Engineering enzymatic assembly lines to produce new antibiotics

Direct α-Hydroxy Acid Loading onto a Bacterial Thiotemplate Assembly Line via a Multienzyme Gateway

Various nonribosomal peptide synthetases (NRPSs) create structural and functional diversity by incorporating α-hydroxy acids into peptide backbones. Trigonic acid, an unusual cyclopropanol-substituted hydroxy acid, is the source of the molecular warhead of malleicyprol, a critical virulence factor of human and animal pathogens of the Burkholderia pseudomallei (BP) group. The process of selecting and loading this building block remained enigmatic as the NRPS module designated for this task is incomplete. Using a combination of bioinformatics, mutational analyses, targeted metabolomics, and in vitro biochemical assays, we show that two trans-acting enzymes are required to load this central building block onto the modular assembly line. An adenylation-thiolation didomain enzyme (BurJ) activates trigonic acid, followed by the translocation of the enzyme-bound α-hydroxy acid thioester by an FkbH-like protein with a mutated phosphatase domain (BurH). This specialized gateway is the first reported direct loading of an α-hydroxy acid onto a bona fide NRPS module in bacteria and expands the synthetic biology toolbox for the site-specific incorporation of non-canonical building blocks. Moreover, insight into the biochemical basis of virulence factor biosynthesis can provide a foundation for developing enzyme inhibitors as anti-virulence therapeutics against BP pathogen infections.

Advances, opportunities, and challenges in methods for interrogating the structure activity relationships of natural products

Time span in literature: 1985-early 2024Natural products play a key role in drug discovery, both as a direct source of drugs and as a starting point for the development of synthetic compounds. Most natural products are not suitable to be used as drugs without further modification due to insufficient activity or poor pharmacokinetic properties. Choosing what modifications to make requires an understanding of the compound's structure-activity relationships. Use of structure-activity relationships is commonplace and essential in medicinal chemistry campaigns applied to human-designed synthetic compounds. Structure-activity relationships have also been used to improve the properties of natural products, but several challenges still limit these efforts. Here, we review methods for studying the structure-activity relationships of natural products and their limitations. Specifically, we will discuss how synthesis, including total synthesis, late-stage derivatization, chemoenzymatic synthetic pathways, and engineering and genome mining of biosynthetic pathways can be used to produce natural product analogs and discuss the challenges of each of these approaches. Finally, we will discuss computational methods including machine learning methods for analyzing the relationship between biosynthetic genes and product activity, computer aided drug design techniques, and interpretable artificial intelligence approaches towards elucidating structure-activity relationships from models trained to predict bioactivity from chemical structure. Our focus will be on these latter topics as their applications for natural products have not been extensively reviewed. We suggest that these methods are all complementary to each other, and that only collaborative efforts using a combination of these techniques will result in a full understanding of the structure-activity relationships of natural products.

Multiple pathways impact the swarming motility of Pseudomonas fluorescens Pf0-1

Swarming motility in pseudomonads typically requires both a functional flagellum and the production/secretion of a biosurfactant. Published work has shown that the wild-type Pseudomonas fluorescens Pf0-1 is swarming deficient due to a point mutation in the gacA gene, which until recently was thought to inactivate rather than attenuate the Gac/Rsm pathway. As a result, little is known about the underlying mechanisms that regulate swarming motility by P. fluorescens Pf0-1. Here, we demonstrate that a ΔrsmA ΔrsmE ΔrsmI mutant, which phenotypically mimics Gac/Rsm pathway overstimulation, is proficient at swarming motility. RsmA and RsmE appear to play a key role in this regulation. Transposon mutagenesis of the ΔrsmA ΔrsmE ΔrsmI mutant identified multiple factors that impact swarming motility, including pathways involved in flagellar synthesis and biosurfactant production/secretion. We find that loss of genes linked to biosurfactant Gacamide A biosynthesis or secretion impacts swarming motility, as does loss of the alternative sigma factor FliA, which results in a defect in flagellar function. Collectively, these findings provide evidence that P. fluorescens Pf0-1 can swarm if the Gac/Rsm pathway is activated, highlight the regulatory complexity of swarming motility in this strain, and demonstrate that the cyclic lipopeptide Gacamide A is utilized as a biosurfactant for swarming motility.IMPORTANCESwarming motility is a coordinated process that allows communities of bacteria to collectively move across a surface. For P. fluorescens Pf0-1, this phenotype is notably absent in the parental strain, and to date, little is known about the regulation of swarming in this strain. Here, we identify RsmA and RsmE as key repressors of swarming motility via modulating the levels of biosurfactant production/secretion. Using transposon mutagenesis and subsequent genetic analyses, we further identify potential regulatory mechanisms of swarming motility and link Gacamide A biosynthesis and transport machinery to swarming motility.

