Epimerization and substrate gating by a TE domain in β-lactam antibiotic biosynthesis

Substrate Trapping in Polyketide Synthase Thioesterase Domains: Structural Basis for Macrolactone Formation

Emerging antibiotic resistance requires continual improvement in the arsenal of antimicrobial drugs, especially the critical macrolide antibiotics. Formation of the macrolactone scaffold of these polyketide natural products is catalyzed by a modular polyketide synthase (PKS) thioesterase (TE). The TE accepts a linear polyketide substrate from the termina PKS acyl carrier protein to generate an acyl-enzyme adduct that is resolved by attack of a substrate hydroxyl group to form the macrolactone. Our limited mechanistic understanding of TE selectivity for a substrate nucleophile and/or water has hampered development of TEs as biocatalysts that accommodate a variety of natural and non-natural substrates. To understand how TEs direct the substrate nucleophile for macrolactone formation, acyl-enzyme intermediates were trapped as stable amides by substituting the natural serine OH with an amino group. Incorporation of the unnatural amino acid, 1,3-diaminopropionic acid (DAP), was tested with five PKS TEs. DAP-modified TEs (TE DAP ) from the pikromycin and erythromycin pathways were purified and tested with six full-length polyketide intermediates from three pathways. The erythromycin TE had permissive substrate selectivity, whereas the pikromycin TE was selective for its native hexaketide and heptaketide substrates. In a crystal structure of a native substrate trapped in pikromycin TE DAP , the linear heptaketide was curled in the active site with the nucleophilic hydroxyl group positioned 4 Å from the amide-enzyme linkage. The curled heptaketide displayed remarkable shape complementarity with the TE acyl cavity. The strikingly different shapes of acyl cavities in TEs of known structure, including those reported here for juvenimicin, tylosin and fluvirucin biosynthesis, provide new insights to facilitate TE engineering and optimization.

The structure of the monobactam-producing thioesterase domain of SulM forms a unique complex with the upstream carrier protein domain

Nonribosomal peptide synthetases (NRPSs) are responsible for the production of important biologically active peptides. The large, multidomain NRPSs operate through an assembly line strategy in which the growing peptide is tethered to carrier domains that deliver the intermediates to neighboring catalytic domains. While most NRPS domains catalyze standard chemistry of amino acid activation, peptide bond formation, and product release, some canonical NRPS catalytic domains promote unexpected chemistry. The paradigm monobactam antibiotic sulfazecin is produced through the activity of a terminal thioesterase domain of SulM, which catalyzes an unusual β-lactam-forming reaction in which the nitrogen of the C-terminal N-sulfo-2,3-diaminopropionate residue attacks its thioester tether to release the monobactam product. We have determined the structure of the thioesterase domain as both a free-standing domain and a didomain complex with the upstream holo peptidyl-carrier domain. The position of variant lid helices results in an active site pocket that is quite constrained, a feature that is likely necessary to orient the substrate properly for β-lactam formation. Modeling of a sulfazecin tripeptide into the active site identifies a plausible binding mode identifying potential interactions for the sulfamate and the peptide backbone with Arg2849 and Asn2819, respectively. The overall structure is similar to the β-lactone-forming thioesterase domain that is responsible for similar ring closure in the production of obafluorin. We further use these insights to enable bioinformatic analysis to identify additional, uncharacterized β-lactam-forming biosynthetic gene clusters by genome mining.

Structural advances toward understanding the catalytic activity and conformational dynamics of modular nonribosomal peptide synthetases

Covering: up to fall 2022.Nonribosomal peptide synthetases (NRPSs) are a family of modular, multidomain enzymes that catalyze the biosynthesis of important peptide natural products, including antibiotics, siderophores, and molecules with other biological activity. The NRPS architecture involves an assembly line strategy that tethers amino acid building blocks and the growing peptides to integrated carrier protein domains that migrate between different catalytic domains for peptide bond formation and other chemical modifications. Examination of the structures of individual domains and larger multidomain proteins has identified conserved conformational states within a single module that are adopted by NRPS modules to carry out a coordinated biosynthetic strategy that is shared by diverse systems. In contrast, interactions between modules are much more dynamic and do not yet suggest conserved conformational states between modules. Here we describe the structures of NRPS protein domains and modules and discuss the implications for future natural product discovery.

Labelling studies in the biosynthesis of polyketides and non-ribosomal peptides

Covering: 2015 to 2022In this review, we discuss the recent advances in the use of isotopically labelled compounds to investigate the biosynthesis of polyketides, non-ribosomally synthesised peptides, and their hybrids. Also, we highlight the use of isotopes in the elucidation of their structures and investigation of enzyme mechanisms. The biosynthetic pathways of selected examples are presented in detail to reveal the principles of the discussed labelling experiments. The presented examples demonstrate that the application of isotopically labelled compounds is still the state of the art and can provide valuable information for the biosynthesis of natural products.

