the peptidyl transferase center (PTC). As the nascent peptide emerges from the ribosome, it is threaded through a long tunnel referred to as a nascent peptide exit tunnel (NPET). The PTC and NPET are the sites targeted by many antibiotics and are thus of tremendous importance from a biomedical perspective and for drug development in the pharmaceutical industry.Researchers have achieved much progress in characterizing ribosomal translation at the molecular level; an impressive number of high-resolution structures of different functional and inhibited states of the ribosome are now available. These structures have significantly contributed to our understanding of how the ribosome interacts with its key substrates, namely, mRNA, tRNAs, and translation factors. In contrast, much less is known about the mechanisms of how small molecules, especially antibiotics, affect ribosomal protein synthesis. This mainly concerns the structural basis of small molecule-NPET interference with cotranslational protein folding and the regulation of protein synthesis. Growing biochemical evidence suggests that NPET plays an active role in the regulation of protein synthesis.Much-needed progress in this field is hampered by the fact that during the preparation of ribosome complexes for structural studies (i.e., X-ray crystallography, cryoelectron microscopy, and NMR spectroscopy) the aminoacyl- or peptidyl-tRNAs are unstable and become hydrolyzed. A solution to this problem is the application of hydrolysis-resistant mimics of aminoacyl- or peptidyl-tRNAs.In this Account, we present an overview of synthetic methods for the generation of peptidyl-tRNA analogs. Modular approaches have been developed that combine (i) RNA and peptide solid-phase synthesis on 3'-aminoacylamino-adenosine resins, (ii) native chemical ligations and Staudinger ligations, (iii) tailoring of tRNAs by the selective cleavage of natural native tRNAs with DNAzymes followed by reassembly with enzymatic ligation to synthetic peptidyl-RNA fragments, and (iv) enzymatic tailing and cysteine charging of the tRNA to obtain modified CCA termini of a tRNA that are chemically ligated to the peptide moiety of interest. With this arsenal of tools, in principle, any desired sequence of a stably linked peptidyl-tRNA mimic is accessible. To underline the significance of the synthetic conjugates, we briefly point to the most critical applications that have shed new light on the molecular mechanisms underlying the context-specific activity of ribosome-targeting antibiotics, ribosome-dependent incorporation of multiple consecutive proline residues, the incorporation of d-amino acids, and tRNA mischarging.Furthermore, we discuss new types of stably charged tRNA analogs, relying on triazole- and squarate (instead of amide)-linked conjugates. Those have pushed forward our mechanistic understanding of nonribosomal peptide synthesis, where aminoacyl-tRNA-dependent enzymes are critically involved in various cellular processes in primary and secondary metabolism and in bacterial cell wall synthesis.

Combining Chemical Protein Synthesis and Random Nonstandard Peptides Integrated Discovery for Modulating Biological Processes

Chemical manipulation of naturally occurring peptides offers a convenient route for generating analogs to screen against different therapeutic targets. However, the limited success of the conventional chemical libraries has urged chemical biologists to adopt alternative methods such as phage and mRNA displays and create libraries of a large number of variants for the screening and selection of novel peptides. Messenger RNA (mRNA) display provides great advantages in terms of the library size and the straightforward recovery of the selected polypeptide sequences. Importantly, the integration of the flexible in vitro translation (FIT) system with the mRNA display provides the basis of the random nonstandard peptides integrated discovery (RaPID) approach for the introduction of diverse nonstandard motifs, such as unnatural side chains and backbone modifications. This platform allows the discovery of functionalized peptides with tight binding against virtually any protein of interest (POI) and therefore shows great potential in the pharmaceutical industry. However, this method has been limited to targets generated by recombinant expression, excluding its applications to uniquely modified proteins, particularly those with post-translational modifications.Chemical protein synthesis allows a wide range of changes to the protein's chemical composition to be performed, including side chain and backbone modifications and access to post-translationally modified proteins, which are often inaccessible or difficult to achieve via recombinant expression methods. Notably, d-proteins can be prepared via chemical synthesis, which has been used in mirror image phase display for the discovery of nonproteolytic d-peptide binders.Combining chemical protein synthesis with the RaPID system allows the production of a library of trillions of cyclic peptides and subsequent selection for novel cyclic peptide binders targeting a uniquely modified protein to assist in studying its unexplored biology and possibly the discovery of new drug candidates.Interestingly, the small post-translational modifier protein ubiquitin (Ub), with its various polymeric forms, regulates directly or indirectly many biochemical processes, e.g., proteasomal degradation, DNA damage repair, cell cycle regulation, etc. In this Account, we discuss combining the RaPID approach against various synthetic Ub chains for selecting effective and specific macrocyclic peptide binders. This offers an advancement in modulating central Ub pathways and provides opportunities in drug discovery areas associated with Ub signaling. We highlight experimental approaches and conceptual adaptations required to design and modulate the activity of Lys48- and Lys63-linked Ub chains by macrocyclic peptides. We also present the applications of these approaches to shed light on related biological activities and ultimately their activity against cancer. Finally, we contemplate future developments still pending in this exciting multidisciplinary field.