Late-stage diversification of bacterial natural products through biocatalysis

Bacterial natural products (BNPs) are very important sources of leads for drug development and chemical novelty. The possibility to perform late-stage diversification of BNPs using biocatalysis is an attractive alternative route other than total chemical synthesis or metal complexation reactions. Although biocatalysis is gaining popularity as a green chemistry methodology, a vast majority of orphan sequenced genomic data related to metabolic pathways for BNP biosynthesis and its tailoring enzymes are underexplored. In this review, we report a systematic overview of biotransformations of 21 molecules, which include derivatization by halogenation, esterification, reduction, oxidation, alkylation and nitration reactions, as well as degradation products as their sub-derivatives. These BNPs were grouped based on their biological activities into antibacterial (5), antifungal (5), anticancer (5), immunosuppressive (2) and quorum sensing modulating (4) compounds. This study summarized 73 derivatives and 16 degradation sub-derivatives originating from 12 BNPs. The highest number of biocatalytic reactions was observed for drugs that are already in clinical use: 28 reactions for the antibacterial drug vancomycin, followed by 18 reactions reported for the immunosuppressive drug rapamycin. The most common biocatalysts include oxidoreductases, transferases, lipases, isomerases and haloperoxidases. This review highlights biocatalytic routes for the late-stage diversification reactions of BNPs, which potentially help to recognize the structural optimizations of bioactive scaffolds for the generation of new biomolecules, eventually leading to drug development.

Evolution-inspired engineering of nonribosomal peptide synthetases

Many clinically used drugs are derived from or inspired by bacterial natural products that often are produced through nonribosomal peptide synthetases (NRPSs), megasynthetases that activate and join individual amino acids in an assembly line fashion. In this work, we describe a detailed phylogenetic analysis of several bacterial NRPSs that led to the identification of yet undescribed recombination sites within the thiolation (T) domain that can be used for NRPS engineering. We then developed an evolution-inspired "eXchange Unit between T domains" (XUT) approach, which allows the assembly of NRPS fragments over a broad range of GC contents, protein similarities, and extender unit specificities, as demonstrated for the specific production of a proteasome inhibitor designed and assembled from five different NRPS fragments.

Trapping of a Polyketide Synthase Module after C-C Bond Formation Reveals Transient Acyl Carrier Domain Interactions

Modular polyketide synthases (PKSs) are giant assembly lines that produce an impressive range of biologically active compounds. However, our understanding of the structural dynamics of these megasynthases, specifically the delivery of acyl carrier protein (ACP)-bound building blocks to the catalytic site of the ketosynthase (KS) domain, remains severely limited. Using a multipronged structural approach, we report details of the inter-domain interactions after C-C bond formation in a chain-branching module of the rhizoxin PKS. Mechanism-based crosslinking of an engineered module was achieved using a synthetic substrate surrogate that serves as a Michael acceptor. The crosslinked protein allowed us to identify an asymmetric state of the dimeric protein complex upon C-C bond formation by cryo-electron microscopy (cryo-EM). The possible existence of two ACP binding sites, one of them a potential "parking position" for substrate loading, was also indicated by AlphaFold2 predictions. NMR spectroscopy showed that a transient complex is formed in solution, independent of the linker domains, and photochemical crosslinking/mass spectrometry of the standalone domains allowed us to pinpoint the interdomain interaction sites. The structural insights into a branching PKS module arrested after C-C bond formation allows a better understanding of domain dynamics and provides valuable information for the rational design of modular assembly lines.

Multiple Pathways Impact Swarming Motility of Pseudomonas fluorescens Pf0-1

Swarming motility in pseudomonads typically requires both a functional flagellum and production/secretion of a biosurfactant. Published work has shown that the wild-type Pseudomonas fluorescens Pf0-1 is swarming-deficient due to a point mutation in the gacA gene, which until recently, was thought to inactivate rather than attenuate the Gac/Rsm pathway. As a result, little is known about the underlying mechanisms that regulate swarming motility by P. fluorescens Pf0-1. Here, we demonstrate that a ΔrsmA ΔrsmE ΔrsmI mutant, which phenotypically mimics Gac/Rsm pathway overstimulation, is proficient at swarming motility. RsmA and RsmE appear to play a key role in this regulation. Transposon mutagenesis of the ΔrsmA ΔrsmE ΔrsmI mutant identified multiple factors that impact swarming motility, including pathways involved in flagellar synthesis and biosurfactant production/secretion. We find that loss of genes linked to biosurfactant Gacamide A biosynthesis or secretion impact swarming motility, as does loss of the alternative sigma factor FliA, which results in a defect in flagellar function. Collectively, these findings provide evidence that P. fluorescens Pf0-1 can swarm if the Gac/Rsm pathway is activated, highlight the regulatory complexity of swarming motility in this strain, and demonstrate that the cyclic lipopeptide Gacamide A is utilized as a biosurfactant for swarming motility.

Genomic characterization of Bacillus cereus isolated from food poisoning cases revealed the mechanism of toxin production

Introduction

Bacillus cereus is a ubiquitous opportunistic human pathogen that causes food intoxications worldwide. However, the genomic characteristics and pathogenic mechanisms of B. cereus are still unclear.

Methods

Here, we isolated and purified nine strains of B. cereus (LY01-LY09) that caused vomiting, diarrhea and other symptoms from four foodborne outbreaks happened in Guizhou Province in southwest China from June to September 2021. After colony observation, Gram staining, microscopic examination and biochemical test, they were identified as B. cereus. The genomic characteristics, phylogenetic relationships and virulence factors of the isolated strains were analyzed at the genome level. Genome sequencing, comparative genomic analysis, secondary metabolite analysis and quantitative PCR were utilized to give a thorough exploration of the strains.