Structural Studies of Modular Nonribosomal Peptide Synthetases

The non-ribosomal peptide synthetases (NRPSs) are a family of modular enzymes involved in the production of peptide natural products. Not restricted by the constraints of ribosomal peptide and protein production, the NRPSs are able to incorporate unusual amino acids and other suitable building blocks into the final product. The NRPSs operate with an assembly line strategy in which peptide intermediates are covalently tethered to a peptidyl carrier protein and transported to different catalytic domains for the multiple steps in the biosynthesis. Often the carrier and catalytic domains are joined into a single large multidomain protein. This chapter serves to introduce the NRPS enzymes, using the nocardicin NRPS system as an example that highlights many common features to NRPS biochemistry. We then describe recent advances in the structural biology of NRPSs focusing on large multidomain structures that have been determined.

In Vitro Reconstitution of Fimsbactin Biosynthesis from Acinetobacter baumannii

Siderophores produced via nonribosomal peptide synthetase (NRPS) pathways serve as critical virulence factors for many pathogenic bacteria. Improved knowledge of siderophore biosynthesis guides the development of inhibitors, vaccines, and other therapeutic strategies. Fimsbactin A is a mixed ligand siderophore derived from human pathogenic Acinetobacter baumannii that contains phenolate-oxazoline, catechol, and hydroxamate metal chelating groups branching from a central l-Ser tetrahedral unit via amide and ester linkages. Fimsbactin A is derived from two molecules of l-Ser, two molecules of 2,3-dihydroxybenzoic acid (DHB), and one molecule of l-Orn and is a product of the fbs biosynthetic operon. Here, we report the complete in vitro reconstitution of fimsbactin A biosynthesis in a cell-free system using purified enzymes. We demonstrate the conversion of l-Orn to N₁-acetyl-N₁-hydroxy-putrescine (ahPutr) via ordered action of FbsJ (decarboxylase), FbsI (flavin N-monooxygenase), and FbsK (N-acetyltransferase). We achieve conversion of l-Ser, DHB, and l-Orn to fimsbactin A using FbsIJK in combination with the NRPS modules FbsEFGH. We also demonstrate chemoenzymatic conversion of synthetic ahPutr to fimsbactin A using FbsEFGH and establish the substrate selectivity for the NRPS adenylation domains in FbsH (DHB) and FbsF (l-Ser). We assign a role for the type II thioesterase FbsM in producing the shunt metabolite 2-(2,3-dihydroxyphenyl)-4,5-dihydrooxazole-4-carboxylic acid (DHB-oxa) via cleavage of the corresponding thioester intermediate that is tethered to NRPS peptidyl carrier domains during biosynthetic assembly. We propose a mechanism for branching NRPS-derived peptides via amide and ester linkages via the dynamic equilibration of N-DHB-Ser and O-DHB-Ser thioester intermediates via hydrolysis of DHB-oxa thioester intermediates. We also propose a genetic signature for NRPS "branching" in the presence of a terminating C-T-C motif (FbsG).

Nonribosomal Peptide Synthesis Definitely Working Out of the Rules

Nonribosomal peptides are microbial secondary metabolites exhibiting a tremendous structural diversity and a broad range of biological activities useful in the medical and agro-ecological fields. They are built up by huge multimodular enzymes called nonribosomal peptide synthetases. These synthetases are organized in modules constituted of adenylation, thiolation, and condensation core domains. As such, each module governs, according to the collinearity rule, the incorporation of a monomer within the growing peptide. The release of the peptide from the assembly chain is finally performed by a terminal core thioesterase domain. Secondary domains with modifying catalytic activities such as epimerization or methylation are sometimes included in the assembly lines as supplementary domains. This assembly line structure is analyzed by bioinformatics tools to predict the sequence and structure of the final peptides according to the sequence of the corresponding synthetases. However, a constantly expanding literature unravels new examples of nonribosomal synthetases exhibiting very rare domains and noncanonical organizations of domains and modules, leading to several amazing strategies developed by microorganisms to synthesize nonribosomal peptides. In this review, through several examples, we aim at highlighting these noncanonical pathways in order for the readers to perceive their complexity.

Exploring the Molecular Basis of Substrate and Product Selectivities of Nocardicin Bifunctional Thioesterase

D-amino acid introduction in peptides can enrich their biological activities and pharmacological properties as potential drugs. This achievement of stereochemical inversion usually owes to an epimerase or racemase. Interestingly, a unique bifunctional thioesterase (NocTE), which is incorporated in nonribosomal peptide synthetase (NRPS) NocA-NocB assembly line for the biosynthesis of monocyclic β-lactam antibiotic nocardicin A, can control the generation of D-products with high stereochemical purity. However, the molecular basis of NocTE selectivity on substrates and products is still unclear. Herein, we constructed a series of systems with different peptides varying in stereochemistry, length, and composition to investigate the substrate selectivity. The studies on binding affinities and loading conformations elucidated the important roles of peptide length and β-lactam ring in substrate selectivity. Through energy decomposition and interaction analyses, some key residues involved in substrate selectivity were captured. On the other hand, natural product undergoing epimerization was found to be liberated from the active pocket more easily in comparison with its diastereomer (epi-nocardicin G), explaining the superiority of nocardicin G. These results provide detailed molecular insights into the exquisite control of substrate and product scopes for NocTE, and encourage to diversification of substrates and final products for NRPS assembly line. The molecular insights into substrate and product selectivities of unique bifunctional thioesterase NocTE were illustrated via several molecular simulations and free energy calculations, contributing to expanding substrate and product scopes of nonribosomal peptide synthetases.