Total Chemical Synthesis of Glycosylated TREM2 Ectodomain

Mutations in a microglia-associated gene TREM2 increase the risk of Alzheimer's disease. Currently, structural and functional studies of TREM2 mainly rely on recombinant TREM2 proteins expressed from mammalian cells. However, using this method, it is difficult to achieve site-specific labeling. Here, we present the total chemical synthesis of the 116 amino acid TREM2 ectodomain. Rigorous structural analysis ensured correct structural fold after refolding. Treating microglial cells with refolded synthetic TREM2 enhanced microglial phagocytosis, proliferation, and survival. We also prepared TREM2 constructs with defined glycosylation patterns and found that glycosylation at N79 is critical to the thermal stability of TREM2. This method will provide access to TREM2 constructs with site-specific labeling, such as fluorescent labeling, reactive chemical handles, and enrichment handles, to further advance our understanding of TREM2 in Alzheimer's disease.

Synthesis and applications of mirror-image proteins

The homochirality of biomolecules in nature, such as DNA, RNA, peptides and proteins, has played a critical role in establishing and sustaining life on Earth. This chiral bias has also given synthetic chemists the opportunity to generate molecules with inverted chirality, unlocking valuable new properties and applications. Advances in the field of chemical protein synthesis have underpinned the generation of numerous 'mirror-image' proteins (those comprised entirely of D-amino acids instead of canonical L-amino acids), which cannot be accessed using recombinant expression technologies. This Review seeks to highlight recent work on synthetic mirror-image proteins, with a focus on modern synthetic strategies that have been leveraged to access these complex biomolecules as well as their applications in protein crystallography, drug discovery and the creation of mirror-image life.

Semisynthesis of homogeneous misfolded glycoprotein interleukin-8

To uncover how cells distinguish between misfolded and correctly-folded glycoproteins, homogeneous misfolded glycoproteins are needed as a probe for analysis of their structure and chemical characteristic nature. In this study, we have synthesized misfolded glycosyl interleukin-8 (IL-8) by combining E. coli expression and chemical synthesis to improve the synthetic efficiency. In order to prepare N-terminal peptide-thioester segment (1-33), we prepared an E. coli expressed peptide and then activated the C-terminal Cys by using an intramolecular N-to-S acyl shift reaction, followed by trans-thioesterification of the Cys-thioester with an external bis(2-sulfanylethyl)amine (SEA). The glycopeptide segment (34-49) was prepared by solid phase peptide synthesis and the C-terminal peptide (50-72) was prepared in E. coli. These peptide and glycopeptide segments were successfully coupled by sequential native chemical ligation. To obtain homogeneous misfolded glycoproteins by shuffling the disulfide bond pattern, folding conditions were optimized to maximize the yield of individual homogeneous misfolded glycoproteins.

Rapid Single-Shot Synthesis of the 214 Amino Acid-Long N-Terminal Domain of Pyocin S2

The impermeable outer membrane of Pseudomonas aeruginosa is bypassed by antibacterial proteins known as S-type pyocins. Because of their properties, pyocins are investigated as a potential new class of antimicrobials against Pseudomonas infections. Their production and modification, however, remain challenging. To address this limitation, we employed automated fast-flow peptide synthesis for the rapid production of a pyocin S2 import domain. The N-terminal domain sequence (PyS2NTD) was synthesized in under 10 h and purified to yield milligram quantities of the desired product. To our knowledge, the 214 amino acid sequence of PyS2NTD is among the longest peptides produced from a "single-shot" synthesis, i.e., made in a single stepwise route without the use of ligation techniques. Biophysical characterization of the PyS2NTD with circular dichroism was consistent with the literature reports. Fluorescently labeled PyS2NTD binds to P. aeruginosa expressing the cognate ferripyoverdine receptor and is taken up into the periplasm. This selective uptake was validated with confocal and super resolution microscopy, flow cytometry, and fluorescence recovery after photobleaching. These modified, synthetic S-type pyocin domains can be used to probe import mechanisms of P. aeruginosa and leveraged to develop selective antimicrobial agents that bypass the outer membrane.

In Vitro Assessment Methods for Antidiabetic Peptides from Legumes: A Review

Almost 65% of the human protein supply in the world originates from plants, with legumes being one of the highest contributors, comprising between 20 and 40% of the protein supply. Bioactive peptides from various food sources including legumes have been reported to show efficacy in modulating starch digestion and glucose absorption. This paper will provide a comprehensive review on recent in vitro studies that have been performed on leguminous antidiabetic peptides, focusing on the α-amylase inhibitor, α-glucosidase inhibitor, and dipeptidyl peptidase-IV (DPP-IV) inhibitor. Variations in legume cultivars and methods affect the release of peptides. Different methods have been used, such as in sample preparation, including fermentation (t, T), germination (t), and pre-cooking; in protein extraction, alkaline extraction, isoelectric precipitation, phosphate buffer extraction, and water extraction; in protein hydrolysis enzyme types and combination, enzyme substrate ratio, pH, and time; and in enzyme inhibitory assays, positive control type and concentration, inhibitor or peptide concentration, and the unit of inhibitory activity. The categorization of the relative scale of inhibitory activities among legume samples becomes difficult because of these method differences. Peptide sequences in samples were identified by means of HPLC/MS. Software and online tools were used in bioactivity prediction and computational modelling. The identification of the types and locations of chemical interactions between the inhibitor peptides and enzymes and the type of enzyme inhibition were achieved through computational modelling and enzyme kinetic studies.

