Enzymatic Macrocyclization of 1,2,3-Triazole Peptide Mimetics

Proteases Involved in Leader Peptide Removal during RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) have received much attention in recent years because of their promising bioactivities and the portability of their biosynthetic pathways. Heterologous expression studies of RiPP biosynthetic enzymes identified by genome mining often leave a leader peptide on the final product to prevent toxicity to the host and to allow the attachment of a genetically encoded affinity purification tag. Removal of the leader peptide to produce the mature natural product is then carried out in vitro with either a commercial protease or a protease that fulfills this task in the producing organism. This review covers the advances in characterizing these latter cognate proteases from bacterial RiPPs and their utility as sequence-dependent proteases. The strategies employed for leader peptide removal have been shown to be remarkably diverse. They include one-step removal by a single protease, two-step removal by two dedicated proteases, and endoproteinase activity followed by aminopeptidase activity by the same protease. Similarly, the localization of the proteolytic step varies from cytoplasmic cleavage to leader peptide removal during secretion to extracellular leader peptide removal. Finally, substrate recognition ranges from highly sequence specific with respect to the leader and/or modified core peptide to nonsequence specific mechanisms.

Identification of a short ACE2-derived stapled peptide targeting the SARS-CoV-2 spike protein

The design and synthesis of a series of peptide derivatives based on a short ACE2 α-helix 1 epitope and subsequent [i - i+4] stapling of the secondary structure resulted in the identification of a 9-mer peptide capable to compete with recombinant ACE2 towards Spike RBD in the micromolar range. Specifically, SARS-CoV-2 Spike inhibitor screening based on colorimetric ELISA assay and structural studies by circular dichroism showed the ring-closing metathesis cyclization being capable to stabilize the helical structure of the 9-mer ³⁴HEAEDLFYQ⁴² epitope better than the triazole stapling via click chemistry. MD simulations showed the stapled peptide being able not only to bind the Spike RBD, sterically interfering with ACE2, but also showing higher affinity to the target as compared to parent epitope.

Emulating nonribosomal peptides with ribosomal biosynthetic strategies

Peptide natural products are important lead structures for human drugs and many nonribosomal peptides possess antibiotic activity. This makes them interesting targets for engineering approaches to generate peptide analogues with, for example, increased bioactivities. Nonribosomal peptides are produced by huge mega-enzyme complexes in an assembly-line like manner, and hence, these biosynthetic pathways are challenging to engineer. In the past decade, more and more structural features thought to be unique to nonribosomal peptides were found in ribosomally synthesised and posttranslationally modified peptides as well. These streamlined ribosomal pathways with modifying enzymes that are often promiscuous and with gene-encoded precursor proteins that can be modified easily, offer several advantages to produce designer peptides. This review aims to provide an overview of recent progress in this emerging research area by comparing structural features common to both nonribosomal and ribosomally synthesised and posttranslationally modified peptides in the first part and highlighting synthetic biology strategies for emulating nonribosomal peptides by ribosomal pathway engineering in the second part.

Possible Functional Roles of Patellamides in the Ascidian-Prochloron Symbiosis

Patellamides are highly bioactive compounds found along with other cyanobactins in the symbiosis between didemnid ascidians and the enigmatic cyanobacterium Prochloron. The biosynthetic pathway of patellamide synthesis is well understood, the relevant operons have been identified in the Prochloron genome and genes involved in patellamide synthesis are among the most highly transcribed cyanobacterial genes in hospite. However, a more detailed study of the in vivo dynamics of patellamides and their function in the ascidian-Prochloron symbiosis is complicated by the fact that Prochloron remains uncultivated despite numerous attempts since its discovery in 1975. A major challenge is to account for the highly dynamic microenvironmental conditions experienced by Prochloron in hospite, where light-dark cycles drive rapid shifts between hyperoxia and anoxia as well as pH variations from pH ~6 to ~10. Recently, work on patellamide analogues has pointed out a range of different catalytic functions of patellamide that could prove essential for the ascidian-Prochloron symbiosis and could be modulated by the strong microenvironmental dynamics. Here, we review fundamental properties of patellamides and their occurrence and dynamics in vitro and in vivo. We discuss possible functions of patellamides in the ascidian-Prochloron symbiosis and identify important knowledge gaps and needs for further experimental studies.