Results

We obtained the genome maps of LY01-LY09 and found that LY01-LY09 had a complex interspecific relationship with B. anthracis and B. thuringiensis. We also observed a contraction of gene families in LY01-LY09, and the contracted families were mainly associated with prophage, which contributed to the species diversity of B. cereus. The Hsp20 gene family underwent a rapid evolution in LY01-LY09, which facilitated the adaptation of the strains to adverse environmental conditions. Moreover, the LY01-LY09 strains exhibited a higher copy number in the non-ribosomal polypeptide synthetase (NRPS) genes and carried the complete cereulide synthetase (ces) gene cluster sequences. Considering that the NRPS system is a classical regulatory mechanism for emetic toxin synthesis, we hypothesized that LY01-LY09 could synthesize emetic toxins through the regulation of ces gene clusters by the NRPS system.

Discussion

These findings are important for further investigation into the evolutionary relationship between B. cereus and their related species, as well as the underlying mechanisms governing the synthesis and secretion of bacterial toxins.

High-Throughput Engineering of Nonribosomal Extension Modules

Nonribosomal peptides constitute an important class of natural products that display a wide range of bioactivities. They are biosynthesized by large assembly lines called nonribosomal peptide synthetases (NRPSs). Engineering NRPS modules represents an attractive strategy for generating customized synthetases for the production of peptide variants with improved properties. Here, we explored the yeast display of NRPS elongation and termination modules as a high-throughput screening platform for assaying adenylation domain activity and altering substrate specificity. Depending on the module, display of A-T bidomains or C-A-T tridomains, which also include an upstream condensation domain, proved to be most effective. Reprograming a tyrocidine synthetase elongation module to accept 4-propargyloxy-phenylalanine, a noncanonical amino acid that is not activated by the native protein, illustrates the utility of this approach for altering NRPS specificity at internal sites.

Analysing Megasynthetase Mutants at High Throughput Using Droplet Microfluidics

Nonribosomal peptide synthetases (NRPSs) are giant enzymatic assembly lines that deliver many pharmaceutically valuable natural products, including antibiotics. As the search for new antibiotics motivates attempts to redesign nonribosomal metabolic pathways, more robust and rapid sorting and screening platforms are needed. Here, we establish a microfluidic platform that reliably detects production of the model nonribosomal peptide gramicidin S. The detection is based on calcein-filled sensor liposomes yielding increased fluorescence upon permeabilization. From a library of NRPS mutants, the sorting platform enriches the gramicidin S producer 14.5-fold, decreases internal stop codons 250-fold, and generates enrichment factors correlating with enzyme activity. Screening for NRPS activity with a reliable non-binary sensor will enable more sophisticated structure-activity studies and new engineering applications in the future.

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.

Biosynthetic incorporation of fluorinated amino acids into the nonribosomal peptide gramicidin S

Fluorine is a key element in medicinal chemistry, as it can significantly enhance the pharmacological properties of drugs. In this study, we aimed to biosynthetically produce fluorinated analogues of the antimicrobial cyclic decapeptide gramicidin S (GS). However, our results show that the A-domain of the NRPS module GrsA rejects 4-fluorinated analogues of its native substrate Phe due to an interrupted T-shaped aromatic interaction in the binding pocket. We demonstrate that GrsA mutant W239S improves the incorporation of 4-fluorinated Phe into GS both in vitro and in vivo. Our findings provide new insights into the behavior of NRPSs towards fluorinated amino acids and strategies for the engineered biosynthesis of fluorinated peptides.

Directed Evolution of Piperazic Acid Incorporation by a Nonribosomal Peptide Synthetase

Engineering of biosynthetic enzymes is increasingly employed to synthesize structural analogues of antibiotics. Of special interest are nonribosomal peptide synthetases (NRPSs) responsible for the production of important antimicrobial peptides. Here, directed evolution of an adenylation domain of a Pro-specific NRPS module completely switched substrate specificity to the non-standard amino acid piperazic acid (Piz) bearing a labile N-N bond. This success was achieved by UPLC-MS/MS-based screening of small, rationally designed mutant libraries and can presumably be replicated with a larger number of substrates and NRPS modules. The evolved NRPS produces a Piz-derived gramicidin S analogue. Thus, we give new impetus to the too-early dismissed idea that widely accessible low-throughput methods can switch the specificity of NRPSs in a biosynthetically useful fashion.

