Immobilized protease-assisted synthesis of engineered cysteine-knot microproteins

Insights into the synthesis strategies of plant-derived cyclotides

Cyclotides are plant peptides characterized with a head-to-tail cyclized backbone and three interlocking disulfide bonds, known as a cyclic cysteine knot. Despite the variations in cyclotides peptide sequences, this core structure is conserved, underlying their most useful feature: stability against thermal and chemical breakdown. Cyclotides are the only natural peptides known to date that are orally bioavailable and able to cross cell membranes. Cyclotides also display bioactivities that have been exploited and expanded to develop as potential therapeutic reagents for a wide range of conditions (e.g., HIV, inflammatory conditions, multiple sclerosis, etc.). As such, in vitro production of cyclotides is of the utmost importance since it could assist further research on this peptide class, specifically the structure-activity relationship and its mechanism of action. The information obtained could be utilized to assist drug development and optimization. Here, we discuss several strategies for the synthesis of cyclotides using both chemical and biological routes.

Engineering the Cyclization Loop of MCoTI-II Generates Targeted Cyclotides that Potently Inhibit Factor XIIa

Factor XIIa (FXIIa) is a promising target for developing new drugs that prevent thrombosis without causing bleeding complications. A native cyclotide (MCoTI-II) is gaining interest for engineering FXIIa-targeted anticoagulants as this peptide inhibits FXIIa but not other coagulation proteases. Here, we engineered the native biosynthetic cyclization loop of MCoTI-II (loop 6) to generate improved FXIIa inhibitors. Decreasing the loop length led to gains in potency up to 7.7-fold, with the most potent variant having five residues in loop 6 (K_i = 25 nM). We subsequently examined sequence changes within loop 6 and an adjacent loop, with substitutions at P4 and P2' producing a potent FXIIa inhibitor (K_i = 2 nM) that displayed more than 700-fold selectivity, was stable in human serum, and blocked the intrinsic coagulation pathway in human plasma. These findings demonstrate that engineering the biosynthetic cyclization loop can generate improved cyclotide variants, expanding their potential for drug discovery.

Circular Permutation of the Native Enzyme-Mediated Cyclization Position in Cyclotides

Cyclotides are a class of cyclic disulfide-rich peptides found in plants that have been adopted as a molecular scaffold for pharmaceutical applications due to their inherent stability and ability to penetrate cell membranes. For research purposes, they are usually produced and cyclized synthetically, but there are concerns around the cost and environmental impact of large-scale chemical synthesis. One strategy to improve this is to combine a recombinant production system with native enzyme-mediated cyclization. Asparaginyl endopeptidases (AEPs) are enzymes that can act as peptide ligases in certain plants to facilitate cyclotide maturation. One of these ligases, OaAEP1b, originates from the cyclotide-producing plant, Oldenlandia affinis, and can be produced recombinantly for use in vitro as an alternative to chemical cyclization of recombinant substrates. However, not all engineered cyclotides are compatible with AEP-mediated cyclization because new pharmaceutical epitopes often replace the most flexible region of the peptide, where the native cyclization site is located. Here we redesign a popular cyclotide grafting scaffold, MCoTI-II, to incorporate an AEP cyclization site located away from the usual grafting region. We demonstrate the incorporation of a bioactive peptide sequence in the most flexible region of MCoTI-II while maintaining AEP compatibility, where the two were previously mutually exclusive. We anticipate that our AEP-compatible scaffold, based on the most popular cyclotide for pharmaceutical applications, will be useful in designing bioactive cyclotides that are compatible with AEP-mediated cyclization and will therefore open up the possibility of larger scale enzyme-mediated production of recombinant or synthetic cyclotides alike.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

Ribosomally-synthesised cyclic peptides from plants as drug leads and pharmaceutical scaffolds

Owing to their exceptional stability and favourable pharmacokinetic properties, plant-derived cyclic peptides have recently attracted significant attention in the field of peptide-based drug design. This article describes the three major classes of ribosomally-synthesised plant peptides - the cyclotides, the PawS-derived peptides and the orbitides - and reviews their applications as leads or scaffolds in drug design. These ribosomally-produced peptides have a range of biological activities, including anti-HIV, cytotoxic and immunomodulatory activity. In addition, recent interest has focused on their use as scaffolds to stabilise bioactive peptide sequences, thereby enhancing their biopharmaceutical properties. There are now more than 30 published papers on such 'grafting' applications, most of which have been reported only in the last few years, and several such studies have reported in vivo activity of orally delivered cyclic peptides. In this article, we describe approaches to the synthesis of cyclic peptides and their pharmaceutically-grafted derivatives as well as outlining their biosynthetic routes. Finally, we describe possible bioproduction routes for pharmaceutically active cyclic peptides, involving plants and plant suspension cultures.

