In Situ Cyclization of Native Proteins: Structure-Based Design of a Bicyclic Enzyme

Sortases: structure, mechanism, and implications for protein engineering

Sortase enzymes are critical cysteine transpeptidases on the surface of bacteria that attach proteins to the cell wall and are involved in the construction of bacterial pili. Due to their ability to recognize specific substrates and covalently ligate a range of reaction partners, sortases are widely used in protein engineering applications via sortase-mediated ligation (SML) strategies. In this review, we discuss recent structural studies elucidating key aspects of sortase specificity and the catalytic mechanism. We also highlight select recent applications of SML, including examples where fundamental studies of sortase structure and function have informed the continued development of these enzymes as tools for protein engineering.

Geometric Antibody Engineering Reveals the Spatial Factor on the Efficacy of Bispecific T Cell Engagers

Bispecific antibodies (BsAbs) represent an emerging class of biologics that can recognize two different antigens or epitopes. T-cell engagers (TcEs) bind two targets in trans on the cell surface of the effector and target cell to induce proximal immune effects, opening exciting windows for immunotherapies. To date, the engineering of BsAbs has been mainly focused on tuning the molecular weight and valency. However, the effects of spatial factors on the biological functions of BsAbs have been less explored due to the lack of biochemical methods to precisely manipulate protein geometry. Here, we studied the geometric effects of the TcEs. First, by genetically inserting rigidly designed ankyrin repeat proteins into TcEs, we revealed that the efficacy progressively decreased as the spacer distance of the two binding domains increased. Then, we constructed 26 pairs of TcEs with the same size but varying orientations using click chemistry-mediated conjugation at different mutation sites. We found that linear ligation sites play a minor role in modulating cell-killing efficacy. Next, we rendered the TcEs' advanced topology by cyclization chemistry using the SpyTag/SpyCatcher pair or sortase ligation approaches. Cyclized TcEs were generally more potent than their linear counterparts. Particularly, sortase A cyclized TcEs, bearing a minimal tagging motif, exhibited better cell-killing efficacy in vitro and improved stability both in vitro and in vivo compared to the linear TcE. This work combines modern bioconjugation chemistry and protein engineering tools for antibody engineering, shedding light on the elusive spatial factors of BsAbs functionality.

Peptide Stapling by Crosslinking Two Amines with α-Ketoaldehydes through Diverse Modified Glyoxal-Lysine Dimer Linkers

α-Ketoaldehydes play versatile roles in the ubiquitous natural processes of protein glycation. However, leveraging the reactivity of α-ketoaldehydes for biomedical applications has been challenging. Previously, the reactivity of α-ketoaldehydes with guanidine has been harnessed to design probes for labeling Arg residues on proteins in an aqueous medium. Herein, a highly effective, broadly applicable, and operationally simple protocol for stapling native peptides by crosslinking two amino groups through diverse imidazolium linkers with various α-ketoaldehyde reagents is described. The use of hexafluoroisopropanol as a solvent facilitates rapid and clean reactions under mild conditions and enables unique selectivity for Lys over Arg. The naturally occurring GOLD/MOLD linkers have been expanded to encompass a wide range of modified glyoxal-lysine dimer (OLD) linkers. In a proof-of-concept trial, these modular stapling reactions enabled a convenient two-round strategy to streamline the structure-activity relationship (SAR) study of the wasp venom peptide anoplin, leading to enhanced biological activities.

Exploring biocompatible chemistry to create stapled and photoswitchable variants of the antimicrobial peptide aurein 1.2