Chain release mechanisms in polyketide and non-ribosomal peptide biosynthesis

Review covering up to mid-2021The structure of polyketide and non-ribosomal peptide natural products is strongly influenced by how they are released from their biosynthetic enzymes. As such, Nature has evolved a diverse range of release mechanisms, leading to the formation of bioactive chemical scaffolds such as lactones, lactams, diketopiperazines, and tetronates. Here, we review the enzymes and mechanisms used for chain release in polyketide and non-ribosomal peptide biosynthesis, how these mechanisms affect natural product structure, and how they could be utilised to introduce structural diversity into the products of engineered biosynthetic pathways.

Acyl Donor Stringency and Dehydroaminoacyl Intermediates in β-Lactam Formation by a Non-ribosomal Peptide Synthetase

Condensation (C) domains in non-ribosomal peptide synthetases catalyze peptide elongation steps whereby activated amino acid or peptidyl acyl donors are coupled with specific amino acid acceptors. In the biosynthesis of the β-lactam antibiotic nocardicin A, an unusual C domain converts a seryl tetrapeptide into its pentapeptide product containing an integrated β-lactam ring. While indirect evidence for the intermediacy of a dehydroalanyl species has been reported, here we describe observation of the elusive enzyme-bound dehydroamino acyl intermediate generated from the corresponding allo-threonyl tetrapeptide and partitioned into pentapeptide products containing either a dehydrobutyrine residue or an embedded β-lactam. Contrary to trends in the literature where condensation domains have been deemed flexible as to acyl donor structure, this β-lactam synthesizing domain is highly discriminating. The observation of dehydrobutyrine formation links this C domain to related clades associated with natural products containing dehydroamino acid and d-configured residues, suggesting a common mechanistic link.

Evolutionary and functional analysis of an NRPS condensation domain integrates β-lactam, ᴅ-amino acid, and dehydroamino acid synthesis

Nonribosomal peptide synthetases (NRPSs) are large, multidomain biosynthetic enzymes involved in the assembly-line-like synthesis of numerous peptide natural products. Among these are clinically useful antibiotics including three classes of β-lactams: the penicillins/cephalosporins, the monobactams, and the monocyclic nocardicins, as well as the vancomycin family of glycopeptides and the depsipeptide daptomycin. During NRPS synthesis, peptide bond formation is catalyzed by condensation (C) domains, which couple the nascent peptide with the next programmed amino acid of the sequence. A growing number of additional functions are linked to the activity of C domains. In the biosynthesis of the nocardicins, a specialized C domain prepares the embedded β-lactam ring from a serine residue. Here, we examine the evolutionary descent of this unique β-lactam-synthesizing C domain. Guided by its ancestry, we predict and demonstrate in vitro that this C domain alternatively performs peptide bond formation when a single stereochemical change is introduced into its peptide starting material. Remarkably, the function of the downstream thioesterase (TE) domain also changes. Natively, the TE directs C terminus epimerization prior to hydrolysis when the β-lactam is made but catalyzes immediate release of the alternative peptide. In addition, we investigate the roles of C-domain histidine residues in light of clade-specific sequence motifs, refining earlier mechanistic proposals of both β-lactam formation and canonical peptide synthesis. Finally, expanded phylogenetic analysis reveals unifying connections between β-lactam synthesis and allied C domains associated with the appearance of ᴅ-amino acid and dehydroamino acid residues in other NRPS-derived natural products.

Co-occurring Congeners Reveal the Position of Enantiomeric Amino Acids in Nonribosomal Peptides

The absolute configuration of the constituent amino acids in microbial nonribosomal peptides is typically determined by Marfey's method after total hydrolysis of the peptide. A challenge to structure elucidation arises when both d and l enantiomeric configurations of an amino acid are present. Determining the actual position of each amino acid enantiomer within the peptide sequence typically requires laborious approaches based on peptide partial hydrolysis or even total synthesis of the possible diastereomers. Herein, an alternative solution is discussed based on the homogeneous backbone chirality that governs all peptides biosynthesized by a common nonribosomal peptide synthetase. The information on configuration provided by Marfey's analysis of co-occurring minor congeners can reveal unequivocally the stereochemical sequence of the whole peptide family.

Thioesterase-mediated side chain transesterification generates potent Gq signaling inhibitor FR900359

The potent and selective Gq protein inhibitor depsipeptide FR900359 (FR), originally discovered as the product of an uncultivable plant endosymbiont, is synthesized by a complex biosynthetic system comprising two nonribosomal peptide synthetase (NRPS) assembly lines. Here we characterize a cultivable bacterial FR producer, enabling detailed investigations into biosynthesis and attachment of the functionally important FR side chain. We reconstitute side chain assembly by the monomodular NRPS FrsA and the non-heme monooxygenase FrsH, and characterize intermolecular side chain transesterification to the final macrocyclic intermediate FR-Core, mediated by the FrsA thioesterase domain. We harness FrsA substrate promiscuity to generate FR analogs with altered side chains and demonstrate indispensability of the FR side chain for efficient Gq inhibition by comparative bioactivity, toxicity and docking studies. Finally, evolution of FR and side chain biosynthesis is discussed based on bioinformatics analyses. Side chain transesterification boosts potency and target affinity of selective Gq inhibitor natural products.