Synthesis and Protein Engineering Applications of Cyclotides

Cyclotides are a group of plant-derived peptides with a head-to-tail cyclized backbone that is stabilized by three knotted disulfide bonds. Their exceptional stability and tolerance for residue substitutions have led to interest in their application as drug design scaffolds. To date, chemical synthesis has been the dominant methodology for producing cyclotides and their analogues. Native chemical ligation is the most common strategy to generate the cyclic backbone and has been highly successful at producing a wide range of cyclotides for studies of structure–activity relationships. Both this and other chemical approaches require a specific linker at the C-terminus and typically involve a non-directed folding (disulfide oxidation) regimen, which can sometimes be a limiting factor in final yields. Following the recent discovery of enzymes involved in peptide cyclization in planta, site-specific and highly efficient enzymatic ligations have been used for synthetic cyclotide backbone cyclization. In this review, chemical synthesis strategies and approaches involving cyclization via enzymes for the production of cyclotides are described.

Synthetic Cystine-Knot Miniproteins - Valuable Scaffolds for Polypeptide Engineering

Peptides with the cystine-knot architecture, often termed knottins, are promising scaffolds for biomolecular engineering. These unique molecules combine diverse bioactivities with excellent structural, thermal, and proteolytical stability. Being different in the composition and structure of their amino acid backbone, knottins share the same core element, namely cystine knot, which is built by six cysteine residues forming three disulfides upon oxidative folding. This motif ensures a notably rigid framework that highly tolerates both rational and combinatorial changes in the primary structure. Being accessible through recombinant production and total chemical synthesis, cystine-knot miniproteins can be endowed with novel bioactivities by variation of surface-exposed loops and incorporation of non-natural elements within their non-conserved regions towards the generation of tailor-made peptidic compounds. In this chapter the topology of cystine-knot peptides, their synthesis and applications for diagnostics and therapy is discussed.

Antimicrobial Peptides from Plants

Plant antimicrobial peptides (AMPs) have evolved differently from AMPs from other life forms. They are generally rich in cysteine residues which form multiple disulfides. In turn, the disulfides cross-braced plant AMPs as cystine-rich peptides to confer them with extraordinary high chemical, thermal and proteolytic stability. The cystine-rich or commonly known as cysteine-rich peptides (CRPs) of plant AMPs are classified into families based on their sequence similarity, cysteine motifs that determine their distinctive disulfide bond patterns and tertiary structure fold. Cystine-rich plant AMP families include thionins, defensins, hevein-like peptides, knottin-type peptides (linear and cyclic), lipid transfer proteins, α-hairpinin and snakins family. In addition, there are AMPs which are rich in other amino acids. The ability of plant AMPs to organize into specific families with conserved structural folds that enable sequence variation of non-Cys residues encased in the same scaffold within a particular family to play multiple functions. Furthermore, the ability of plant AMPs to tolerate hypervariable sequences using a conserved scaffold provides diversity to recognize different targets by varying the sequence of the non-cysteine residues. These properties bode well for developing plant AMPs as potential therapeutics and for protection of crops through transgenic methods. This review provides an overview of the major families of plant AMPs, including their structures, functions, and putative mechanisms.