Guidelines for Optimizing Type S Nonribosomal Peptide Synthetases

Bacterial biosynthetic assembly lines, such as nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs), play a crucial role in the synthesis of natural products that have significant therapeutic potential. The ability to engineer these biosynthetic assembly lines offers opportunities to produce artificial nonribosomal peptides, polyketides, and their hybrids with improved properties. In this study, we introduced a synthetic NRPS variant, termed type S NRPS, which simplifies the engineering process and enables biocombinatorial approaches for generating nonribosomal peptide libraries in a parallelized high-throughput manner. However, initial generations of type S NRPSs exhibited a bottleneck that led to significantly reduced production yields. To address this challenge, we employed two optimization strategies. First, we truncated SYNZIPs from the N- and/or C-terminus of the NRPS. SYNZIPs comprise a large set of well-characterized synthetic protein interaction reagents. Second, we incorporated a structurally flexible glycine-serine linker between the NRPS protein and the attached SYNZIP, aiming to improve dynamic domain-domain interactions. Through an iterative optimization process, we achieved remarkable improvements in production yields, with titer increases of up to 55-fold compared to the nonoptimized counterparts. These optimizations successfully restored production levels of type S NRPSs to those observed in wild-type NRPSs and even surpassed them. Overall, our findings demonstrate the potential of engineering bacterial biosynthetic assembly lines for the production of artificial nonribosomal peptides. In addition, optimizing the SYNZIP toolbox can have valuable implications for diverse applications in synthetic biology, such as metabolic engineering, cell signaling studies, or engineering of other multienzyme complexes, such as PKSs.

Engineering Siderophore Biosynthesis and Regulation Pathways to Increase Diversity and Availability

Siderophores are small metal chelators synthesized by numerous organisms to access iron. These secondary metabolites are ubiquitously present on Earth, and because their production represents the main strategy to assimilate iron, they play an important role in both positive and negative interactions between organisms. In addition, siderophores are used in biotechnology for diverse applications in medicine, agriculture and the environment. The generation of non-natural siderophore analogs provides a new opportunity to create new-to-nature chelating biomolecules that can offer new properties to expand applications. This review summarizes the main strategies of combinatorial biosynthesis that have been used to generate siderophore analogs. We first provide a brief overview of siderophore biosynthesis, followed by a description of the strategies, namely, precursor-directed biosynthesis, the design of synthetic or heterologous pathways and enzyme engineering, used in siderophore biosynthetic pathways to create diversity. In addition, this review highlights the engineering strategies that have been used to improve the production of siderophores by cells to facilitate their downstream utilization.

Advances in the adenylation domain: discovery of diverse non-ribosomal peptides

Non-ribosomal peptide synthetases are mega-enzyme assembly lines that synthesize many clinically useful compounds. As a gatekeeper, they have an adenylation (A)-domain that controls substrate specificity and plays an important role in product structural diversity. This review summarizes the natural distribution, catalytic mechanism, substrate prediction methods, and in vitro biochemical analysis of the A-domain. Taking genome mining of polyamino acid synthetases as an example, we introduce research on mining non-ribosomal peptides based on A-domains. We discuss how non-ribosomal peptide synthetases can be engineered based on the A-domain to obtain novel non-ribosomal peptides. This work provides guidance for screening non-ribosomal peptide-producing strains, offers a method to discover and identify A-domain functions, and will accelerate the engineering and genome mining of non-ribosomal peptide synthetases. KEY POINTS: • Introducing adenylation domain structure, substrate prediction, and biochemical analysis methods • Advances in mining homo polyamino acids based on adenylation domain analysis • Creating new non-ribosomal peptides by engineering adenylation domains.

Investigating and Optimizing the Lysate-Based Expression of Nonribosomal Peptide Synthetases Using a Reporter System

Lysate-based cell-free expression (CFE) systems are accessible platforms for expressing proteins that are difficult to synthesize in vivo, such as nonribosomal peptide synthetases (NRPSs). NRPSs are large (>100 kDa), modular enzyme complexes that synthesize bioactive peptide natural products. This synthetic process is analogous to transcription/translation (TX/TL) in lysates, resulting in potential resource competition between NRPS expression and NRPS activity in cell-free environments. Moreover, CFE conditions depend on the size and structure of the protein. Here, a reporter system for rapidly investigating and optimizing reaction environments for NRPS CFE is described. This strategy is demonstrated in E. coli lysate reactions using blue pigment synthetase A (BpsA), a model NRPS, carrying a C-terminal tetracysteine (TC) tag which forms a fluorescent complex with the biarsenical dye, FlAsH. A colorimetric assay was adapted for lysate reactions to detect the blue pigment product, indigoidine, of cell-free expressed BpsA-TC, confirming that the tagged enzyme is catalytically active. An optimized protocol for end point TC/FlAsH complex measurements in reactions enables quick comparisons of full-length BpsA-TC expressed under different reaction conditions, defining unique requirements for NRPS expression that are related to the protein's catalytic activity and size. Importantly, these protein-dependent CFE conditions enable higher indigoidine titer and improve the expression of other monomodular NRPSs. Notably, these conditions differ from those used for the expression of superfolder GFP (sfGFP), a common reporter for optimizing lysate-based CFE systems, indicating the necessity for tailored reporters to optimize expression for specific enzyme classes. The reporter system is anticipated to advance lysate-based CFE systems for complex enzyme synthesis, enabling natural product discovery.

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.