Atg8-PE protein-based in vitro biochemical approaches to autophagy studies

Macroautophagy/autophagy is an evolutionarily conserved intracellular degradation pathway that maintains cellular homeostasis. Over the past two decades, a series of scientific breakthroughs have helped explain autophagy-related molecular mechanisms and physiological functions. This tremendous progress continues to depend largely on powerful research methods, specifically, various autophagy marker Atg8-PE protein-based methods for studying membrane dynamics and monitoring autophagic activity. Recently, several biochemical approaches have been successfully developed to produce the lipidated protein Atg8-PE or its mimics in vitro, including enzyme-mediated reconstitution systems, chemically defined reconstitution systems, cell-free lipidation systems and protein chemical synthesis. These approaches have contributed important insights into the mechanisms underlying Atg8-mediated membrane dynamics and protein-protein interactions, creating a new perspective in autophagy studies. In this review, we comprehensively summarize Atg8-PE protein-based in vitro biochemical approaches and recent advances to facilitate a better understanding of autophagy mechanisms. In addition, we highlight the advantages and disadvantages of various Atg8-PE protein-based approaches to provide general guidance for their use in studying autophagy.Abbreviations: ATG: autophagy related; ATP: adenosine triphosphate; COPII: coat protein complex II; DGS-NTA: 1,2-dioleoyl-sn-glycero-3-[(N-(5-amino-1-carboxypentyl)iminodiacetic acid)succinyl] (nickel salt); DPPE: 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine; DSPE: 1,2-distearoyl-sn-glycero-3-phosphoethanolamine; E. coli: Escherichia coli; EPL: expressed protein ligation; ERGIC: ER-Golgi intermediate compartment; GABARAP: GABA type A receptor-associated protein; GABARAPL1: GABA type A receptor associated protein like 1; GABARAPL2: GABA type A receptor associated protein like 2; GFP: green fluorescent protein; GUVs: giant unilamellar vesicles; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MBP: maltose binding protein; MEFs: mouse embryonic fibroblasts; MESNa: 2-mercaptoethanesulfonic acid sodium salt; NCL: native chemical ligation; NTA: nitrilotriacetic acid; PE: phosphatidylethanolamine; PS: phosphatidylserine; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; SPPS: solid-phase peptide synthesis; TEV: tobacco etch virus; WT: wild-type.

Therapeutic Potential of Marine Bioactive Peptides against Human Immunodeficiency Virus: Recent Evidence, Challenges, and Future Trends

Acquired immunodeficiency syndrome (AIDS) is a chronic and potentially fatal ailment caused by the human immunodeficiency virus (HIV) and remains a major health problem worldwide. In recent years, the research focus has shifted to a greater emphasis on complementing treatment regimens involving conventional antiretroviral (ARV) drug therapies with novel lead structures isolated from various marine organisms that have the potential to be utilized as therapeutics for the management of HIV-AIDS. The present review summarizes the recent developments regarding bioactive peptides sourced from various marine organisms. This includes a discussion encompassing the potential of these novel marine bioactive peptides with regard to antiretroviral activities against HIV, preparation, purification, and processing techniques, in addition to insight into the future trends with an emphasis on the potential of exploration and evaluation of novel peptides to be developed into effective antiretroviral drugs.

Dissecting the role of protein phosphorylation: a chemical biology toolbox

Protein phosphorylation is a crucial regulator of protein and cellular function, yet, despite identifying an enormous number of phosphorylation sites, the role of most is still unclear. Each phosphoform, the particular combination of phosphorylations, of a protein has distinct and diverse biological consequences. Aberrant phosphorylation is implicated in the development of many diseases. To investigate their function, access to defined protein phosphoforms is essential. Materials obtained from cells often are complex mixtures. Recombinant methods can provide access to defined phosphoforms if site-specifically acting kinases are known, but the methods fail to provide homogenous material when several amino acid side chains compete for phosphorylation. Chemical and chemoenzymatic synthesis has provided an invaluable toolbox to enable access to previously unreachable phosphoforms of proteins. In this review, we selected important tools that enable access to homogeneously phosphorylated protein and discuss examples that demonstrate how they can be applied. Firstly, we discuss the synthesis of phosphopeptides and proteins through chemical and enzymatic means and their advantages and limitations. Secondly, we showcase illustrative examples that applied these tools to answer biological questions pertaining to proteins involved in signal transduction, control of transcription, neurodegenerative diseases and aggregation, apoptosis and autophagy, and transmembrane proteins. We discuss the opportunities and challenges in the field.