Antibiotic resistance is an escalating global health threat. Due to their diverse mechanisms of action and evasion of traditional resistance mechanisms, peptides hold promise as future antibiotics. Their ability to disrupt bacterial membranes presents a potential strategy to combat drug-resistant infections and address the increasing need for effective antimicrobial treatments. Amphipathic α-helical peptides possess a distinctive molecular structure with both charged/hydrophilic and hydrophobic regions that interact with the bacterial cell membrane, disrupting its structural integrity. The α-helical amphipathic peptide aurein 1.2, secreted by the Australian frog Litoria aurea, is one of the shortest known antimicrobial peptides, spanning only 13 amino acids. The primary objective of this study was to investigate stapled and photoswitchable modifications of short helical peptides employing biocompatible chemistry, utilising aurein 1.2 as a model system. We developed various stapled versions of aurein 1.2 using biocompatible conjugation chemistry between dicyanopyridine and 1,2-aminothiols. While the commonly employed stapling pattern for longer staples is i, i + 7, we observed superior helicity in peptides stapled at positions i, i + 8. Molecular dynamics simulations confirmed both stapling patterns to support an α-helical peptide conformation. Additionally, we utilised a cysteine-selective photosensitive staple, perfluoro azobenzene, to explore photoswitchable variants of aurein 1.2. A double-cysteine variant stapled at i, i + 7 indeed exhibited a change in overall helicity induced by light. We further demonstrated the applicability of this staple to attach to cysteine residues in i, i + 7 positions of a helix in a model protein. While some of the stapled variants displayed substantial increase in helicity, minimal inhibitory concentration assays revealed that none of the stapled aurein 1.2 variants exhibited increased antimicrobial activity compared to the wildtype.

Empowering Site-Specific Bioconjugations In Vitro and In Vivo: Advances in Sortase Engineering and Sortase-Mediated Ligation

Sortase-mediated ligation (SML) has emerged as a powerful and versatile methodology for site-specific protein conjugation, functionalization/labeling, immobilization, and design of biohybrid molecules and systems. However, the broader application of SML faces several challenges, such as limited activity and stability, dependence on calcium ions, and reversible reactions caused by nucleophilic side-products. Over the past decade, protein engineering campaigns and particularly directed evolution, have been extensively employed to overcome sortase limitations, thereby expanding the potential application of SML in multiple directions, including therapeutics, biorthogonal chemistry, biomaterials, and biosensors. This review provides an overview of achieved advancements in sortase engineering and highlights recent progress in utilizing SML in combination with other state-of-the-art chemical and biological methodologies. The aim is to encourage scientists to employ sortases in their conjugation experiments.

Teraryl Braces in Macrocycles: Synthesis and Conformational Landscape Remodeling of Peptides

The three-dimensional structure of medium-sized cyclic peptides accounts for their biological activity and other important physiochemical properties. Despite significant advances in the past few decades, chemists' ability to fine-tune the structure, in particular, the backbone conformation, of short peptides made of canonical amino acids is still quite limited. Nature has shown that cross-linking the aromatic side chains of linear peptide precursors via enzyme catalysis can generate cyclophane-braced products with unusual structures and diverse activities. However, the biosynthetic path to these natural products is challenging to replicate in the synthetic laboratory using practical chemical modifications of peptides. Herein, we report a broadly applicable strategy to remodel the structure of homodetic peptides by cross-linking the aromatic side chains of Trp, His, and Tyr residues with various aryl linkers. The aryl linkers can be easily installed via copper-catalyzed double heteroatom-arylation reactions of peptides with aryl diiodides. These aromatic side chains and aryl linkers can be combined to form a large variety of assemblies of heteroatom-linked multi-aryl units. The assemblies can serve as tension-bearable multijoint braces to modulate the backbone conformation of peptides as an entry to previously inaccessible conformational space.

oxSTEF Reagents Are Tunable and Versatile Electrophiles for Selective Disulfide-Rebridging of Native Proteins

Site-selective disulfide rebridging has emerged as a powerful strategy to modulate the structural and functional properties of proteins. Here, we introduce a novel class of electrophilic reagents, designated oxSTEF, that demonstrate excellent efficiency in disulfide rebridging via double thiol exchange. The oxSTEF reagents are prepared using an efficient synthetic sequence which may be diverted to obtain a range of derivatives allowing for tuning of reactivity or steric bulk. We demonstrate highly selective rebridging of cyclic peptides and native proteins, such as human growth hormone, and the absence of cross-reactivity with other nucleophilic amino acid residues. The oxSTEF conjugates undergo glutathione-mediated disintegration under tumor-relevant glutathione concentrations, which highlights their potential for use in targeted drug delivery. Finally, the α-dicarbonyl motif of the oxSTEF reagents enables "second phase" oxime ligation, which furthermore increases the thiol stability of the conjugates significantly.