Metabolic pathway assembly using docking domains from type I cis-AT polyketide synthases

Engineered metabolic pathways in microbial cell factories often have no natural organization and have challenging flux imbalances, leading to low biocatalytic efficiency. Modular polyketide synthases (PKSs) are multienzyme complexes that synthesize polyketide products via an assembly line thiotemplate mechanism. Here, we develop a strategy named mimic PKS enzyme assembly line (mPKSeal) that assembles key cascade enzymes to enhance biocatalytic efficiency and increase target production by recruiting cascade enzymes tagged with docking domains from type I cis-AT PKS. We apply this strategy to the astaxanthin biosynthetic pathway in engineered Escherichia coli for multienzyme assembly to increase astaxanthin production by 2.4-fold. The docking pairs, from the same PKSs or those from different cis-AT PKSs evidently belonging to distinct classes, are effective enzyme assembly tools for increasing astaxanthin production. This study addresses the challenge of cascade catalytic efficiency and highlights the potential for engineering enzyme assembly.

Assembly artificial pathway in design connecting media can increase biosynthetic efficiency, but the choice of connecting media is limited. Here, the authors develop a new protein assembly strategy using a pool of docking peptides from polyketide synthase and show its application in astaxanthin biosynthesis in E. coli.

NMR as a readout to monitor and restore the integrity of complex chemoenzymatic reactions

The non-invasive nature of NMR offers a means to monitor biochemical reactions in situ at the atomic level. We harness this advantage to monitor a complex chemoenzymatic reaction that sequentially modifies reagents and loads the product on a nonribosomal peptide synthetase carrier protein. We present a protocol including a pulse sequence that permits to assess both the integrity of reagents and the completion of each step in the reaction, thus alleviating otherwise time-consuming and costly approaches to debug and repeat inefficient reactions. This study highlights the importance of NMR as a tool to establish reliable and reproducible experimental conditions in biochemical studies.

Bifurcation drives the evolution of assembly-line biosynthesis

Reprogramming biosynthetic assembly-lines is a topic of intense interest. This is unsurprising as the scaffolds of most antibiotics in current clinical use are produced by such pathways. The modular nature of assembly-lines provides a direct relationship between the sequence of enzymatic domains and the chemical structure of the product, but rational reprogramming efforts have been met with limited success. To gain greater insight into the design process, we wanted to examine how Nature creates assembly-lines and searched for biosynthetic pathways that might represent evolutionary transitions. By examining the biosynthesis of the anti-tubercular wollamides, we uncover how whole gene duplication and neofunctionalization can result in pathway bifurcation. We show that, in the case of the wollamide biosynthesis, neofunctionalization is initiated by intragenomic recombination. This pathway bifurcation leads to redundancy, providing the genetic robustness required to enable large structural changes during the evolution of antibiotic structures. Should the new product be non-functional, gene loss can restore the original genotype. However, if the new product confers an advantage, depreciation and eventual loss of the original gene creates a new linear pathway. This provides the blind watchmaker equivalent to the design, build, test cycle of synthetic biology.

Type S Non‐Ribosomal Peptide Synthetases for the Rapid Generation of Tailormade Peptide Libraries**

Bacterial natural products in general, and non‐ribosomally synthesized peptides in particular, are structurally diverse and provide us with a broad range of pharmaceutically relevant bioactivities. Yet, traditional natural product research suffers from rediscovering the same scaffolds and has been stigmatized as inefficient, time‐, labour‐ and cost‐intensive. Combinatorial chemistry, on the other hand, can produce new molecules in greater numbers, cheaper and in less time than traditional natural product discovery, but also fails to meet current medical needs due to the limited biologically relevant chemical space that can be addressed. Consequently, methods for the high throughput generation of new natural products would offer a new approach to identifying novel bioactive chemical entities for the hit to lead phase of drug discovery programs. As a follow‐up to our previously published proof‐of‐principle study on generating bipartite type S non‐ribosomal peptide synthetases (NRPSs), we now envisaged the de novo generation of non‐ribosomal peptides (NRPs) on an unreached scale. Using synthetic zippers, we split NRPSs in up to three subunits and rapidly generated different bi‐ and tripartite NRPS libraries to produce 49 peptides, peptide derivatives, and de novo peptides at good titres up to 145 mg L⁻¹. A further advantage of type S NRPSs not only is the possibility to easily expand the created libraries by re‐using previously created type S NRPS, but that functions of individual domains as well as domain‐domain interactions can be studied and assigned rapidly.

Recent Advances in Biocatalysis for Drug Synthesis

Biocatalysis is constantly providing novel options for the synthesis of active pharmaceutical ingredients (APIs). In addition to drug development and manufacturing, biocatalysis also plays a role in drug discovery and can support many active ingredient syntheses at an early stage to build up entire scaffolds in a targeted and preparative manner. Recent progress in recruiting new enzymes by genome mining and screening or adapting their substrate, as well as product scope, by protein engineering has made biocatalysts a competitive tool applied in academic and industrial spheres. This is especially true for the advances in the field of nonribosomal peptide synthesis and enzyme cascades that are expanding the capabilities for the discovery and synthesis of new bioactive compounds via biotransformation. Here we highlight some of the most recent developments to add to the portfolio of biocatalysis with special relevance for the synthesis and late-stage functionalization of APIs, in order to bypass pure chemical processes.