Diabetes-Associated Mutations in Proinsulin Provide a "Molecular Rheostat" of Nascent Foldability

PURPOSE OF REVIEW

Diabetes mellitus (DM) due to toxic misfolding of proinsulin variants provides a monogenic model of endoplasmic reticulum (ER) stress. The mutant proinsulin syndrome (also designated MIDY; Mutant INS-gene-induced Diabetes of Youth or Maturity-onset diabetes of the young 10 (MODY10)) ordinarily presents as permanent neonatal-onset DM, but specific amino-acid substitutions may also present later in childhood or adolescence. This review highlights structural mechanisms of proinsulin folding as inferred from phenotype-genotype relationships.

RECENT FINDINGS

MIDY mutations most commonly add or remove a cysteine, leading to a variant polypeptide containing an odd number of thiol groups. Such variants are associated with aberrant intermolecular disulfide pairing, ER stress, and neonatal β-cell dysfunction. Non-cysteine-related (NCR) mutations (occurring in both the B and A domains of proinsulin) define distinct determinants of foldability and vary in severity. The range of ages of onset, therefore, reflects a "molecular rheostat" connecting protein biophysics to quality-control ER checkpoints. Because in most mammalian cell lines even wild-type proinsulin exhibits limited folding efficiency, molecular barriers to folding uncovered by NCR MIDY mutations may pertain to β-cell dysfunction in non-syndromic type 2 DM due to INS-gene overexpression in the face of peripheral insulin resistance. Recent studies of MIDY mutations and related NCR variants, combining molecular and cell-based approaches, suggest that proinsulin has evolved at the edge of non-foldability. Chemical protein synthesis promises to enable comparative studies of "non-foldable" proinsulin variants to define key steps in wild-type biosynthesis. Such studies may create opportunities for novel therapeutic approaches to non-syndromic type 2 DM.

Bioactive Peptides: Synthesis, Sources, Applications, and Proposed Mechanisms of Action

Bioactive peptides are a group of biological molecules that are normally buried in the structure of parent proteins and become active after the cleavage of the proteins. Another group of peptides is actively produced and found in many microorganisms and the body of organisms. Today, many groups of bioactive peptides have been marketed chemically or recombinantly. This article reviews the various production methods and sources of these important/ubiquitous and useful biomolecules. Their applications, such as antimicrobial, antihypertensive, antioxidant activities, blood-lipid-lowering effect, opioid role, antiobesity, ability to bind minerals, antidiabetic, and antiaging effects, will be explored. The types of pathways proposed for bioactive applications will be in the next part of the article, and at the end, the future perspectives of bioactive peptides will be reviewed. Reading this article is recommended for researchers interested in various fields of physiology, microbiology, biochemistry, and nanotechnology and food industry professionals.

Synthesis of the l- and d-SH2 domain of the leukaemia oncogene Bcr-Abl

The d- and l-versions of the Bcr-Abl SH2 domain (12.7 kDa) were synthesized. Key optimizations included pseudoproline incorporation, N-terminal hydrophilic tail addition and mild N-acetoxy succinimide acetylation. Their folding and activity are as for the recombinant protein. Our results will enable engineering of mirror-image monobody antagonists of the central oncoprotein Bcr-Abl.

The d- and l-versions of the Bcr-Abl SH2 domain (12.7 kDa) were synthesized successfully and show similar folding as compared to a recombinantly expressed version.

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.

Traceless Click-Assisted Native Chemical Ligation Enabled by Protecting Dibenzocyclooctyne from Acid-Mediated Rearrangement with Copper(I)

The scope of proteins accessible to total chemical synthesis via native chemical ligation (NCL) is often limited by slow ligation kinetics. Here we describe Click-Assisted NCL (CAN), in which peptides are incorporated with traceless "helping hand" lysine linkers that enable addition of dibenzocyclooctyne (DBCO) and azide handles. The resulting strain-promoted alkyne-azide cycloaddition (SPAAC) increases their effective concentration to greatly accelerate ligations. We demonstrate that copper(I) protects DBCO from acid-mediated rearrangement during acidic peptide cleavage, enabling direct production of DBCO synthetic peptides. Excitingly, triazole-linked model peptides ligated rapidly and accumulated little side product due to the fast reaction time. Using the E. coli ribosomal subunit L32 as a model protein, we further demonstrate that SPAAC, ligation, desulfurization, and linker cleavage steps can be performed in one pot. CAN is a useful method for overcoming challenging ligations involving sterically hindered junctions. Additionally, CAN is anticipated to be an important stepping stone toward a multisegment, one-pot, templated ligation system.