FR900359 (FR) is a Gq protein inhibitor depsipeptide isolated from an uncultivable plant endosymbiont and synthesized by non-ribosomal peptide synthetases. Here, the authors discover a cultivable bacterial FR producer and show that FrsA thioesterase domain catalyses intermolecular transesterification of the FR side chain to the depsipeptide core during biosynthesis, improving Gq inhibition properties.

Understanding the early stages of peptide formation during the biosynthesis of teicoplanin and related glycopeptide antibiotics

The biosynthesis of the glycopeptide antibiotics (GPAs) demonstrates the exceptional ability of nonribosomal peptide (NRP) synthesis to generate diverse and complex structures from an expanded array of amino acid precursors. Whilst the heptapeptide cores of GPAs share a conserved C terminus, including the aromatic residues involved cross-linking and that are essential for the antibiotic activity of GPAs, most structural diversity is found within the N terminus of the peptide. Furthermore, the origin of the (D)-stereochemistry of residue 1 of all GPAs is currently unclear, despite its importance for antibiotic activity. Given these important features, we have now reconstituted modules (M) 1-4 of the NRP synthetase (NRPS) assembly lines that synthesise the clinically relevant type IV GPA teicoplanin and the related compound A40926. Our results show that important roles in amino acid modification during the NRPS-mediated biosynthesis of GPAs can be ascribed to the actions of condensation domains present within these modules, including the incorporation of (D)-amino acids at position 1 of the peptide. Our results also indicate that hybrid NRPS assembly lines can be generated in a facile manner by mixing NRPS proteins from different systems and that uncoupling of peptide formation due to different rates of activity seen for NRPS modules can be controlled by varying the ratio of NRPS modules. Taken together, this indicates that NRPS assembly lines function as dynamic peptide assembly lines and not static megaenzyme complexes, which has significant implications for biosynthetic redesign of these important biosynthetic systems.

Cariogenic Streptococcus mutans Produces Tetramic Acid Strain-Specific Antibiotics That Impair Commensal Colonization

Streptococcus mutans is a common constituent of dental plaque and a major etiologic agent of dental caries (tooth decay). In this study, we elucidated the biosynthetic pathway encoded by muc, a hybrid polyketide synthase and nonribosomal peptide synthetase (PKS/NRPS) biosynthetic gene cluster (BGC), present in a number of globally distributed S. mutans strains. The natural products synthesized by muc included three N-acyl tetramic acid compounds (reutericyclin and two novel analogues) and an unacylated tetramic acid (mutanocyclin). Furthermore, the enzyme encoded by mucF was identified as a novel class of membrane-associated aminoacylases and was responsible for the deacylation of reutericyclin to mutanocyclin. A large number of hypothetical proteins across a broad diversity of bacteria were homologous to MucF, suggesting that this may represent a large family of unexplored acylases. Finally, S. mutans utilized the reutericyclin produced by muc to impair the growth of neighboring oral commensal bacteria. Since S. mutans must be able to out-compete these health-associated organisms to persist in the oral microbiota and cause disease, the competitive advantage conferred by muc suggests that this BGC is likely to be involved in S. mutans ecology and therefore dental plaque dysbiosis and the resulting caries pathogenesis.

Non-heme iron enzyme-catalyzed complex transformations: Endoperoxidation, cyclopropanation, orthoester, oxidative C-C and C-S bond formation reactions in natural product biosynthesis

Non-heme iron enzymes catalyze a wide range of chemical transformations, serving as one of the key types of tailoring enzymes in the biosynthesis of natural products. Hydroxylation reaction is the most common type of reactions catalyzed by these enzymes and hydroxylation reactions have been extensively investigated mechanistically. However, the mechanistic details for other types of transformations remain largely unknown or unexplored. In this paper, we present some of the most recently discovered transformations, including endoperoxidation, orthoester formation, cyclopropanation, oxidative C-C and C-S bond formation reactions. In addition, many of them are multi-functional enzymes, which further complicate their mechanistic investigations. In this work, we summarize their biosynthetic pathways, with special emphasis on the mechanistic details available for these newly discovered enzymes.

Structure of a bound peptide phosphonate reveals the mechanism of nocardicin bifunctional thioesterase epimerase-hydrolase half-reactions

Nonribosomal peptide synthetases (NRPSs) underlie the biosynthesis of many natural products that have important medicinal utility. Protection of the NRPS peptide products from proteolysis is critical to these pathways and is often achieved by structural modification, principally the introduction of d-amino acid residues into the elongating peptide. These amino acids are generally formed in situ from their l-stereoisomers by epimerization domains or dual-function condensation/epimerization domains. In singular contrast, the thioesterase domain of nocardicin biosynthesis mediates both the effectively complete l- to d-epimerization of its C-terminal amino acid residue (≥100:1) and hydrolytic product release. We report herein high-resolution crystal structures of the nocardicin thioesterase domain in ligand-free form and reacted with a structurally precise fluorophosphonate substrate mimic that identify the complete peptide binding pocket to accommodate both stereoisomers. These structures combined with additional functional studies provide detailed mechanistic insight into this unique dual-function NRPS domain.