Construction of Complex Macromulticyclic Peptides via Stitching with Formaldehyde and Guanidine

There is a growing interest in constructing multicyclic peptide structures to expand the chemical space of peptides. Conventional strategies for constructing large peptide structures are limited by the typical reliance on the inflexible coupling between premade templates equipped with fixed reactive handles and peptide substrates via cysteine anchors. Herein, we report the development of a facile three-component condensation reaction of primary alkyl amine, formaldehyde, and guanidine for construction of complex macromulticyclic peptides with novel topologies via lysine anchors. Moreover, the reaction sequences can be orchestrated in different anchor combinations and spatial arrangements to generate various macrocyclic structures crosslinked by distinct fused tetrahydrotriazine linkages. The macrocyclization reactions are selective, efficient, versatile, and workable in both organic and aqueous media. Thus, the condensation reaction provides a smart tool for stitching native peptides in situ using simple methylene threads and guanidine joints in a flexible and programmable manner.

Tuning Enzyme Thermostability via Computationally Guided Covalent Stapling and Structural Basis of Enhanced Stabilization

Enhancing the thermostability of enzymes without impacting their catalytic function represents an important yet challenging goal in protein engineering and biocatalysis. We recently introduced a novel method for enzyme thermostabilization that relies on the computationally guided installation of genetically encoded thioether "staples" into a protein via cysteine alkylation with the noncanonical amino acid O-2-bromoethyl tyrosine (O2beY). Here, we demonstrate the functionality of an expanded set of electrophilic amino acids featuring chloroacetamido, acrylamido, and vinylsulfonamido side-chain groups for protein stapling using this strategy. Using a myoglobin-based cyclopropanase as a model enzyme, our studies show that covalent stapling with p-chloroacetamido-phenylalanine (pCaaF) provides higher stapling efficiency and enhanced stability (thermodynamic and kinetic) compared to the other stapled variants and the parent protein. Interestingly, molecular simulations of conformational flexibility of the cross-links show that the pCaaF staple allows fewer energetically feasible conformers than the other staples, and this property may be a broader indicator of stability enhancement. Using this strategy, pCaaF-stapled variants with significantly enhanced stability against thermal denaturation (ΔTm' = +27 °C) and temperature-induced heme loss (ΔT50 = +30 °C) were obtained while maintaining high levels of catalytic activity and stereoselectivity. Crystallographic analyses of singly and doubly stapled variants provide key insights into the structural basis for stabilization, which includes both direct interactions of the staples with protein residues and indirect interactions through adjacent residues involved in heme binding. This work expands the toolbox of protein stapling strategies available for protein stabilization.

Challenges in the use of sortase and other peptide ligases for site-specific protein modification

Site-specific protein modification is a widely-used biochemical tool. However, there are many challenges associated with the development of protein modification techniques, in particular, achieving site-specificity, reaction efficiency and versatility. The engineering of peptide ligases and their substrates has been used to address these challenges. This review will focus on sortase, peptidyl asparaginyl ligases (PALs) and variants of subtilisin; detailing how their inherent specificity has been utilised for site-specific protein modification. The review will explore how the engineering of these enzymes and substrates has led to increased reaction efficiency mainly due to enhanced catalytic activity and reduction of reversibility. It will also describe how engineering peptide ligases to broaden their substrate scope is opening up new opportunities to expand the biochemical toolkit, particularly through the development of techniques to conjugate multiple substrates site-specifically onto a protein using orthogonal peptide ligases.

We highlight chemical and biochemical strategies taken to optimise peptide and protein modification using peptide ligases.

Unprotected peptide macrocyclization and stapling via a fluorine-thiol displacement reaction

We report the discovery of a facile peptide macrocyclization and stapling strategy based on a fluorine thiol displacement reaction (FTDR), which renders a class of peptide analogues with enhanced stability, affinity, cellular uptake, and inhibition of cancer cells. This approach enabled selective modification of the orthogonal fluoroacetamide side chains in unprotected peptides in the presence of intrinsic cysteines. The identified benzenedimethanethiol linker greatly promoted the alpha helicity of a variety of peptide substrates, as corroborated by molecular dynamics simulations. The cellular uptake of benzenedimethanethiol stapled peptides appeared to be universally enhanced compared to the classic ring-closing metathesis (RCM) stapled peptides. Pilot mechanism studies suggested that the uptake of FTDR-stapled peptides may involve multiple endocytosis pathways in a distinct pattern in comparison to peptides stapled by RCM. Consistent with the improved cell permeability, the FTDR-stapled lead Axin and p53 peptide analogues demonstrated enhanced inhibition of cancer cells over the RCM-stapled analogues and the unstapled peptides.