Macrocylases as synthetic tools for ligand synthesis: enzymatic synthesis of cyclic peptides containing metal-binding amino acids

Improving the sustainability of synthesis is a major goal in green chemistry, which has been greatly aided by the development of asymmetric transition metal catalysis. Recent advances in asymmetric catalysis show that the ability to control the coordination sphere of substrates can lead to improvements in enantioselectivity and activity, in a manner resembling the operation of enzymes. Peptides can be used to mimic enzyme structures and their secondary interactions and they are easily accessible through solid-phase peptide synthesis. Despite this, cyclic peptides remain underexplored as chiral ligands for catalysis due to synthetic complications upon macrocyclization. Here, we show that the solid-phase synthesis of peptides containing metal-binding amino acids, bipyridylalanine (1), phenyl pyridylalanine (2) and N,N-dimethylhistidine (3) can be combined with peptide macrocylization using peptide cyclase 1 (PCY1) to yield cyclic peptides under mild conditions. High conversions of the linear peptides were observed (approx. 90%) and the Cu-bound cyclo(FSAS(1)SSKP) was shown to be a competent catalyst in the Friedel-Crafts/conjugate addition of indole. This study shows that PCY1 can tolerate peptides containing amino acids with classic inorganic and organometallic ligands as side chains, opening the door to the streamlined and efficient development of cyclic peptides as metal ligands.

Biocatalytic synthesis of peptidic natural products and related analogues

Peptidic natural products (PNPs) represent a rich source of lead compounds for the discovery and development of therapeutic agents for the treatment of a variety of diseases. However, the chemical synthesis of PNPs with diverse modifications for drug research is often faced with significant challenges, including the unavailability of constituent nonproteinogenic amino acids, inefficient cyclization protocols, and poor compatibility with other functional groups. Advances in the understanding of PNP biosynthesis and biocatalysis provide a promising, sustainable alternative for the synthesis of these compounds and their analogues. Here we discuss current progress in using native and engineered biosynthetic enzymes for the production of both ribosomally and nonribosomally synthesized peptides. In addition, we highlight new in vitro and in vivo approaches for the generation and screening of PNP libraries.

Synthesis of 4-Arylselanyl-1H-1,2,3-triazoles from Selenium-Containing Carbinols

In this work, we present a simple way to achieve 4-arylselanyl-1H-1,2,3-triazoles from selenium-containing carbinols in a one-pot strategy. The selenium-containing carbinols were used as starting materials to produce a range of selanyl-triazoles in moderate to good yields, including a quinoline and Zidovudine derivatives. One-pot protocols are crucial to the current concerns about waste production and solvent consumption, avoiding the isolation and purification steps of the reactive terminal selanylalkynes. We could also isolate an interesting and unprecedented by-product with one alkynylselenium moiety connected to the triazole.

New developments in RiPP discovery, enzymology and engineering

Covering: up to June 2020Ribosomally-synthesized and post-translationally modified peptides (RiPPs) are a large group of natural products. A community-driven review in 2013 described the emerging commonalities in the biosynthesis of RiPPs and the opportunities they offered for bioengineering and genome mining. Since then, the field has seen tremendous advances in understanding of the mechanisms by which nature assembles these compounds, in engineering their biosynthetic machinery for a wide range of applications, and in the discovery of entirely new RiPP families using bioinformatic tools developed specifically for this compound class. The First International Conference on RiPPs was held in 2019, and the meeting participants assembled the current review describing new developments since 2013. The review discusses the new classes of RiPPs that have been discovered, the advances in our understanding of the installation of both primary and secondary post-translational modifications, and the mechanisms by which the enzymes recognize the leader peptides in their substrates. In addition, genome mining tools used for RiPP discovery are discussed as well as various strategies for RiPP engineering. An outlook section presents directions for future research.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Expanding the chemical space of synthetic cyclic peptides using a promiscuous macrocyclase from prenylagaramide biosynthesis