NocTE is a nonribosomal peptide synthetase thioesterase that completes the biosynthesis of pro-nocardicin G, the precursor for nocardicin β-lactam antibiotics. Here the authors provide mechanistic insights into NocTE by determining its crystal structures in the ligand-free form and covalently linked to a fluorophosphonate substrate mimic.

Unrivalled diversity: the many roles and reactions of bacterial cytochromes P450 in secondary metabolism

Covering: 2000 up to 2018 The cytochromes P450 (P450s) are a superfamily of heme-containing monooxygenases that perform diverse catalytic roles in many species, including bacteria. The P450 superfamily is widely known for the hydroxylation of unactivated C-H bonds, but the diversity of reactions that P450s can perform vastly exceeds this undoubtedly impressive chemical transformation. Within bacteria, P450s play important roles in many biosynthetic and biodegradative processes that span a wide range of secondary metabolite pathways and present diverse chemical transformations. In this review, we aim to provide an overview of the range of chemical transformations that P450 enzymes can catalyse within bacterial secondary metabolism, with the intention to provide an important resource to aid in understanding of the potential roles of P450 enzymes within newly identified bacterial biosynthetic pathways.

The hidden enzymology of bacterial natural product biosynthesis

Bacterial natural products display astounding structural diversity, which, in turn, endows them with a remarkable range of biological activities that are of significant value to modern society. Such structural features are generated by biosynthetic enzymes that construct core scaffolds or perform peripheral modifications, and can thus define natural product families, introduce pharmacophores and permit metabolic diversification. Modern genomics approaches have greatly enhanced our ability to access and characterize natural product pathways via sequence-similarity-based bioinformatics discovery strategies. However, many biosynthetic enzymes catalyse exceptional, unprecedented transformations that continue to defy functional prediction and remain hidden from us in bacterial (meta)genomic sequence data. In this Review, we highlight exciting examples of unusual enzymology that have been uncovered recently in the context of natural product biosynthesis. These suggest that much of the natural product diversity, including entire substance classes, awaits discovery. New approaches to lift the veil on the cryptic chemistries of the natural product universe are also discussed.

Late-Stage Conversion of Diphenylphosphonate to Fluorophosphonate Probes for the Investigation of Serine Hydrolases

Diphenylphosphonates (DPPs) have been used for 50 years to inactivate serine hydrolases, creating adducts representative of tetrahedral intermediates of this important class of enzymes. Failure to react at active site serine residues, however, can thwart their usefulness. Here, we describe a facile route and allied mechanistic studies to highly reactive, structurally complex organofluorophosphonates (FPs) by direct fluorinative hydrolysis of DPPs. Advantages over current preparations of FPs are exemplified by the synthesis of a β-lactam-containing peptide substrate analog capable of modifying the C-terminal, dual-function thioesterase involved in nocardicin A biosynthesis. Although this serine hydrolase was found to resist modification by classic DPP inhibitors, active site selective phosphonylation by the corresponding FP occurs rapidly to form a stable adduct. This simple, late-stage method enables the ready preparation of FP probes that retain important structural motifs of native substrates, thus promoting efforts in mechanistic enzymology by accessing biologically relevant enzyme-inhibitor co-structures.

Chemical Diversification Based on Substrate Promiscuity of a Standalone Adenylation Domain in a Reconstituted NRPS System

A nonribosomal peptide synthetase (NRPS) assembly line ( sfa) in Streptomyces thioluteus that directs the formation of the diisonitrile chalkophore SF2768 (1) has been characterized by heterologous expression and directed gene knockouts. Herein, differential metabolic analysis of the heterologous expression strain and the original host led to the isolation of an SF2768 analogue (2, a byproduct of sfa) that possesses N-isovaleryl rather than 3-isocyanobutyryl side chains. The proposed biosynthetic logic of sfa and the structural difference between 1 and 2 suggested substrate promiscuity of the adenylate-forming enzyme SfaB. Further substrate scope investigation of SfaB and a successfully reconstituted NRPS system including a four-enzyme cascade enabled incorporation of diverse carboxylic acid building blocks into peptide scaffolds, and 30 unnatural products were thus generated. This structural diversification strategy based on substrate flexibility of the adenylation domain and in vitro reconstitution can be applied to other adenylation-priming pathways, thus providing a supplementary method for diversity-oriented total synthesis. Additionally, the biocatalytic process of the putative lysine δ-hydroxylase SfaE was validated through the derivatization of two key aldehyde intermediates (2a and 2b), thereby expanding the toolkit of enzymatic C-H bond activation.

Built to bind: biosynthetic strategies for the formation of small-molecule protease inhibitors

The discovery and characterization of natural product protease inhibitors has inspired the development of numerous pharmaceutical agents.