Strategies capable of stapling unprotected peptides in a straightforward, chemoselective, and clean manner, as well as promoting cellular uptake are of great interest. Here the authors report a peptide macrocyclization and stapling strategy which satisfies those criteria, based on a fluorine thiol displacement reaction.

Extendable stapling of unprotected peptides by crosslinking two amines with o-phthalaldehyde

Peptide modification methods that do not rely on the cysteine residue are underdeveloped, and their development could greatly expand the current toolbox for peptide chemistry. During the course of preliminary investigations into the classical ortho-phthalaldehyde (OPA)-amine-thiol condensation reaction, we found that in the absence of thiol, OPA readily condenses with two primary alkyl amines to form a class of underexplored isoindolin-1-imine compounds under mild aqueous conditions. From the intramolecular version of this OPA-2amines reaction, an efficient and selective methodology using mild reaction conditions has been developed for stapling unprotected peptides via crosslinking of two amino groups in both an end-to-side and side-to-side fashion. The stapling method is superfast and broadly applicable for various peptide substrates with the reacting amino groups separated by a wide range of different amino acid units. The macrocyclization reactions of selected substrates are completed within 10 seconds at 5 mM concentration and within 2 minutes at 50 μM concentration. Importantly, the resulting cyclized peptides with an isoindolinimine linkage can be extended in a one-pot sequential addition manner with several different electron-deficient π electrophiles, thereby generating more complex structures.

Methods for peptide stapling, or covalently linking amino acid residues to create a non-linear structure, mostly rely on cysteine residues, which imposes a significant practical limitation. Here the authors disclose a method to chemoselectively macrocyclize two free-amine-containing residues in mild, peptide-relevant conditions, using a commercially available reagent.

Structural guidelines for stabilization of α-helical coiled coils via PEG stapling

Macrocyclization or stapling is one of the most well-known and generally applicable strategies for enhancing peptide/protein conformational stability and target binding affinity. However, there are limited structure- or sequence-based guidelines for the incorporation of optimal interhelical staples within coiled coils: the location and length of an interhelical staple is either arbitrarily chosen or requires significant optimization. Here we explore the impact of interhelical PEG stapling on the conformational stability and proteolytic resistance of a model disulfide-bound heterodimeric coiled coil. We demonstrate that (1) interhelical PEG staples are more stabilizing when placed farther from an existing disulfide crosslink; (2) e/g′ staples are more stabilizing than f/b′ or b/c′ staples; (3) PEG staples between different positions have different optimal staple lengths; (4) PEG stapling tolerates variation in the structure of the PEG linker and in the mode of conjugation; and (5) the guidelines developed here enable the rational design of a stabilized PEG-stapled HER-2 affibody with enhanced conformational stability and proteolytic resistance.

Here we identify key criteria for designing PEG-stapled coiled coils with increased conformational and proteolytic stability.

Diels-Alder Cycloadditions for Peptide Macrocycle Formation

Macrocyclization can confer enhanced stability, target affinity, and membrane permeability to peptide scaffolds, all of which are desirable properties for chemical probes and therapeutics. A wide array of macrocyclization chemistries have been reported over the last few decades; however, these often have limited compatibility with each other and across chemical environments, thus restricting access to specific molecular properties. In an effort to address some of these limitations, we recently described the use of Diels-Alder [4 + 2] cycloadditions for peptide macrocyclization. Among the attributes of this chemistry, we demonstrated that Diels-Alder cyclization can template diverse peptide secondary structures, proceed in organic or aqueous environments, and endow improved pharmacologic properties on cyclized peptides. Here, we present synthetic processes and characterization methods for the synthesis of Diels-Alder cyclized peptides.

Protein Macrocyclization for Tertiary Structure Stabilization

Proteins possess unique molecular recognition capabilities and enzymatic activities, features that are usually tied to a particular tertiary structure. To make use of proteins for biotechnological and biomedical purposes, it is often required to enforce their tertiary structure in order to ensure sufficient stability under the conditions inherent to the application of interest. The introduction of intramolecular crosslinks has proven efficient in stabilizing native protein folds. Herein, we give an overview of methods that allow the macrocyclization of expressed proteins, discussing involved reaction mechanisms and structural implications.