Recent advances in chemoenzymatic peptide syntheses

Chemoenzymatic peptide synthesis is the hydrolase-catalyzed stereoselective formation of peptide bonds. It is a clean and mild procedure, unlike conventional chemical synthesis, which involves complicated and laborious protection-deprotection procedures and harsh reaction conditions. The chemoenzymatic approach has been utilized for several decades because determining the optimal conditions for conventional synthesis is often time-consuming. The synthesis of poly- and oligopeptides comprising various amino acids longer than a dipeptide continues to pose a challenge owing to the lack of knowledge about enzymatic mechanisms and owing to difficulty in optimizing the pH, temperature, and other reaction conditions. These drawbacks limit the applications of the chemoenzymatic approach. Recently, a variety of enzymes and substrates produced using recombinant techniques, substrate mimetics, and optimal reaction conditions (e.g., frozen aqueous media and ionic liquids) have broadened the scope of chemoenzymatic peptide syntheses. In this review, we highlight the recent advances in the chemoenzymatic syntheses of various peptides and their use in developing new materials and biomedical applications.

Potent and specific inhibition of the biological activity of the type-II transmembrane serine protease matriptase by the cyclic microprotein MCoTI-II

Matriptase is a type-II transmembrane serine protease involved in epithelial homeostasis in both health and disease, and is implicated in the development and progression of a variety of cancers. Matriptase mediates its biological effects both via as yet undefined substrates and pathways, and also by proteolytic cleavage of a variety of well-defined protein substrates, several of which it shares with the closely-related protease hepsin. Development of targeted therapeutic strategies will require discrimination between these proteases. Here we have investigated cyclic microproteins of the squash Momordica cochinchinensis trypsin-inhibitor family (generated by total chemical synthesis) and found MCoTI-II to be a high-affinity (Ki 9 nM) and highly selective (> 1,000-fold) inhibitor of matriptase. MCoTI-II efficiently inhibited the proteolytic activation of pro-hepatocyte growth factor (HGF) by matriptase but not by hepsin, in both purified and cell-based systems, and inhibited HGF-dependent cell scattering. MCoTI-II also selectively inhibited the invasion of matriptase-expressing prostate cancer cells. Using a model of epithelial cell tight junction assembly, we also found that MCoTI-II could effectively inhibit the re-establishment of tight junctions and epithelial barrier function in MDCK-I cells after disruption, consistent with the role of matriptase in regulating epithelial integrity. Surprisingly, MCoTI-II was unable to inhibit matriptase-dependent proteolytic activation of prostasin, a GPI-anchored serine protease also implicated in epithelial homeostasis. These observations suggest that the unusually high selectivity afforded by MCoTI-II and its biological effectiveness might represent a useful starting point for the development of therapeutic inhibitors, and further highlight the role of matriptase in epithelial maintenance.

Combinatorial optimization of cystine-knot peptides towards high-affinity inhibitors of human matriptase-1

Cystine-knot miniproteins define a class of bioactive molecules with several thousand natural members. Their eponymous motif comprises a rigid structured core formed by six disulfide-connected cysteine residues, which accounts for its exceptional stability towards thermic or proteolytic degradation. Since they display a remarkable sequence tolerance within their disulfide-connected loops, these molecules are considered promising frameworks for peptide-based pharmaceuticals. Natural open-chain cystine-knot trypsin inhibitors of the MCoTI (Momordica cochinchinensis trypsin inhibitor) and SOTI (Spinacia oleracea trypsin inhibitor) families served as starting points for the generation of inhibitors of matriptase-1, a type II transmembrane serine protease with possible clinical relevance in cancer and arthritic therapy. Yeast surface-displayed libraries of miniproteins were used to select unique and potent matriptase-1 inhibitors. To this end, a knowledge-based library design was applied that makes use of detailed information on binding and folding behavior of cystine-knot peptides. Five inhibitor variants, four of the MCoTI family and one of the SOTI family, were identified, chemically synthesized and oxidatively folded towards the bioactive conformation. Enzyme assays revealed inhibition constants in the low nanomolar range for all candidates. One subnanomolar binder (K_i = 0.83 nM) with an inverted selectivity towards trypsin and matriptase-1 was identified.

Cyclotide biosynthesis

Cyclotides are bioactive macrocyclic peptides from plants that are characterized by their exceptional stability and potential applications as protein engineering or drug design frameworks. Their stability arises from their unique cyclic cystine knot structure, which combines a head-to-tail cyclic peptide backbone with three conserved disulfide bonds having a knotted topology. Cyclotides are ribosomally synthesized by plants and expressed in a wide range of tissues, including leaves, flowers, stems and roots. Here we describe recent studies that have examined the biosynthesis of cyclotides and in particular the mechanism associated with post-translational backbone cyclization.