Isotope-Edited Amide II Mode: A New Label for Site-Specific Vibrational Spectroscopy

Vibrational spectroscopy is a powerful tool used to analyze biological and chemical samples. However, in proteins, the most predominant peaks that arise from the backbone amide groups overlap one another, hampering site-specific analyses. Isotope editing has provided a robust, noninvasive approach to overcome this hurdle. In particular, the 1-¹³C═¹⁶O and 1-¹³C═¹⁸O labels that shift the amide I vibrational mode have enabled 1D- and 2D-IR spectroscopy to characterize proteins with excellent site-specific resolution. Herein, we expand the vibrational spectroscopy toolkit appreciably by introducing the 1-¹³C[Formula: see text]¹⁵N probe at specific locations along the protein backbone. A new, isotopically edited amide II peak is observed clearly in the spectra despite the presence of unlabeled modes arising from the rest of the protein. The experimentally determined shift of -30 cm^-1 is reproduced by DFT calculations providing further credence to the mode assignment. Since the amide II mode arises from different elements than the amide I mode, it affords molecular insights that are both distinct and complementary. Moreover, multiple labeling schemes may be used simultaneously, enhancing vibrational spectroscopy's ability to provide detailed molecular insights.

Through the looking glass: milestones on the road towards mirroring life

Naturally occurring DNA, RNA, and proteins predominantly exist in only one enantiomeric form (homochirality). Advances in biotechnology and chemical synthesis allow the production of the respective alternate enantiomeric form, enabling access to mirror-image versions of these natural biopolymers. Exploiting the unique properties of such mirror molecules has already led to many applications, such as biostable and nonimmunogenic therapeutics or sensors. However, a 'roadblock' for unlocking the mirror world is the lack of biological systems capable of synthesizing critical building blocks including mirror oligonucleotides and oligopeptides to reducing cost and improve purity. Here, we provide an overview of the current progress, applications, and challenges of the molecular mirror world by identifying milestones towards mirroring life.

Sensitive pH-responsive point-of-care electrochemical immunoassay for influenza A (H1N1) virus using glucose oxidase-functionalized Ti3C2-MXene nanosheets

Influenza A (H1N1) virus is a serious health threat and potential leading cause of death around the world during the processes of immunity and inflammation. Herein a sensitive pH-responsive point-of-care (POC) electrochemical immunoassay was designed for the quantitative monitoring of H1N1 influenza virus using glucose oxidase (GOx) and secondary antibody-functionalized Ti₃C₂-MXene nanosheets. The assay was carried out on the basis of the sandwich-type immunoreaction in the capture antibody-coated microplate. Two-dimensional (2D) Ti₃C₂-MXene nanosheets with a large surface area could efficiently enhance the loading amount of GOx molecules, thereby resulting in the signal amplification. Accompanying the formed immunocomplexes, labeled GOx molecules catalyzed glucose into gluconic acid and hydrogen peroxide. The generated gluconic acid caused a pH change of the detection solution, which was quantitatively determined on a handheld pH meter. Two labeling strategies with and without Ti₃C₂-MXene nanosheets were investigated to determine the target H1N1 influenza virus, and improved properties were acquired with the Ti₃C₂-MXene-labeled system. Under optimum conditions, the Ti₃C₂-MXene-based immunoassay gave good dynamic responses toward the target H1N1 influenza virus from 0.01 μg mL^-1 to 100 μg mL^-1 with a detection limit of 1.3 ng mL^-1. Good reproducibility, high specificity, and acceptable stability were also achieved in the analysis of the target H1N1 influenza virus. Significantly, measurements of the H1N1 influenza virus from clinical human samples were demonstrated to further confirm the method reliability and accuracy of the Ti₃C₂-MXene-based electrochemical immunoassay. Importantly, such a pH-meter-based immunoassay can be suitable for use in point-of-care applications and opens new opportunities for diagnostics.

Total Synthesis and Structural Characterization of Caveolin-1

Caveolin-1, which is an essential protein for caveola formation, was chemically synthesized. It is composed of 177 amino acid residues, is triply palmitoylated at the C-terminal region, and is inserted into the lipid bilayer to form a V-shaped structure in the middle of the polypeptide chain. The entire sequence was divided into five peptide segments, each of which was synthesized by the solid-phase method. To improve the solubility of the C-terminal region, O-acyl isopeptide structures were incorporated. After ligation by the thioester method and the introduction of the palmitoyl groups, all the protecting groups were removed and the isopeptide structures were converted into the native peptide bond. Finally, the obtained polypeptide was successfully inserted into bicelles, thus showing the success of the synthesis.