Developing crosslinkers specific for epimerization domain in NRPS initiation modules to evaluate mechanism

Nonribosomal peptide synthetases (NRPSs) are complex multi-modular enzymes containing catalytic domains responsible for the loading and incorporation of amino acids into natural products. These unique molecular factories can produce peptides with nonproteinogenic d-amino acids in which the epimerization (E) domain catalyzes the conversion of l-amino acids to d-amino acids, but its mechanism remains not fully understood. Here, we describe the development of pantetheine crosslinking probes that mimic the natural substrate l-Phe of the initiation module of tyrocidine synthetase, TycA, to elucidate and study the catalytic residues of the E domain. Mechanism-based crosslinking assays and MALDI-TOF MS were used to identify both H743 and E882 as the crosslinking site residues, demonstrating their roles as catalytic bases. Mutagenesis studies further validated these results and allowed the comparison of reactivity between the catalytic residues, concluding that glutamate acts as the dominant nucleophile in the crosslinking reaction, resembling the deprotonation of the Cα-H of amino acids in the epimerization reaction. The crosslinking probes employed in these studies provide new tools for studying the molecular details of E domains, as well as the potential to study C domains. In particular, they would elucidate key information for how these domains function and interact with their substrates in nature, further enhancing the knowledge needed to assist combinatorial biosynthetic efforts of NRPS systems to produce novel compounds.

Complex peptide natural products: Biosynthetic principles, challenges and opportunities for pathway engineering

Complex peptide natural products exhibit diverse biological functions and a wide range of physico-chemical properties. As a result, many peptides have entered the clinics for various applications. Two main routes for the biosynthesis of complex peptides have evolved in nature: ribosomally synthesized and post-translationally modified peptide (RiPP) biosynthetic pathways and non-ribosomal peptide synthetases (NRPSs). Insights into both bioorthogonal peptide biosynthetic strategies led to the establishment of universal principles for each of the two routes. These universal rules can be leveraged for the targeted identification of novel peptide biosynthetic blueprints in genome sequences and used for the rational engineering of biosynthetic pathways to produce non-natural peptides. In this review, we contrast the key principles of both biosynthetic routes and compare the different biochemical strategies to install the most frequently encountered peptide modifications. In addition, the influence of the fundamentally different biosynthetic principles on past, current and future engineering approaches is illustrated. Despite the different biosynthetic principles of both peptide biosynthetic routes, the arsenal of characterized peptide modifications encountered in RiPP and NRPS systems is largely overlapping. The continuous expansion of the biocatalytic toolbox of peptide modifying enzymes for both routes paves the way towards the production of complex tailor-made peptides and opens up the possibility to produce NRPS-derived peptides using the ribosomal route and vice versa.

Anti-Zika candidates from a marine fungus with a remarkable biosynthetic repertoire

The study of natural products provides exciting opportunities for the discovery of novel biologically active molecules and biosynthetic pathways. Recently, Yuan and colleagues described 30 cyclic depsipeptides that are biosynthesized by proteins encoded by three distinct gene clusters in the marine fungus, Beauveria felina. Genetic and biochemical studies confirmed the involvement of nonribosomal peptide synthetases in the production of multiple compounds, some of which inhibit Zika virus replication.

Synthetic Zippers as an Enabling Tool for Engineering of Non-Ribosomal Peptide Synthetases*

Non‐ribosomal peptide synthetases (NRPSs) are the origin of a wide range of natural products, including many clinically used drugs. Efficient engineering of these often giant biosynthetic machineries to produce novel non‐ribosomal peptides (NRPs) is an ongoing challenge. Here we describe a cloning and co‐expression strategy to functionally combine NRPS fragments of Gram‐negative and ‐positive origin, synthesising novel peptides at titres up to 220 mg L⁻¹. Extending from the recently introduced definition of eXchange Units (XUs), we inserted synthetic zippers (SZs) to split single protein NRPSs into independently expressed and translated polypeptide chains. These synthetic type of NRPS (type S) enables easier access to engineering, overcomes cloning limitations, and provides a simple and rapid approach to building peptide libraries via the combination of different NRPS subunits.

Recent trends in biocatalysis

Biocatalysis has undergone revolutionary progress in the past century. Benefited by the integration of multidisciplinary technologies, natural enzymatic reactions are constantly being explored. Protein engineering gives birth to robust biocatalysts that are widely used in industrial production. These research achievements have gradually constructed a network containing natural enzymatic synthesis pathways and artificially designed enzymatic cascades. Nowadays, the development of artificial intelligence, automation, and ultra-high-throughput technology provides infinite possibilities for the discovery of novel enzymes, enzymatic mechanisms and enzymatic cascades, and gradually complements the lack of remaining key steps in the pathway design of enzymatic total synthesis. Therefore, the research of biocatalysis is gradually moving towards the era of novel technology integration, intelligent manufacturing and enzymatic total synthesis.

The inherent flexibility of type I non-ribosomal peptide synthetase multienzymes drives their catalytic activities

Non-ribosomal peptide synthetases (NRPSs) are multienzymes that produce complex natural metabolites with many applications in medicine and agriculture. They are composed of numerous catalytic domains that elongate and chemically modify amino acid substrates or derivatives and of non-catalytic carrier protein domains that can tether and shuttle the growing products to the different catalytic domains. The intrinsic flexibility of NRPSs permits conformational rearrangements that are required to allow interactions between catalytic and carrier protein domains. Their large size coupled to this flexibility renders these multi-domain proteins very challenging for structural characterization. Here, we summarize recent studies that offer structural views of multi-domain NRPSs in various catalytically relevant conformations, thus providing an increased comprehension of their catalytic cycle. A better structural understanding of these multienzymes provides novel perspectives for their re-engineering to synthesize new bioactive metabolites.