Synthesis of and Metal Complexation with a Chiral Cyclam

Tetraazamacrocycles, like cyclam 1, are well-studied polyamine ligands for metal ions that were first developed to model biological processes. Despite being studied for nearly 60 years, the development of chiral variants of 1 has been limited. We report the synthesis of a chiral variant of 1, the tetraazamacrocycle 2. Outlined herein are the synthesis of 2, a preliminary study of its complexation with metal ions, and the properties of the resulting metal complexes.

Genetic Fusion of Transacting Activator of Transcription Peptide to Cyclized Green Fluorescence Protein Improves Stability, Intracellular Delivery, and Tumor Retention

Therapeutic proteins such as enzymes, hormones, and cytokines suffer from poor stability, inefficient cellular penetration, and rapid clearance from circulation. Conjugation with polymers (such as poly(ethylene glycol)) and fusion with long-acting proteins (such as albumin and Fc fragments) have been utilized to partially address the delivery issues, but these strategies require the introduction of new macromolecular substances, resulting in potential immunogenicity and toxicity. Herein, we report an easy strategy to increase the intracellular delivery efficiency and stability of proteins by combining of sortase-mediated protein cyclization and cell-penetrating peptide (CPP)-mediated intracellular delivery. We, for the first time, genetically constructed a green fluorescence protein (GFP) fused with a CPP, a transacting activator of transcription (TAT) peptide, at its C-terminus for intracellular internalization, and two sortase recognition sequences, pentaglycine and LPETG, at its N- and C-termini for cyclization. Notably, the cyclized GFP-TAT (cGFP-TAT) not only highly retained the photophysical properties of the protein but also significantly improved the in vitro stability compared with the native linear GFP (lGFP) and linear TAT peptide-fused GFP (lGFP-TAT).Moreover, cGFP-TAT showed better cellular internalization ability compared with lGFP. In C26 tumor-inoculated mice, cGFP-TAT exhibited enhanced in vivo tumor retention, with increases of 7.79- and 6.52-fold relative to lGFP and lGFP-TAT in tumor retention 3 h after intratumor administration. This proof-of-concept study has provided an easy strategy to increase the in vitro stability, intracellular delivery efficiency, and in vivo tumor retention of GFP, which would be applicable to numerous therapeutic proteins and peptides for clinical practice.

Enhancing Robustness of Sortase A by Loop Engineering and Backbone Cyclization

Staphylococcus aureus sortase A (SaSrtA) is widely used for site-specific protein modifications, but it lacks the robustness for performing bioconjugation reactions at elevated temperatures or in presence of denaturing agents. Loop engineering and subsequent head-to-tail backbone cyclization of SaSrtA yielded the cyclized variant CyM6 that has a 7.5 °C increased melting temperature and up to 4.6-fold increased resistance towards denaturants when compared to the parent rM4. CyM6 gained up to 2.6-fold (vs. parent rM4) yield of conjugate in ligation of peptide and primary amine under denaturing conditions.

Automated Design of Macrocycles for Therapeutic Applications: From Small Molecules to Peptides and Proteins

Macrocycles and cyclic peptides are increasingly attractive therapeutic modalities as they often have improved affinity, are able to bind to extended protein surfaces, and otherwise have favorable properties. Macrocyclization of a known binder may stabilize its bioactive conformation and improve its metabolic stability, cell permeability, and in certain cases oral bioavailability. Herein, we present implementation and application of an approach that automatically generates, evaluates, and proposes cyclizations utilizing a library of well-established chemical reactions and reagents. Using the three-dimensional (3D) conformation of the linear molecule in complex with a target protein as the starting point, this approach identifies attachment points, generates linkers, evaluates their geometric compatibility, and ranks the resulting molecules with respect to their predicted conformational stability and interactions with the target protein. As we show here with prospective and retrospective case studies, this procedure can be applied for the macrocyclization of small molecules and peptides and even PROteolysis TArgeting Chimeras (PROTACs) and proteins.