Acyl Transfer Catalytic Activity in De Novo Designed Protein with N-Terminus of α-Helix As Oxyanion-Binding Site

The design of catalytic proteins with functional sites capable of specific chemistry is gaining momentum and a number of artificial enzymes have recently been reported, including hydrolases, oxidoreductases, retro-aldolases, and others. Our goal is to develop a peptide ligase for robust catalysis of amide bond formation that possesses no stringent restrictions to the amino acid composition at the ligation junction. We report here the successful completion of the first step in this long-term project by building a completely de novo protein with predefined acyl transfer catalytic activity. We applied a minimalist approach to rationally design an oxyanion hole within a small cavity that contains an adjacent thiol nucleophile. The N-terminus of the α-helix with unpaired hydrogen-bond donors was exploited as a structural motif to stabilize negatively charged tetrahedral intermediates in nucleophilic addition-elimination reactions at the acyl group. Cysteine acting as a principal catalytic residue was introduced at the second residue position of the α-helix N-terminus in a designed three-α-helix protein based on structural informatics prediction. We showed that this minimal set of functional elements is sufficient for the emergence of catalytic activity in a de novo protein. Using peptide-αthioesters as acyl-donors, we demonstrated their catalyzed amidation concomitant with hydrolysis and proved that the environment at the catalytic site critically influences the reaction outcome. These results represent a promising starting point for the development of efficient catalysts for protein labeling, conjugation, and peptide ligation.

Late-Stage Solubilization of Poorly Soluble Peptides Using Hydrazide Chemistry

A novel late-stage solubilization of peptides using hydrazides is described. A solubilizing tag was attached through a selective N-alkylation at a hydrazide moiety with the aid of a 2-picoline-borane complex in 50% acetic acid-hexafluoro-2-propanol. The tag, which tolerates ligation and desulfurization conditions, can be detached by a Cu-mediated selective oxidative hydrolysis of the N-alkyl hydrazide. This new method was validated through the synthesis of HIV-1 protease.

Solid-Phase Protein Modifications: Towards Precision Protein Hybrids for Biological Applications

Proteins have attracted increasing attention as biopharmaceutics and diagnostics due to their high specificity, biocompatibility, and biodegradability. The biopharmaceutical sector in particular is experiencing rapid growth, which has led to an increase in the production and sale of protein drugs and diagnostics over the last two decades. Since the first-generation biopharmaceutics dominated by native proteins, both recombinant and chemical technologies have evolved and transformed the outlook of this rapidly developing field. This review article presents updates on the fabrication of covalent and supramolecular fusion hybrids, as well as protein-polymer hybrids using solid-phase approaches that hold great promise for preparing protein hybrids with precise control at the macromolecular level to incorporate additional features. In addition, the applications of the resultant protein hybrids in medicine and diagnostics are highlighted where possible.

Organoruthenium-catalyzed chemical protein synthesis to elucidate the functions of epigenetic modifications on heterochromatin factors

The application of organometallic compounds for protein science has received attention. Recently, total chemical protein synthesis using transition metal complexes has been developed to produce various proteins bearing site-specific posttranslational modifications (PTMs). However, in general, significant amounts of metal complexes were required to achieve chemical reactions of proteins bearing a large number of nucleophilic functional groups. Moreover, syntheses of medium-size proteins (>20 kDa) were plagued by time-consuming procedures due to cumbersome purification and isolation steps, which prevented access to variously decorated proteins. Here, we report a one-pot multiple peptide ligation strategy assisted by an air-tolerant organoruthenium catalyst that showed more than 50-fold activity over previous palladium complexes, leading to rapid and quantitative deprotection on a protein with a catalytic amount (20 mol%) of the metal complex even in the presence of excess thiol moieties. Utilizing the organoruthenium catalyst, heterochromatin factors above 20 kDa, such as linker histone H1.2 and heterochromatin protein 1α (HP1α), bearing site-specific PTMs including phosphorylation, ubiquitination, citrullination, and acetylation have been synthesized. The biochemical assays using synthetic proteins revealed that the citrullination at R53 in H1.2 resulted in the reduced electrostatic interaction with DNA and the reduced binding affinity to nucleosomes. Furthermore, we identified a key phosphorylation region in HP1α to control its DNA-binding ability. The ruthenium chemistry developed here will facilitate the preparation of a variety of biologically and medically significant proteins containing PTMs and non-natural amino acids.

Chemical protein synthesis assisted by an organoruthenium catalyst streamlined the production of heterochromatin factors bearing various patterns of epigenetic modifications, and their biological significance was elucidated.

Mirror-image antiparallel β-sheets organize water molecules into superstructures of opposite chirality

Biomolecular hydration is fundamental to biological functions. Using phase-resolved chiral sum-frequency generation spectroscopy (SFG), we probe molecular architectures and interactions of water molecules around a self-assembling antiparallel β-sheet protein. We find that the phase of the chiroptical response from the O-H stretching vibrational modes of water flips with the absolute chirality of the (l-) or (d-) antiparallel β-sheet. Therefore, we can conclude that the (d-) antiparallel β-sheet organizes water solvent into a chiral supermolecular structure with opposite handedness relative to that of the (l-) antiparallel β-sheet. We use molecular dynamics to characterize the chiral water superstructure at atomic resolution. The results show that the macroscopic chirality of antiparallel β-sheets breaks the symmetry of assemblies of surrounding water molecules. We also calculate the chiral SFG response of water surrounding (l-) and (d-) LK₇β to confirm the presence of chiral water structures. Our results offer a different perspective as well as introduce experimental and computational methodologies for elucidating hydration of biomacromolecules. The findings imply potentially important but largely unexplored roles of water solvent in chiral selectivity of biomolecular interactions and the molecular origins of homochirality in the biological world.