Cyclic peptides are excellent drug candidates, placing macrocyclization reactions at the apex of drug development. PatG and related dual-action proteases from cyanobactin biosynthesis are responsible for cleaving off the C-terminal recognition sequence and macrocyclizing the substrate to provide cyclic peptides. This reaction has found use in the enzymatic synthesis of diverse macrocycles. However, these enzymes function best on substrates that terminate with the non-proteinogenic thiazole/thiazoline residue, complicating synthetic strategies. Here, we biochemically characterize a new class of PatG-like macrocyclases that natively use proline, obviating the necessity of additional chemical or biochemical steps. We experimentally define the biochemical steps involved in synthesizing the widespread prenylagaramide-like natural products, including macrocyclization and prenylation. Using saturation mutagenesis, we show that macrocyclase PagG and prenyltransferase PagF are highly promiscuous, producing a library of more than 100 cyclic peptides and their prenylated derivatives in vitro. By comparing our results to known cyanobactin macrocyclases, we catalog a series of enzymes from this family that should synthesize most small macrocycles. Collectively, these data reveal that, by selecting the right cyanobactin macrocyclase, a large array of enzymatically synthesized macrocycles are accessible.

Mannosylated hemagglutinin peptides bind cyanovirin-N independent of disulfide-bonds in complementary binding sites

Cyanovirin-N (CV-N) has been shown to reveal broad neutralizing activity against human immunodeficiency virus (HIV) and to specifically bind Manα(1→2)Manα units exposed on various glycoproteins of enveloped viruses, such as influenza hemagglutinin (HA) and Ebola glycoprotein. Chemically synthesized dimannosylated HA peptides bound domain-swapped and dimeric CV-N with either four disulfide-bonds (Cys–Cys), or three Cys–Cys bonds and an intact fold of the high-affinity binding site at an equilibrium dissociation constant K_D of 10 μM. Cys–Cys mutagenesis with ion-pairing amino-acids glutamic acid and arginine was calculated by in silico structure-based protein design and allowed for recognizing dimannose and dimannosylated peptide binding to low-affinity binding sites (K_D ≈ 11 μM for one C58–C73 bond, and binding to dimannosylated peptide). In comparison, binding to HA was achieved based on one ion-pairing C58E–C73R substitution at K_D = 275 nM, and K_D = 5 μM for two C58E–C73R substitutions. We were utilizing a triazole bioisostere linkage to form the respective mannosylated-derivative on the HA peptide sequence of residues glutamine, glycine, and glutamic acid. Thus, mono- and dimannosylated peptides with N-terminal cysteine facilitated site-specific interactions with HA peptides, mimicking a naturally found N-linked glycosylation site on the HA head domain.

Di-mannosylated peptides reveal mannose binding to cyanovirin-N (CV-N) low-affinity binding sites.

Porous Nanosheet Assembly for Macrocyclization and Self-Release

Macrocyclic structures are challenging synthetic targets owing to various potential applications ranging from drug discovery to nanomaterials. Their use, however, is highly limited due to synthetic difficulties arising from an entropic penalty for folding of linear chains. Here, we report single-layered porous nanosheets with 2D ordered internal cavities that act as a highly efficient macrocycle generator, changing linear substrates to release as macrocycles in aqueous methanol solution. The nanosheets with hydrophobic cavities encapsulate a linear substrate with nearly perfect uptake, perform clean cyclization, and then spontaneously release as a pure macrocycle. The self-separation of the macrocycle that precipitates from the solution leads to repeated cycles of macrocycle generation; thereby, the single-layered porous materials enabling catch and release offer a powerful novel strategy for repeated macrocycle generation.