Joseph Rudinger memorial lecture: discovery and applications of cyclotides

Cyclotides are plant-derived peptides of approximately 30 amino acids that have the characteristic structural features of a head-to-tail cyclized backbone and a cystine knot arrangement of their three conserved disulfide bonds. This article gives a personal account of the discovery of cyclotides, their characterization and their applications, based on work carried out in my laboratory over the last 20 years. It describes some of the background to their discovery and focuses on how their unique structural features lead to exceptional stability. This stability and their amenability to chemical synthesis have made it possible to use cyclotides as templates in protein engineering and drug design applications. These applications complement the interest in cyclotides deriving from their unique structures and natural function as host defense molecules.

Chemical synthesis, backbone cyclization and oxidative folding of cystine-knot peptides: promising scaffolds for applications in drug design

Cystine-knot peptides display exceptional structural, thermal, and biological stability. Their eponymous motif consists of six cysteine residues that form three disulfide bonds, resulting in a notably rigid structural core. Since they highly tolerate either rational or combinatorial changes in their primary structure, cystine knots are considered to be promising frameworks for the development of peptide-based pharmaceuticals. Despite their relatively small size (two to three dozens amino acid residues), the chemical synthesis route is challenging since it involves critical steps such as head-to-tail cyclization and oxidative folding towards the respective bioactive isomer. Herein we describe the topology of cystine-knot peptides, their synthetic availability and briefly discuss potential applications of engineered variants in diagnostics and therapy.

Cyclotides as a basis for drug design

INTRODUCTION

Cyclotides are plant-made defence proteins with a head-to-tail cyclic backbone combined with a conserved, six cystine knot. They have a range of biological activities, including uterotonic and anti-HIV activity, which have attracted attention to their potential pharmaceutical applications. Furthermore, their unique structures and high stability make them appealing as peptide-based templates for drug design applications. Methods have been developed for their production, including solid phase peptide synthesis as well as recombinant methods.

AREAS COVERED

This article reviews the recent literature associated with therapeutic applications of naturally occurring and synthetically modified cyclotides. It includes applications of cyclotides and cyclotide-like molecules as peptide-based drug leads and diagnostic agents.

EXPERT OPINION

The ultra-stable cyclotides are promising templates for drug development applications and are currently being assessed for the potential breadth of their applications. For synthetic versions of cyclotides to enter human clinical trials further studies to examine their biopharmaceutical properties and toxicities are required. However, several promising proof-of-concept studies have established that pharmaceutically relevant bioactive peptide sequences can be grafted into cyclotide frameworks and thereby stabilised, while maintaining biological activity. These studies include examples directed at cancer, cardiovascular disease and infectious diseases. Solid phase peptide synthesis has been the preferred approach for making pharmaceutically modified cyclotides so far, but promising progress is being made in biological approaches to cyclotide production.

Host-defense activities of cyclotides

Cyclotides are plant mini-proteins whose natural function is thought to be to protect plants from pest or pathogens, particularly insect pests. They are approximately 30 amino acids in size and are characterized by a cyclic peptide backbone and a cystine knot arrangement of three conserved disulfide bonds. This article provides an overview of the reported pesticidal or toxic activities of cyclotides, discusses a possible common mechanism of action involving disruption of biological membranes in pest species, and describes methods that can be used to produce cyclotides for potential applications as novel pesticidal agents.

Cyclotides

In recent years, a number of peptides containing a cyclic structural fold have been described. Among them, the cyclotides family was widely reported in different plant tissues, being composed of small cyclic peptides containing 6 conserved cysteine residues connected by disulfide bonds and forming a cysteine-binding cyclic structure known as a cyclic cysteine knot. This structural scaffold is responsible for an enhanced structural stability against chemical, thermal, and proteolytic degradation. Because of the observed stability and multifunctionality, including insecticidal, antimicrobial, and anti-HIV (human immunodeficiency virus) action, much effort has gone into trying to elucidate the structural-function relations of cyclotide compounds. This review focuses on the novelties involving gene structure, precursor formation and processing, and protein folding of the cyclotide family, shedding some light on molecular mechanisms of cyclotide production. Because cyclotides are clear targets for drug development and also biotechnology applications, their chemical synthesis, heterologous systems production, and protein grafting are also addressed.