Chemical Protein Synthesis: Advances, Challenges, and Outlooks

Contemporary chemical protein synthesis has been dramatically advanced over the past few decades, which has enabled chemists to reach the landscape of synthetic biomacromolecules. Chemical synthesis can produce synthetic proteins with precisely controlled structures which are difficult or impossible to obtain via gene expression systems. Herein, we summarize the key enabling ligation technologies, major strategic developments, and some selected representative applications of synthetic proteins and provide an outlook for future development.

In Cellulo Protein Semi-Synthesis from Endogenous and Exogenous Fragments Using the Ultra-Fast Split Gp41-1 Intein

Protein semi-synthesis inside live cells from exogenous and endogenous parts offers unique possibilities for studying proteins in their native context. Split-intein-mediated protein trans-splicing is predestined for such endeavors and has seen some successes, but a much larger variety of established split inteins and associated protocols is urgently needed. We characterized the association and splicing parameters of the Gp41-1 split intein, which favorably revealed a nanomolar affinity between the intein fragments combined with the exceptionally fast splicing rate. Following bead-loading of a chemically modified intein fragment precursor into live mammalian cells, we fluorescently labeled target proteins on their N- and C-termini with short peptide tags, thus ensuring minimal perturbation of their structure and function. In combination with a nuclear-entrapment strategy to minimize cytosolic fluorescence background, we applied our technique for super-resolution imaging and single-particle tracking of the outer mitochondrial protein Tom20 in HeLa cells.

Efficient Chemical Protein Synthesis using Fmoc-Masked N-Terminal Cysteine in Peptide Thioester Segments

We report an operationally simple method to facilitate chemical protein synthesis by fully convergent and one-pot native chemical ligations utilizing the fluorenylmethyloxycarbonyl (Fmoc) moiety as an N-masking group of the N-terminal cysteine of the middle peptide thioester segment(s). The Fmoc group is stable to the harsh oxidative conditions frequently used to generate peptide thioesters from peptide hydrazide or o-aminoanilide. The ready availability of Fmoc-Cys(Trt)-OH, which is routinely used in Fmoc solid-phase peptide synthesis, where the Fmoc group is pre-installed on cysteine residue, minimizes additional steps required for the temporary protection of the N-terminal cysteinyl peptides. The Fmoc group is readily removed after ligation by short exposure (<7 min) to 20 % piperidine at pH 11 in aqueous conditions at room temperature. Subsequent native chemical ligation reactions can be performed in presence of piperidine in the same solution at pH 7.

Fmoc-Compatible and C-terminal-Sequence-Independent Peptide Alkyl Thioester Formation Using Cysteinylprolyl Imide

We report an Fmoc-compatible and external-thiol-free method of peptide C-terminus thioesterification with cysteinylprolyl imide. The newly synthesized structure, i.e., cysteinylprolyl-thiazolidinone, provided high conversion and sequence-independent fast kinetics (90 min) in the diketopiperazine thioester formation under relatively mild conditions: pH 6.0, 37 °C. Employing this thioesterification method, we synthesized histone H3.2 bearing K56 acetylation.

Proteomimetics as protein-inspired scaffolds with defined tertiary folding patterns

Proteins have evolved as a variable platform that provides access to molecules with diverse shapes, sizes and functions. These features have inspired chemists for decades to seek artificial mimetics of proteins with improved or novel properties. Such work has focused primarily on small protein fragments, often isolated secondary structures; however, there has lately been a growing interest in the design of artificial molecules that mimic larger, more complex tertiary folds. In this Perspective, we define these agents as 'proteomimetics' and discuss the recent advances in the field. Proteomimetics can be divided into three categories: protein domains with side-chain functionality that alters the native linear-chain topology; protein domains in which the chemical composition of the polypeptide backbone has been partially altered; and protein-like folded architectures that are composed entirely of non-natural monomer units. We give an overview of these proteomimetic approaches and outline remaining challenges facing the field.

Methods and Applications of Expressed Protein Ligation

Expressed protein ligation is a method of protein semisynthesis and typically involves the reaction of recombinant protein C-terminal thioesters with N-cysteine containing synthetic peptides in a chemoselective ligation. The recombinant protein C-terminal thioesters are produced by exploiting the action of nature's inteins which are protein modules that catalyze protein splicing. This chapter discusses the basic principles of expressed protein ligation and recent advances and applications in this protein semisynthesis field. Comparative strengths and weaknesses of the method and future challenges are highlighted.