Novofumigatonin biosynthesis involves a non-heme iron-dependent endoperoxide isomerase for orthoester formation

Novofumigatonin (1), isolated from the fungus Aspergillus novofumigatus, is a heavily oxygenated meroterpenoid containing a unique orthoester moiety. Despite the wide distribution of orthoesters in nature and their biological importance, little is known about the biogenesis of orthoesters. Here we show the elucidation of the biosynthetic pathway of 1 and the identification of key enzymes for the orthoester formation by a series of CRISPR-Cas9-based gene-deletion experiments and in vivo and in vitro reconstitutions of the biosynthesis. The novofumigatonin pathway involves endoperoxy compounds as key precursors for the orthoester synthesis, in which the Fe(II)/α-ketoglutarate-dependent enzyme NvfI performs the endoperoxidation. NvfE, the enzyme catalyzing the orthoester synthesis, is an Fe(II)-dependent, but cosubstrate-free, endoperoxide isomerase, despite the fact that NvfE shares sequence homology with the known Fe(II)/α-ketoglutarate-dependent dioxygenases. NvfE thus belongs to a class of enzymes that gained an isomerase activity by losing the α-ketoglutarate-binding ability.

Mechanism of Integrated β-Lactam Formation by a Nonribosomal Peptide Synthetase during Antibiotic Synthesis

Modular nonribosomal peptide synthetases (NRPSs) are large, multidomain engines of bioactive natural product biosynthesis that function as molecular "assembly lines" in which monomer units are selectively bound, modified, and linked in a specific order and number dictated by their mega-enzyme templates. Recently, a condensation domain in an NRPS was discovered to carry out the synthesis of an integrated β-lactam ring from a substrate seryl residue during antibiotic biosynthesis. We report here a series of experiments supporting a mechanism that involves C-N bond formation by stepwise elimination/addition reactions followed by canonical NRPS-catalyzed amide bond synthesis to achieve β-lactam formation. Partitioning of reactive intermediates formed during the multistep catalytic cycle provided insight into the ability of the NRPS to overcome the reversibility of corresponding reactions in solution and enforce directionality during synthesis.

The Thioesterase Domain in Glycopeptide Antibiotic Biosynthesis Is Selective for Cross-Linked Aglycones

The biosynthesis of the glycopeptide antibiotics (GPAs)-which include teicoplanin and vancomycin-is a complex enzymatic process relying on the interplay of nonribosomal peptide synthesis and a cytochrome P450-mediated cyclization cascade. This unique cyclization cascade generates the highly cross-linked state of these nonribosomal peptides, which is crucial for their antimicrobial activity. Given that these essential oxidative transformations occur while the peptide remains bound to the terminal module of the nonribosomal peptide synthetase (NRPS) machinery, it is important to assess the selectivity of the terminal thioesterase (TE) domain and how this domain contributes to the maintenance of an efficient biosynthetic pathway while at the same time ensuring GPA maturation is completed. In this study, we report the in vitro characterization of the thioesterase domain from teicoplanin biosynthesis, the first GPA thioesterase to be characterized. Our results show that the activity of this TE domain relies on the presence of an unusual extended N-terminal linker region that appears to be unique to the NRPS machineries found in GPA biosynthesis. In addition, we show that the activity of this domain against carrier protein bound substrates is dramatically enhanced for mature GPA aglycones as opposed to linear peptides and partially cyclized intermediates. These results demonstrate how the interplay between NRPS and P450s during late stage GPA biosynthesis is not only maintained but also leads to the efficient production of mature GPA aglycones. Thus, GPA TE domains represent another impressive example of the ability of TE domains to act as logic gates during NRPS biosynthesis, ensuring that essential late-stage peptide modifications are completed before catalyzing the release of the mature, bioactive peptide product.

A route to diastereomerically pure phenylglycine thioester peptides: crucial intermediates for investigating glycopeptide antibiotic biosynthesis

Non-ribosomal peptides contain an array of amino acid building blocks that can present challenges for the synthesis of important intermediates. Here we report a route to incorporate phenylglycine residues in peptide thioesters without significant racemisation.

Monobactam formation in sulfazecin by a nonribosomal peptide synthetase thioesterase

The N-sulfonated monocyclic β-lactam ring characteristic of the monobactams confers resistance to zinc metallo-β-lactamases and affords the most effective class to combat carbapenem-resistant enterobacteria (CRE). Here we report unprecedented nonribosomal peptide synthetase activities, wherein an assembled tripeptide is N-sulfonated in trans before direct synthesis of the β-lactam ring in a noncanonical, cysteine-containing thioesterase domain. This means of azetidinone synthesis is distinct from the three others known in nature.

Halogenation of glycopeptide antibiotics occurs at the amino acid level during non-ribosomal peptide synthesis

Halogenation plays a significant role in the activity of the glycopeptide antibiotics (GPAs), although up until now the timing and therefore exact substrate involved was unclear. Here, we present results combined from in vivo and in vitro studies that reveal the substrates for the halogenase enzymes from GPA biosynthesis as amino acid residues bound to peptidyl carrier protein (PCP)-domains from the non-ribosomal peptide synthetase machinery: no activity was detected upon either free amino acids or PCP-bound peptides. Furthermore, we show that the selectivity of GPA halogenase enzymes depends upon both the structure of the bound amino acid and the PCP domain, rather than being driven solely via the PCP domain. These studies provide the first detailed understanding of how halogenation is performed during GPA biosynthesis and highlight the importance and versatility of trans-acting enzymes that operate during peptide assembly by non-ribosomal peptide synthetases.