Synthesis of the natural product descurainolide and cyclic peptides from lignin-derived aromatics

Alternative sources of potential feedstock chemicals are of increasing importance as the availability of oil decreases. The biopolymer lignin is viewed as a source of useful mono-aromatic compounds as exemplified by the industrial scale production of vanillin from this biomass. Alternative lignin-derived aromatics are available in pure form but to date examples of the use of these types of compounds are rare. Here we address this issue by reporting the conversion of an aromatic keto-alcohol to the anti- and syn-isomers of Descurainolide A. The key step involves a rhodium-catalyzed allylic substitution reaction. Enantio-enriched allylic alcohols were generated via an isothiourea-catalyzed kinetic resolution enabling access to both the (2R,3R) and (2S,3S) enantiomers of anti-Descurainolide A. In addition we show that the lignin-derived keto-alcohols can be converted into unnatural amino acid derivatives of tyrosine. Finally, these amino acids were incorporated into cyclic peptide scaffolds through the use of both chemical and an enzyme-mediated macrocylisation.

On resin click-chemistry-mediated synthesis of novel enkephalin analogues with potent anti-nociceptive activity

Here, we report the chemical synthesis of two DPDPE analogues 7a (NOVA1) and 7b (NOVA2). This entailed the solid-phase synthesis of two enkephalin precursor chains followed by a Cu^I-catalyzed azide-alkyne cycloaddition, with the aim of improving in vivo analgesic efficacy versus DPDPE. NOVA2 showed good affinity and selectivity for the μ-opioid receptor (K_I of 59.2 nM, EC₅₀ of 12.9 nM, E_Max of 87.3%), and long lasting anti-nociceptive effects in mice when compared to DPDPE.

Peptide Cyclization Catalyzed by Cyanobactin Macrocyclases

Cyclic peptides are an emerging class of therapeutics that can modulate targets not amenable to traditional small molecule intervention (e.g., protein-protein interactions). However, N-to-C macrocyclization of peptides is a challenging and often a low yielding chemical transformation. Several macrocyclases from cyanobactin biosynthetic clusters have been used to catalyze this reaction.This chapter provides practical guidance to the processes of heterologous expression and purification of these enzymes as well as performing in vitro biochemical reactions. Finally, approaches to recover the final product from an enzymatic reaction mixture are also discussed.

Roads to Rome: Role of Multiple Cassettes in Cyanobactin RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are ubiquitous natural products. Bioactive RiPPs are produced from a precursor peptide, which is modified by enzymes. Usually, a single product is encoded in a precursor peptide. However, in cyanobactins and several other RiPP pathways, a single precursor peptide encodes multiple bioactive products flanking with recognition sequences known as "cassettes". The role of multiple cassettes in one peptide is mysterious, but in general their presence is a marker of biosynthetic plasticity. Here, we show that in cyanobactin biosynthesis the presence of multiple cassettes confers distributive enzyme processing to multiple steps of the pathway, a feature we propose to be a hallmark of multicassette RiPPs. TruD heterocyclase is stochastic and distributive. Although a canonical biosynthetic route is favored with certain substrates, every conceivable biosynthetic route is accepted. Together, these factors afford greater plasticity to the biosynthetic pathway by equalizing the processing of each cassette, enabling access to chemical diversity.

The Biochemistry and Structural Biology of Cyanobactin Pathways: Enabling Combinatorial Biosynthesis

Cyanobactin biosynthetic enzymes have exceptional versatility in the synthesis of natural and unnatural products. Cyanobactins are ribosomally synthesized and posttranslationally modified peptides synthesized by multistep pathways involving a broad suite of enzymes, including heterocyclases/cyclodehydratases, macrocyclases, proteases, prenyltransferases, methyltransferases, and others. Here, we describe the enzymology and structural biology of cyanobactin biosynthetic enzymes, aiming at the twin goals of understanding biochemical mechanisms and biosynthetic plasticity. We highlight how this common suite of enzymes may be utilized to generate a large array or structurally and chemically diverse compounds.