Sortase mutants with improved protein thermostability and enzymatic activity obtained by consensus design

Staphylococcus aureus sortase A (SaSrtA) is an enzyme that anchors proteins to the cell surface of Gram-positive bacteria. During the transpeptidation reaction performed by SaSrtA, proteins containing an N-terminal glycine can be covalently linked to another protein with a C-terminal LPXTG motif (X being any amino acid). Since the sortase reaction can be performed in vitro as well, it has found many applications in biotechnology. Although sortase-mediated ligation has many advantages, SaSrtA is limited by its low enzymatic activity and dependence on Ca2+. In our study, we evaluated the thermodynamic stability of the SaSrtA wild type and found the enzyme to be stable. We applied consensus analysis to further improve the enzyme's stability while at the same time enhancing the enzyme's activity. As a result, we found thermodynamically improved, more active and Ca2+-independent mutants. We envision that these new variants can be applied in conjugation reactions in low Ca2+ environments.

Long-range PEG Stapling: Macrocyclization for Increased Protein Conformational Stability and Resistance to Proteolysis

We previously showed that long-range stapling of two Asn-linked O-allyl PEG oligomers via olefin metathesis substantially increases the conformational stability of the WW domain through an entropic effect. The impact of stapling was more favorable when the staple connected positions that were far apart in primary sequence but close in the folded tertiary structure. Here we validate these criteria by identifying new stabilizing PEG-stapling sites within the WW domain and the SH3 domain, both β-sheet proteins. We find that stapling via olefin metathesis vs. the copper(i)-catalyzed azide/alkyne cycloaddition (CuAAC) results in similar energetic benefits, suggesting that olefin and triazole staples can be used interchangeably. Proteolysis assays of selected WW variants reveal that the observed staple-based increases in conformational stability lead to enhanced proteolytic resistance. Finally, we find that an intermolecular staple dramatically increases the quaternary structural stability of an α-helical GCN4 coiled-coil heterodimer.

Long-range stapling of two Asn-linked PEG oligomers via olefin metathesis substantially increases the conformational stability of the WW and SH3 domain tertiary structures and the GCN4 coiled-coil quaternary structure.

Engineered peptide ligases for cell signaling and bioconjugation

Enzymes that catalyze peptide ligation are powerful tools for site-specific protein bioconjugation and the study of cellular signaling. Peptide ligases can be divided into two classes: proteases that have been engineered to favor peptide ligation, and protease-related enzymes with naturally evolved peptide ligation activity. Here, we provide a review of key natural peptide ligases and proteases engineered to favor peptide ligation activity. We cover the protein engineering approaches used to generate and improve these tools, along with recent biological applications, advantages, and limitations associated with each enzyme. Finally, we address future challenges and opportunities for further development of peptide ligases as tools for biological research.

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.

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.

Natural Occurring and Engineered Enzymes for Peptide Ligation and Cyclization

The renaissance of peptides as prospective therapeutics has fostered the development of novel strategies for their synthesis and modification. In this context, besides the development of new chemical peptide ligation approaches, especially the use of enzymes as a versatile tool has gained increased attention. Nowadays, due to their inherent properties such as excellent regio- and chemoselectivity, enzymes represent invaluable instruments in both academic and industrial laboratories. This mini-review focuses on natural- and engineered peptide ligases that can form a new peptide (amide) bond between the C-terminal carboxy and N-terminal amino group of a peptide and/or protein. The pro's and cons of several enzyme classes such as Sortases, Asparaginyl Endoproteases, Trypsin related enzymes and as a central focus subtilisin-derived variants are summarized. Most recent developments with regards to ligation and cyclization are highlighted.

Vinylphosphonites for Staudinger-induced chemoselective peptide cyclization and functionalization

In this paper, we introduce vinylphosphonites for chemoselective Staudinger-phosphonite reactions (SPhR) with azides to form vinylphosphonamidates for the subsequent modification of cysteine residues in peptides and proteins. An electron-rich alkene is turned into an electron-deficient vinylphosphonamidate, thereby inducing electrophilic reactivity for a following thiol addition. We show that by varying the phosphonamidate ester substituent we can fine-tune the reactivity of the thiol addition and even control the functional properties of the final conjugate. Furthermore, we observed a drastic increase in thiol addition efficiency when the SPhR is carried out in the presence of a thiol substrate in a one-pot reaction. Hence, we utilize vinylphosphonites for the chemoselective intramolecular cyclization of peptides carrying an azide-containing amino acid and a cysteine in high yields. Our concept was demonstrated for the stapling of a cell-permeable peptidic inhibitor for protein-protein interaction (PPI) between BCL9 and beta-catenin, which is known to create a transcription factor complex playing a role in embryonic development and cancer origin, and for macrocyclization of cell-penetrating peptides (CPPs) to enhance the cellular uptake of proteins.