Identification and characterization of a new family of cell-penetrating peptides: cyclic cell-penetrating peptides

Cell-penetrating peptides can translocate across the plasma membrane of living cells and thus are potentially useful agents in drug delivery applications. Disulfide-rich cyclic peptides also have promise in drug design because of their exceptional stability, but to date only one cyclic peptide has been reported to penetrate cells, the Momordica cochinchinensis trypsin inhibitor II (MCoTI-II). MCoTI-II belongs to the cyclotide family of plant-derived cyclic peptides that are characterized by a cyclic cystine knot motif. Previous studies in fixed cells showed that MCoTI-II could penetrate cells but kalata B1, a prototypic cyclotide from a separate subfamily of cyclotides, was bound to the plasma membrane and did not translocate into cells. Here, we show by live cell imaging that both MCoTI-II and kalata B1 can enter cells. Kalata B1 has the same cyclic cystine knot structural motif as MCoTI-II but differs significantly in sequence, and the mechanism by which these two peptides enter cells also differs. MCoTI-II appears to enter via macropinocytosis, presumably mediated by interaction of positively charged residues with phosphoinositides in the cell membrane, whereas kalata B1 interacts directly with the membrane by targeting phosphatidylethanolamine phospholipids, probably leading to membrane bending and vesicle formation. We also show that another plant-derived cyclic peptide, SFTI-1, can penetrate cells. SFTI-1 includes just 14 amino acids and, with the exception of its cyclic backbone, is structurally very different from the cyclotides, which are twice the size. Intriguingly, SFTI-1 does not interact with any of the phospholipids tested, and its mechanism of penetration appears to be distinct from MCoTI-II and kalata B1. The ability of diverse disulfide-rich cyclic peptides to penetrate cells enhances their potential in drug design, and we propose a new classification for them, i.e. cyclic cell-penetrating peptides.

Novel inhibitors of surface layer processing in Clostridium difficile

Clostridium difficile, a leading cause of hospital-acquired bacterial infection, is coated in a dense surface layer (S-layer) that is thought to provide both physicochemical protection and a scaffold for host-pathogen interactions. The key structural components of the S-layer are two proteins derived from a polypeptide precursor, SlpA, via proteolytic cleavage by the protease Cwp84. Here, we report the design, synthesis and in vivo characterization of a panel of protease inhibitors and activity-based probes (ABPs) designed to target S-layer processing in live C. difficile cells. Inhibitors based on substrate-mimetic peptides bearing a C-terminal Michael acceptor warhead were found to be promising candidates for further development.

Cyclotides from an extreme habitat: characterization of cyclic peptides from Viola abyssinica of the Ethiopian highlands

As part of ongoing explorations of the structural diversity of cyclotides, the cyclotide content of a native violet of the East African highlands, Viola abyssinica (which grows at altitudes up to 3400 m), was studied. Six new cyclotides, vaby A-E (1-5) and varv E (6), were isolated and characterized by employing HPLC and MS techniques and quantitative amino acid analysis. Cyclotides 1-5 were found to have new sequences, and 1-3 have a further novel feature in their sequences, an alanine moiety in loop 2. Two of the cyclotides (1 and 4) also exhibited cytotoxic properties in a flourometric microculture cytotoxicity assay. The findings corroborate the hypothesis that investigating the cyclotide contents of violets growing in diverse environments is a promising approach for extending our knowledge of both the structural and biological diversity of cyclotides.

Discovery of cyclotides in the fabaceae plant family provides new insights into the cyclization, evolution, and distribution of circular proteins