Characterization of the Fast and Promiscuous Macrocyclase from Plant PCY1 Enables the Use of Simple Substrates

Cyclic ribosomally derived peptides possess diverse bioactivities and are currently of major interest in drug development. However, it can be chemically challenging to synthesize these molecules, hindering the diversification and testing of cyclic peptide leads. Enzymes used in vitro offer a solution to this; however peptide macrocyclization remains the bottleneck. PCY1, involved in the biosynthesis of plant orbitides, belongs to the class of prolyl oligopeptidases and natively displays substrate promiscuity. PCY1 is a promising candidate for in vitro utilization, but its substrates require an 11 to 16 residue C-terminal recognition tail. We have characterized PCY1 both kinetically and structurally with multiple substrate complexes revealing the molecular basis of recognition and catalysis. Using these insights, we have identified a three residue C-terminal extension that replaces the natural recognition tail permitting PCY1 to operate on synthetic substrates. We demonstrate that PCY1 can macrocyclize a variety of substrates with this short tail, including unnatural amino acids and nonamino acids, highlighting PCY1's potential in biocatalysis.

Internal Peptide Late-Stage Diversification: Peptide-Isosteric Triazoles for Primary and Secondary C(sp3 )-H Activation

Secondary C(sp³ )-H arylations were accomplished by palladium catalysis with triazoles as peptide bond isosteres. The unique power of this approach is highlighted by the possibility of achieving secondary C(sp³ )-H functionalizations on terminal peptides as well as the unprecedented positional-selective C(sp³ )-H functionalization of internal peptide positions, setting the stage for modular peptide late-stage diversification.

Enzyme-catalyzed peptide cyclization

The recent advancement of peptide macrocycles as promising therapeutics creates a need for novel methodologies for their efficient synthesis and (large scale) production. Within this context, due to the favorable properties of biocatalysts, enzyme-mediated methodologies have gained great interest. Enzymes such as sortase A, butelase 1, peptiligase and omniligase-1 represent extremely powerful and valuable enzymatic tools for peptide ligation, since they can be applied to generate complex cyclic peptides with exquisite biological activity. Therefore, the use of enzymatic strategies will effectively supplement the scope of existing chemical methodologies and will accelerate the development of future cyclic peptide therapeutics. The advantages and disadvantages of the different enzymatic methodologies will be discussed in this review.

Bypassing the proline/thiazoline requirement of the macrocyclase PatG

Biocatalysis is a fast developing field in which an enzyme's natural capabilities are harnessed or engineered for synthetic chemistry. The enzyme PatG is an extremely promiscuous macrocyclase enzyme tolerating both non-natural amino acids and non-amino acids within the substrate. It does, however, require a proline or thiazoline at the C-terminal position of the core peptide which means the final product must contain this group. Here, we show guided by structural insight we have identified two synthetic routes, triazole and a double cysteine, that circumvent this requirement. With the triazole, we show PatGmac can macrocyclise substrates that do not contain any amino acids in the final product.

Characterization of a dual function macrocyclase enables design and use of efficient macrocyclization substrates

Peptide macrocycles are promising therapeutic molecules because they are protease resistant, structurally rigid, membrane permeable, and capable of modulating protein-protein interactions. Here, we report the characterization of the dual function macrocyclase-peptidase enzyme involved in the biosynthesis of the highly toxic amanitin toxin family of macrocycles. The enzyme first removes 10 residues from the N-terminus of a 35-residue substrate. Conformational trapping of the 25 amino-acid peptide forces the enzyme to release this intermediate rather than proceed to macrocyclization. The enzyme rebinds the 25 amino-acid peptide in a different conformation and catalyzes macrocyclization of the N-terminal eight residues. Structures of the enzyme bound to both substrates and biophysical analysis characterize the different binding modes rationalizing the mechanism. Using these insights simpler substrates with only five C-terminal residues were designed, allowing the enzyme to be more effectively exploited in biotechnology.

The intramolecular click reaction using 'carbocontiguous' precursors

The synthesis and utilization of all carbon-chain 'carbocontiguous' azidoalkynyl precursors for an intramolecular click reaction is described. The substrates contain both azidoalkyl and ethynylmethyl groups which are conjoined by a 2-(phenylsulfonylmethyl)-4,5-diphenyloxazole lynchpin and are suitably disposed for ring closure. On promotion by copper salts, a number of cyclic click products having the 1,4-disubstituted endo-fused triazole component and the 4,5-diphenyloxazole component are obtained. In one case, removal of the phenylsulfonylmethyl group from the substrate prior to cyclization gave the 1,5-disubstituted exo-fused triazole. The utilization of CuSO4/sodium ascorbate system appears to be the optimal conditions for closure/cyclization and afforded the cyclized products in yields of 84-95%.