A "Quenchergenic" Chemoselective Protein Labeling Strategy

Dynamic changes in protein structure can be monitored by using a fluorescent probe and a dark quencher. This approach is contingent upon the ability to precisely introduce a fluorophore/quencher pair into two specific sites of a protein of interest. Despite recent advances, there is continued demand for new and convenient approaches to site-selectively label proteins with such optical probes. We have recently developed a chemoselectively rapid azo-coupling reaction (CRACR) for site-specific protein labeling; it relies on rapid coupling between a genetically encoded 5-hydroxytryptophan residue and various aromatic diazonium ions. Herein, it is reported that the product of this conjugation reaction, a highly chromophoric biarylazo group, is a potent fluorescence quencher. The absorption properties of this azo product can be tuned by systematically altering the structure of the aryldiazonium species. A particular "quenchergenic" aryldiazonium has been identified that, upon conjugation, efficiently quenches the fluorescence of green fluorescent protein, which is a widely used genetically encoded fluorescent probe that can be terminally attached to target proteins. This fluorophore/quencher pair was used to evaluate the protein-labeling kinetics of CRACR, as well as to monitor the proteolysis of a fusion protein.

Broadening the scope of sortagging

Sortases are enzymes occurring in the cell wall of Gram-positive bacteria. Sortase A (SrtA), the best studied sortase class, plays a key role in anchoring surface proteins with the recognition sequence LPXTG covalently to oligoglycine units of the bacterial cell wall. This unique transpeptidase activity renders SrtA attractive for various purposes and motivated researchers to study multiple in vivo and in vitro ligations in the last decades. This ligation technique is known as sortase-mediated ligation (SML) or sortagging and developed to a frequently used method in basic research. The advantages are manifold: extremely high substrate specificity, simple access to substrates and enzyme, robust nature and easy handling of sortase A. In addition to the ligation of two proteins or peptides, early studies already included at least one artificial (peptide equipped) substrate into sortagging reactions - which demonstrates the versatility and broad applicability of SML. Thus, SML is not only a biology-related technique, but has found prominence as a major interdisciplinary research tool. In this review, we provide an overview about the use of sortase A in interdisciplinary research, mainly for protein modification, synthesis of protein-polymer conjugates and immobilization of proteins on surfaces.

Coiled-Coil Peptide Beacon: A Tunable Conformational Switch for Protein Detection

The understanding of protein folding and assembly is of central importance for the design of proteins and enzymes with novel or improved functions. Minimalistic model systems, such as coiled-coils, provide an excellent platform to improve this understanding and to construct novel molecular devices. Along those lines, we designed a conformational switch that is composed of two coiled-coil forming peptides and a central binding epitope. In the absence of a binding partner, this switch adopts a hairpin-like conformation that opens upon receptor binding. Variation of the coiled-coil length modulates the strength of the intramolecular constraint. The two conformational states of this switch have been linked with characteristic fluorescent properties, which enables the detection of the receptor in real-time.

In Situ Cyclization of Native Proteins: Structure-Based Design of a Bicyclic Enzyme

Increased tolerance of enzymes towards thermal and chemical stress is required for many applications and can be achieved by macrocyclization of the enzyme resulting in the stabilizing of its tertiary structure. Thus far, macrocyclization approaches utilize a very limited structural diversity, which complicates the design process. Herein, we report an approach that enables cyclization through the installation of modular crosslinks into native proteins composed entirely of proteinogenic amino acids. Our stabilization procedure involves the introduction of three surface-exposed cysteine residues, which are reacted with a triselectrophile, resulting in the in situ cyclization of the protein (INCYPRO). A bicyclic version of sortase A was designed that exhibits increased tolerance towards thermal as well as chemical denaturation, and proved to be efficient in protein labeling under denaturing conditions. In addition, we applied INCYPRO to the KIX domain, resulting in up to 24 °C increased thermal stability.