Diversity of nature's assembly lines - recent discoveries in non-ribosomal peptide synthesis

The biosynthesis of complex natural products by non-ribosomal peptide synthetases (NRPSs) and the related polyketide synthases (PKSs) represents a major source of important bioactive compounds. These large, multi-domain machineries are able to produce a fascinating range of molecules due to the nature of their modular architectures, which allows natural products to be assembled and tailored in a modular, step-wise fashion. In recent years there has been significant progress in characterising the important domains and underlying mechanisms of non-ribosomal peptide synthesis. More significantly, several studies have uncovered important examples of novel activity in many NRPS domains. These discoveries not only greatly increase the structural diversity of the possible products of NRPS machineries but - possibly more importantly - they improve our understanding of what is a highly important, yet complex, biosynthetic apparatus. In this review, several recent examples of novel NRPS function will be introduced, which highlight the range of previously uncharacterised activities that have now been detected in the biosynthesis of important natural products by these mega-enzyme synthetases.

β-Lactone formation during product release from a nonribosomal peptide synthetase

Nonribosomal peptide synthetases (NRPSs) are multidomain modular biosynthetic assembly lines that polymerize amino acids into a myriad of biologically active nonribosomal peptides (NRPs). NRPS thioesterase (TE) domains employ diverse release strategies for off-loading thioester-tethered polymeric peptides from termination modules typically via hydrolysis, aminolysis, or cyclization to provide mature antibiotics as carboxylic acids/esters, amides, and lactams/lactones, respectively. Here we report the enzyme-catalyzed formation of a highly strained β-lactone ring during TE-mediated cyclization of a β-hydroxythioester to release the antibiotic obafluorin (Obi) from an NRPS assembly line. The Obi NRPS (ObiF) contains a type I TE domain with a rare catalytic cysteine residue that plays a direct role in β-lactone ring formation. We present a detailed genetic and biochemical characterization of the entire Obi biosynthetic gene cluster in plant-associated Pseudomonas fluorescens ATCC 39502 that establishes a general strategy for β-lactone biogenesis.

Heteroatom-Heteroatom Bond Formation in Natural Product Biosynthesis

Natural products that contain functional groups with heteroatom-heteroatom linkages (X-X, where X = N, O, S, and P) are a small yet intriguing group of metabolites. The reactivity and diversity of these structural motifs has captured the interest of synthetic and biological chemists alike. Functional groups containing X-X bonds are found in all major classes of natural products and often impart significant biological activity. This review presents our current understanding of the biosynthetic logic and enzymatic chemistry involved in the construction of X-X bond containing functional groups within natural products. Elucidating and characterizing biosynthetic pathways that generate X-X bonds could both provide tools for biocatalysis and synthetic biology, as well as guide efforts to uncover new natural products containing these structural features.

Nonribosomal Peptide Synthesis-Principles and Prospects

Nonribosomal peptide synthetases (NRPSs) are large multienzyme machineries that assemble numerous peptides with large structural and functional diversity. These peptides include more than 20 marketed drugs, such as antibacterials (penicillin, vancomycin), antitumor compounds (bleomycin), and immunosuppressants (cyclosporine). Over the past few decades biochemical and structural biology studies have gained mechanistic insights into the highly complex assembly line of nonribosomal peptides. This Review provides state-of-the-art knowledge on the underlying mechanisms of NRPSs and the variety of their products along with detailed analysis of the challenges for future reprogrammed biosynthesis. Such a reprogramming of NRPSs would immediately spur chances to generate analogues of existing drugs or new compound libraries of otherwise nearly inaccessible compound structures.

Linking Biosynthetic Gene Clusters to their Metabolites via Pathway- Targeted Molecular Networking

The connection of microbial biosynthetic gene clusters to the small molecule metabolites they encode is central to the discovery and characterization of new metabolic pathways with ecological and pharmacological potential. With increasing microbial genome sequence information being deposited into publicly available databases, it is clear that microbes have the coding capacity for many more biologically active small molecules than previously realized. Of increasing interest are the small molecules encoded by the human microbiome, as these metabolites likely mediate a variety of currently uncharacterized human-microbe interactions that influence health and disease. In this mini-review, we describe the ongoing biosynthetic, structural, and functional characterizations of the genotoxic colibactin pathway in gut bacteria as a thematic example of linking biosynthetic gene clusters to their metabolites. We also highlight other natural products that are produced through analogous biosynthetic logic and comment on some current disconnects between bioinformatics predictions and experimental structural characterizations. Lastly, we describe the use of pathway-targeted molecular networking as a tool to characterize secondary metabolic pathways within complex metabolomes and to aid in downstream metabolite structural elucidation efforts.

Convergent biosynthetic pathways to β-lactam antibiotics

Five naturally-occurring families of β-lactams have inspired a class of drugs that constitute >60% of the antimicrobials used in human medicine. Their biosynthetic pathways reveal highly individualized synthetic strategies that yet converge on a common azetidinone ring assembled in structural contexts that confer selective binding and inhibition of d,d-transpeptidases that play essential roles in bacterial cell wall (peptidoglycan) biosynthesis. These enzymes belong to a single 'clan' of evolutionarily distinct serine hydrolases whose active site geometry and mechanism of action is specifically matched by these antibiotics for inactivation that is kinetically competitive with their native function. Unusual enzyme-mediated reactions and catalytic multitasking in these pathways are discussed with particular attention to the diverse ways the β-lactam itself is generated, and more broadly how the intrinsic reactivity of this core structural element is modulated in natural systems through the introduction of ring strain and electronic effects.