Automatic reconstruction of metabolic pathways from identified biosynthetic gene clusters

Background

A wide range of bioactive compounds is produced by enzymes and enzymatic complexes encoded in biosynthetic gene clusters (BGCs). These BGCs can be identified and functionally annotated based on their DNA sequence. Candidates for further research and development may be prioritized based on properties such as their functional annotation, (dis)similarity to known BGCs, and bioactivity assays. Production of the target compound in the native strain is often not achievable, rendering heterologous expression in an optimized host strain as a promising alternative. Genome-scale metabolic models are frequently used to guide strain development, but large-scale incorporation and testing of heterologous production of complex natural products in this framework is hampered by the amount of manual work required to translate annotated BGCs to metabolic pathways. To this end, we have developed a pipeline for an automated reconstruction of BGC associated metabolic pathways responsible for the synthesis of non-ribosomal peptides and polyketides, two of the dominant classes of bioactive compounds.

Results

The developed pipeline correctly predicts 72.8% of the metabolic reactions in a detailed evaluation of 8 different BGCs comprising 228 functional domains. By introducing the reconstructed pathways into a genome-scale metabolic model we demonstrate that this level of accuracy is sufficient to make reliable in silico predictions with respect to production rate and gene knockout targets. Furthermore, we apply the pipeline to a large BGC database and reconstruct 943 metabolic pathways. We identify 17 enzymatic reactions using high-throughput assessment of potential knockout targets for increasing the production of any of the associated compounds. However, the targets only provide a relative increase of up to 6% compared to wild-type production rates.

Conclusion

With this pipeline we pave the way for an extended use of genome-scale metabolic models in strain design of heterologous expression hosts. In this context, we identified generic knockout targets for the increased production of heterologous compounds. However, as the predicted increase is minor for any of the single-reaction knockout targets, these results indicate that more sophisticated strain-engineering strategies are necessary for the development of efficient BGC expression hosts.

Structure and biosynthesis of deoxy-polyamine in Xenorhabdus bovienii

Polyamine moieties have been described as part of the fabclavine and zeamine family of natural products. While the corresponding biosynthetic gene clusters have been found in many different proteobacteria, a unique BGC was identified in the entomopathogenic bacterium Xenorhabdus bovienii. Mass spectrometric analysis of a X. bovienii mutant strain revealed a new deoxy-polyamine. The corresponding biosynthesis includes two additional reductive steps, initiated by an additional dehydratase (DH) domain, which was not found in any other Xenorhabdus strain. Moreover, this DH domain could be successfully integrated into homologous biosynthesis pathways, leading to the formation of other deoxy-polyamines. Additional heterologous production experiments revealed that the DH domain could act in cis as well as in trans.

Combinatorial biosynthesis for the generation of new-to-nature peptide antimicrobials

Natural peptide products are a valuable source of important therapeutic agents, including antibiotics, antivirals and crop protection agents. Aided by an increased understanding of structure-activity relationships of these complex molecules and the biosynthetic machineries that produce them, it has become possible to re-engineer complete machineries and biosynthetic pathways to create novel products with improved pharmacological properties or modified structures to combat antimicrobial resistance. In this review, we will address the progress that has been made using non-ribosomally produced peptides and ribosomally synthesized and post-translationally modified peptides as scaffolds for designed biosynthetic pathways or combinatorial synthesis for the creation of novel peptide antimicrobials.

Natural polyenic macrolactams and polycyclic derivatives generated by transannular pericyclic reactions: optimized biogenesis challenging chemical synthesis

Covering from 1992 to the end of 2020-11-20.Genetically-encoded polyenic macrolactams, which are constructed by Nature using hybrid polyketide synthase/nonribosomal peptide synthase (PKSs/NRPSs) assembly lines, are part of the large collection of natural products isolated from bacteria. Activation of cryptic (i.e., silent) gene clusters in these microorganisms has more recently allowed to generate and eventually isolate additional members of the family. Having two unsaturated fragments separated by short saturated chains, the primary macrolactam is posited to undergo transannular reactions and further rearrangements thus leading to the generation of a structurally diverse collection of polycyclic (natural) products and oxidized derivatives. The review will cover the challenges that scientists face on the isolation of these unstable compounds from the cultures of the producing microorganisms, their structural characterization, biological activities, optimized biogenetic routes, as well as the skeletal rearrangements of the primary structures of the natural macrolactams derived from pericyclic reactions of the polyenic fragments. The efforts of the synthetic chemists to emulate Nature on the successful generation and structural confirmation of these natural products will also be reported.