Kinetic Landscape of a Peptide Bond-Forming Prolyl Oligopeptidase

Prolyl oligopeptidase B from Galerina marginata (GmPOPB) has recently been discovered as a peptidase capable of breaking and forming peptide bonds to yield a cyclic peptide. Despite the relevance of prolyl oligopeptidases in human biology and disease, a kinetic analysis pinpointing rate-limiting steps for a member of this enzyme family is not available. Macrocyclase enzymes are currently exploited to produce cyclic peptides with potential therapeutic applications. Cyclic peptides are promising druglike molecules because of their stability and conformational rigidity. Here we describe an in-depth kinetic characterization of a prolyl oligopeptidase acting as a macrocyclase enzyme. By combining steady-state and pre-steady-state kinetics, we propose a kinetic sequence in which a step after macrocyclization limits steady-state turnover. Additionally, product release is ordered, where the cyclic peptide departs first followed by the peptide tail. Dissociation of the peptide tail is slow and significantly contributes to the turnover rate. Furthermore, trapping of the enzyme by the peptide tail becomes significant beyond initial rate conditions. The presence of a burst of product formation and a large viscosity effect further support the rate-limiting nature of a physical step occurring after macrocyclization. This is the first detailed description of the kinetic sequence of a macrocyclase enzyme from this class. GmPOPB is among the fastest macrocyclases described to date, and this work is a necessary step toward designing broad-specificity efficient macrocyclases.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Synthesis of Hybrid Cyclopeptides through Enzymatic Macrocyclization

Natural products comprise a diverse array of molecules, many of which are biologically active. Most natural products are derived from combinations of polyketides, peptides, sugars, and fatty-acid building blocks. Peptidic macrocycles have attracted attention as potential therapeutics possessing cell permeability, stability, and easy-to-control variability. Here, we show that enzymes from the patellamide biosynthetic pathway can be harnessed to make macrocycles that are hybrids of amino acids and a variety of manmade chemical building blocks, including aryl rings, polyethers, and alkyl chains. We have made macrocycles with only three amino acids, one of which can be converted to a thiazoline or a thiazole ring. We report the synthesis of 18 peptide hybrid macrocycles, nine of which have been isolated and fully characterized.

Mechanisms of cyanobactin biosynthesis

Cyanobactins are a diverse collection of natural products that originate from short peptides made on a ribosome. The amino acids are modified in a series of transformations catalyzed by multiple enzymes. The patellamide pathway is the most well studied and characterized example. Here we review the structures and mechanisms of the enzymes that cleave peptide bonds, macrocyclise peptides, heterocyclise cysteine (as well as threonine and serine) residues, oxidize five-membered heterocycles and attach prenyl groups. Some enzymes operate by novel mechanisms which is of interest and in addition the enzymes uncouple recognition from catalysis. The normally tight relationship between these factors hinders biotechnology. The cyanobactin pathway may be particularly suitable for exploitation, with progress observed with in vivo and in vitro approaches.

Enzymatic Macrocyclization of 1,2,3-Triazole Peptide Mimetics

The macrocyclization of linear peptides is very often accompanied by significant improvements in their stability and biological activity. Many strategies are available for their chemical macrocyclization, however, enzyme-mediated methods remain of great interest in terms of synthetic utility. To date, known macrocyclization enzymes have been shown to be active on both peptide and protein substrates. Here we show that the macrocyclization enzyme of the cyanobactin family, PatGmac, is capable of macrocyclizing substrates with one, two, or three 1,4-substituted 1,2,3-triazole moieties. The introduction of non-peptidic scaffolds into macrocycles is highly desirable in tuning the activity and physical properties of peptidic macrocycles. We have isolated and fully characterized nine non-natural triazole-containing cyclic peptides, a further ten molecules are also synthesized. PatGmac has now been shown to be an effective and versatile tool for the ring closure by peptide bond formation.