New Structural Data Reveal the Motion of Carrier Proteins in Nonribosomal Peptide Synthesis

The nonribosomal peptide synthetases (NRPSs) are one of the most promising resources for the production of new bioactive molecules. The mechanism of NRPS catalysis is based around sequential catalytic domains: these are organized into modules, where each module selects, modifies, and incorporates an amino acid into the growing peptide. The intermediates formed during NRPS catalysis are delivered between enzyme centers by peptidyl carrier protein (PCP) domains, which makes PCP interactions and movements crucial to NRPS mechanism. PCP movement has been linked to the domain alternation cycle of adenylation (A) domains, and recent complete NRPS module structures provide support for this hypothesis. However, it appears as though the A domain alternation alone is insufficient to account for the complete NRPS catalytic cycle and that the loaded state of the PCP must also play a role in choreographing catalysis in these complex and fascinating molecular machines.

Cyclization of polyketides and non-ribosomal peptides on and off their assembly lines

Modular polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are multifunctional megaenzymes that serve as templates to program the assembly of short carboxylic acids and amino acids in a primarily co-linear manner. The variation, combination, permutation and evolution of their functional units (e.g., modules, domains and proteins) along with their association with external enzymes have resulted in the generation of numerous versions of templates, the roles of which have not been fully recognized in the structural diversification of polyketides, non-ribosomal peptides and their hybrids present in nature. In this Highlight, we focus on the assembly-line enzymology and associated chemistry by providing examples of some newly characterized cyclization reactions that occur on and off the assembly lines during and after chain elongation for the purpose of elucidating the template effects of PKSs and NRPSs. A fundamental understanding of the underlying biosynthetic logic would facilitate the elucidation of chemical information contained within the PKS or NRPS templates and benefit the development of strategies for genome mining, biosynthesis-inspired chemical synthesis and combinatorial biosynthesis.

A Peptidyl-Transesterifying Type I Thioesterase in Salinamide Biosynthesis

Salinamide A belongs to a rare class of bicyclic depsipeptide antibiotics in which the installation of a (4-methylhexa-2,4-dienoyl)glycine handle across a hexadepsipeptide core contributes to its chemical complexity and biological properties. Herein, we report the genetic and biochemical basis for salinamide construction in the marine bacterium Streptomyces sp. CNB-091, which involves a novel intermolecular transesterification reaction catalyzed by a type I thioesterase. Heterologous expression studies revealed the central role of the nonribosomal peptide synthetase Sln9 in constructing and installing the distinctive acylglycine "basket handle" of salinamide. Biochemical characterization of the Sln9 thioesterase domain established that transesterification of the serine residue of desmethylsalinamide E with acylated glycyl thioesters yields desmethylsalinamide C.

Polyketide synthase and non-ribosomal peptide synthetase thioesterase selectivity: logic gate or a victim of fate?

Thioesterases (TEs) are product offloading enzymes from FAS, PKS, and NRPS complexes. We review the diversity, structure, and mechanism of PKS and NRPS TEs and analyze TE loading and release steps as possible logic gates with a view to predicting TE function in new pathways.

β-Lactam formation by a non-ribosomal peptide synthetase during antibiotic biosynthesis

Non-ribosomal peptide synthetases (NRPSs) are giant enzymes comprised of modules that house repeated sets of functional domains, which select, activate and couple amino acids drawn from a pool of nearly 500 potential building blocks.¹ The structurally and stereochemically diverse peptides generated in this manner underlie the biosynthesis of a large sector of natural products. Many of their derived metabolites are bioactive such as the antibiotics vancomycin, bacitracin, daptomycin and the β-lactam-containing penicillins, cephalosporins and nocardicins. Although penicillins and cephalosporins are synthesised from a classically derived NRPS tripeptide (from ACVS, δ-(L-α-aminoadipyl)–L-cysteinyl–D-valine synthetase)², we now report an unprecedented NRPS activity to both assemble a serine-containing peptide and mediate its cyclisation to the critical β-lactam ring of the nocardicin family of antibiotics. A histidine-rich condensation (C) domain, which typically carries out peptide bond formation during product assembly, was found to also synthesise the embedded 4-membered ring. Here, a mechanism is proposed and supporting experiments are described, which is distinct from the pathways that have evolved to the three other β-lactam antibiotic families: penicillin/cephalosporins, clavams and carbapenems. These findings raise the possibility that β-lactam rings can be regio- and stereospecifically integrated into engineered peptides for application as, for example, targeted protease inactivators.^3,4

Structural aspects of phenylglycines, their biosynthesis and occurrence in peptide natural products

Phenylglycine-type amino acids occur in a wide variety of peptide natural products. Herein structures and properties of these peptides as well as the biosynthetic origin and incorporation of phenylglycines are discussed.