Cyclotides are plant proteins whose defining structural features are a head-to-tail cyclized backbone and three interlocking disulfide bonds, which in combination are known as a cyclic cystine knot. This unique structural motif confers cyclotides with exceptional resistance to proteolysis. Their endogenous function is thought to be as plant defense agents, associated with their insecticidal and larval growth-inhibitory properties. However, in addition, an array of pharmaceutically relevant biological activities has been ascribed to cyclotides, including anti-HIV, anthelmintic, uterotonic, and antimicrobial effects. So far, >150 cyclotides have been elucidated from members of the Rubiaceae, Violaceae, and Cucurbitaceae plant families, but their wider distribution among other plant families remains unclear. Clitoria ternatea (Butterfly pea) is a member of plant family Fabaceae and through its usage in traditional medicine to aid childbirth bears similarity to Oldenlandia affinis, from which many cyclotides have been isolated. Using a combination of nanospray and matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) analyses, we examined seed extracts of C. ternatea and discovered cyclotides in the Fabaceae, the third-largest family of flowering plants. We characterized 12 novel cyclotides, thus expanding knowledge of cyclotide distribution and evolution within the plant kingdom. The discovery of cyclotides containing novel sequence motifs near the in planta cyclization site has provided new insights into cyclotide biosynthesis. In particular, MS analyses of the novel cyclotides from C. ternatea suggest that Asn to Asp variants at the cyclization site are more common than previously recognized. Moreover, this study provides impetus for the examination of other economically and agriculturally significant species within Fabaceae, now the largest plant family from which cyclotides have been described.

Cyclotides: macrocyclic peptides with applications in drug design and agriculture

Cyclotides are disulfide-rich peptides from plants that are exceptionally stable as a result of their unique cyclic cystine knot structural motif. Their natural role is thought to be as plant defence agents, most notably against insect pests, but they also have potential applications in drug design and agriculture. This article identifies gaps in current knowledge on cyclotides and suggests future directions for research into this fascinating family of ultra-stable mini-proteins.

Progress in kalata peptide production via plant cell bioprocessing

Cyclotides are disulfide-rich mini-proteins with the unique structural features of a circular backbone and knotted arrangement of three conserved disulfide bonds. They typically comprise 28-37 amino acids and are produced from linear precursors, and translational modification via oxidative folding, proteolytic processing and N-C cyclization. Because these plant-derived peptides are resistant to degradation and do exhibit a diverse range of biological activities, they have become important agronomic and industrial objectives. Due to its tolerance to sequence variation, the cyclotide backbone is also potentially useful as a molecular scaffold for protein-engineering applications. Several production options are available for bioactive plant metabolites including natural harvesting, total chemical synthesis, and expression of plant pathways in microbial systems. For the cyclotides with low yields in nature, chemical complexity and lack of knowledge of the complete biosynthetic pathway, however, many of these options are precluded. Plant cell-culture technology shows promise towards the goal of producing therapeutically active cyclotides in quality and quantities required for drug development as they are amenable to process optimization, scale-up, and metabolic engineering. It is conceivable that plant-based production systems may ultimately prove to be the preferred route for the production of native or designed cyclotides, and will contribute towards the development of target-specific drugs.

Circling the enemy: cyclic proteins in plant defence

Cyclotides are ultra-stable plant proteins that have a circular peptide backbone crosslinked by a cystine knot of disulfide bonds. They are produced in large quantities by plants of the Violaceae and Rubiaceae families and have a role in plant defence against insect predation. As I discuss here, recent studies have begun to reveal how their unique circular topology evolved. Cyclization is achieved by hijacking existing plant proteolytic enzymes and operating them in 'reverse' to form a peptide bond between the N- and C-termini of a linear precursor. Such studies suggest that circular proteins are more common in the plant kingdom than was previously thought, and their exceptional stability has led to their application as protein-engineering templates in drug design.

Ultra-stable peptide scaffolds for protein engineering-synthesis and folding of the circular cystine knotted cyclotide cycloviolacin O2

The cyclic cystine knot motif, as defined by the cyclotide peptide family, is an attractive scaffold for protein engineering. To date, however, the utilisation of this scaffold has been limited by the inability to synthesise members of the most diverse and biologically active subfamily, the bracelet cyclotides. This study describes the synthesis and first direct oxidative folding of a bracelet cyclotide-cycloviolacin O2-and thus provides an efficient method for exploring the most potent cyclic cystine knot peptides. The linear chain of cycloviolacin O2 was assembled by solid-phase Fmoc peptide synthesis and cyclised by thioester-mediated native chemical ligation, and the inherent difficulties of folding bracelet cyclotides were successfully overcome in a single-step reaction. The folding pathway was characterised and was found to include predominating fully oxidised intermediates that slowly converted to the native peptide structure.