Chemical synthesis of Shiga toxin subunit B using a next-generation traceless "helping hand" solubilizing tag

The application of solid-phase peptide synthesis and native chemical ligation in chemical protein synthesis (CPS) has enabled access to synthetic proteins that cannot be produced recombinantly, such as site-specific post-translationally modified or mirror-image proteins (D-proteins). However, CPS is commonly hampered by aggregation and insolubility of peptide segments and assembly intermediates. Installation of a solubilizing tag consisting of basic Lys or Arg amino acids can overcome these issues. Through the introduction of a traceless cleavable linker, the solubilizing tag can be selectively removed to generate native peptide. Here we describe the synthesis of a next-generation amine-reactive linker N-Fmoc-2-(7-amino-1-hydroxyheptylidene)-5,5-dimethylcyclohexane-1,3-dione (Fmoc-Ddap-OH) that can be used to selectively introduce semi-permanent solubilizing tags ("helping hands") onto Lys side chains of difficult peptides. This linker has improved stability compared to its predecessor, a property that can increase yields for multi-step syntheses with longer handling times. We also introduce a new linker cleavage protocol using hydroxylamine that greatly accelerates removal of the linker. The utility of this linker in CPS was demonstrated by the preparation of the synthetically challenging Shiga toxin subunit B (StxB) protein. This robust and easy-to-use linker is a valuable addition to the CPS toolbox for the production of challenging synthetic proteins.

In Situ Cyclization of Proteins (INCYPRO): Cross-Link Derivatization Modulates Protein Stability

Protein macrocyclization represents a very efficient strategy to increase the stability of protein tertiary structures. Here, we describe a panel of novel C3-symmetric tris-electrophilic agents and their use for the cyclization of proteins. These electrophiles are reacted with a protein domain harboring three solvent-exposed cysteine residues, resulting in the in situ cyclization of the protein (INCYPRO). We observe a clear dependency of cross-linking rates on the electrophilicity. All nine obtained cross-linked protein versions show considerably increased thermal stability (up to 29 °C increased melting temperature) when compared to that of the linear precursor. Most interestingly, the degree of stabilization correlates with the hydrophilicity of the cross-link. These results will support the development of novel cross-linked proteins and enable a more rational design process.

Benzimidazolinone-Free Peptide o-Aminoanilides for Chemical Protein Synthesis

The thioester surrogate 3,4-diaminobenzoic acid (Dbz) facilitates the efficient synthesis of peptide thioesters by Fmoc chemistry solid phase peptide synthesis and the optional attachment of a solubility tag at the C-terminus. The protection of the partially deactivated ortho-amine of Dbz is necessary to obtain contamination-free peptide synthesis. The reported carbamate protecting groups promote a serious side reaction, benzimidazolinone formation. Herein we introduce the Boc-protected Dbz that prevents the benzimidazolinone formation, leading to clean peptide o-aminoanilides suitable for the total chemical synthesis of proteins.

The Ancestral N-Terminal Domain of Big Defensins Drives Bacterially Triggered Assembly into Antimicrobial Nanonets

β-Defensins are host defense peptides controlling infections in species ranging from humans to invertebrates. However, the antimicrobial activity of most human β-defensins is impaired at physiological salt concentrations. We explored the properties of big defensins, the β-defensin ancestors, which have been conserved in a number of marine organisms, mainly mollusks. By focusing on a big defensin from oyster (Cg-BigDef1), we showed that the N-terminal domain lost during evolution toward β-defensins confers bactericidal activity to Cg-BigDef1, even at high salt concentrations. Cg-BigDef1 killed multidrug-resistant human clinical isolates of Staphylococcus aureus. Moreover, the ancestral N-terminal domain drove the assembly of the big defensin into nanonets in which bacteria are entrapped and killed. This discovery may explain why the ancestral N-terminal domain has been maintained in diverse marine phyla and creates a new path of discovery to design β-defensin derivatives active at physiological and high salt concentrations.

Big defensins, ancestors of β-defensins, are composed of a β-defensin-like C-terminal domain and a globular hydrophobic ancestral N-terminal domain. This unique structure is found in a limited number of phylogenetically distant species, including mollusks, ancestral chelicerates, and early-branching cephalochordates, mostly living in marine environments. One puzzling evolutionary issue concerns the advantage for these species of having maintained a hydrophobic domain lost during evolution toward β-defensins. Using native ligation chemistry, we produced the oyster Crassostrea gigas BigDef1 (Cg-BigDef1) and its separate domains. Cg-BigDef1 showed salt-stable and broad-range bactericidal activity, including against multidrug-resistant human clinical isolates of Staphylococcus aureus. We found that the ancestral N-terminal domain confers salt-stable antimicrobial activity to the β-defensin-like domain, which is otherwise inactive. Moreover, upon contact with bacteria, the N-terminal domain drives Cg-BigDef1 assembly into nanonets that entrap and kill bacteria. We speculate that the hydrophobic N-terminal domain of big defensins has been retained in marine phyla to confer salt-stable interactions with bacterial membranes in environments where electrostatic interactions are impaired. Those remarkable properties open the way to future drug developments when physiological salt concentrations inhibit the antimicrobial activity of vertebrate β-defensins.