Pinacho D. Personalized Medicine for Antibiotics: The Role of Nanobiosensors in Therapeutic Drug Monitoring

Due to the high bacterial resistance to antibiotics (AB), it has become necessary to adjust the dose aimed at personalized medicine by means of therapeutic drug monitoring (TDM). TDM is a fundamental tool for measuring the concentration of drugs that have a limited or highly toxic dose in different body fluids, such as blood, plasma, serum, and urine, among others. Using different techniques that allow for the pharmacokinetic (PK) and pharmacodynamic (PD) analysis of the drug, TDM can reduce the risks inherent in treatment. Among these techniques, nanotechnology focused on biosensors, which are relevant due to their versatility, sensitivity, specificity, and low cost. They provide results in real time, using an element for biological recognition coupled to a signal transducer. This review describes recent advances in the quantification of AB using biosensors with a focus on TDM as a fundamental aspect of personalized medicine.

Cell-Free Synthesis of Natural Compounds from Genomic DNA of Biosynthetic Gene Clusters

A variety of chemicals can be produced in a living host cell via optimized and engineered biosynthetic pathways. Despite the successes, pathway engineering remains demanding because of the lack of specific functions or substrates in the host cell, the cell's sensitivity in vital physiological processes to the heterologous components, or constrained mass transfer across the membrane. In this study, we show that complex multidomain proteins involved in natural compound biosynthesis can be produced from encoding DNA in vitro in a minimal complex PURE system to directly run multistep reactions. Specifically, we synthesize indigoidine and rhabdopeptides with the in vitro produced multidomain nonribosomal peptide synthetases BpsA and KJ12ABC from the organisms Streptomyces lavendulae and Xenorhabdus KJ12.1, respectively. These in vitro produced proteins are analyzed in yield, post-translational modification and in their ability to synthesize the natural compounds, and compared to recombinantly produced proteins. Our study highlights cell-free PURE system as suitable setting for the characterization of biosynthetic gene clusters that can potentially be harnessed for the rapid engineering of biosynthetic pathways.

Efficient rational modification of non-ribosomal peptides by adenylation domain substitution

Non-ribosomal peptide synthetase (NRPS) enzymes form modular assembly-lines, wherein each module governs the incorporation of a specific monomer into a short peptide product. Modules are comprised of one or more key domains, including adenylation (A) domains, which recognise and activate the monomer substrate; condensation (C) domains, which catalyse amide bond formation; and thiolation (T) domains, which shuttle reaction intermediates between catalytic domains. This arrangement offers prospects for rational peptide modification via substitution of substrate-specifying domains. For over 20 years, it has been considered that C domains play key roles in proof-reading the substrate; a presumption that has greatly complicated rational NRPS redesign. Here we present evidence from both directed and natural evolution studies that any substrate-specifying role for C domains is likely to be the exception rather than the rule, and that novel non-ribosomal peptides can be generated by substitution of A domains alone. We identify permissive A domain recombination boundaries and show that these allow us to efficiently generate modified pyoverdine peptides at high yields. We further demonstrate the transferability of our approach in the PheATE-ProCAT model system originally used to infer C domain substrate specificity, generating modified dipeptide products at yields that are inconsistent with the prevailing dogma.

Ketosynthase Domain Constrains the Design of Polyketide Synthases

Modular polyketide synthases (PKSs) produce complex, bioactive secondary metabolites in assembly line-like multistep reactions. Longstanding efforts to produce novel, biologically active compounds by recombining intact modules to new modular PKSs have mostly resulted in poorly active chimeras and decreased product yields. Recent findings demonstrate that the low efficiencies of modular chimeric PKSs also result from rate limitations in the transfer of the growing polyketide chain across the noncognate module:module interface and further processing of the non-native polyketide substrate by the ketosynthase (KS) domain. In this study, we aim at disclosing and understanding the low efficiency of chimeric modular PKSs and at establishing guidelines for modular PKSs engineering. To do so, we work with a bimodular PKS testbed and systematically vary substrate specificity, substrate identity, and domain:domain interfaces of the KS involved reactions. We observe that KS domains employed in our chimeric bimodular PKSs are bottlenecks with regards to both substrate specificity as well as interaction with the acyl carrier protein (ACP). Overall, our systematic study can explain in quantitative terms why early oversimplified engineering strategies based on the plain shuffling of modules mostly failed and why more recent approaches show improved success rates. We moreover identify two mutations of the KS domain that significantly increased turnover rates in chimeric systems and interpret this finding in mechanistic detail.

Nonribosomal Peptides Produced by Minimal and Engineered Synthetases with Terminal Reductase Domains

Nonribosomal peptide synthetases (NRPSs) use terminal reductase domains for 2‐electron reduction of the enzyme‐bound thioester releasing the generated peptides as C‐terminal aldehydes. Herein, we reveal the biosynthesis of a pyrazine that originates from an aldehyde‐generating minimal NRPS termed ATRed in entomopathogenic Xenorhabdus indica. Reductase domains were also investigated in terms of NRPS engineering and, although no general applicable approach was deduced, we show that they can indeed be used for the production of similar natural and unnatural pyrazinones.

Bacillus Responses to Plant-Associated Fungal and Bacterial Communities

Some members of root-associated Bacillus species have been developed as biocontrol agents due to their contribution to plant protection by directly interfering with the growth of pathogens or by stimulating systemic resistance in their host. As rhizosphere-dwelling bacteria, these bacilli are surrounded and constantly interacting with other microbes via different types of communications. With this review, we provide an updated vision of the molecular and phenotypic responses of Bacillus upon sensing other rhizosphere microorganisms and/or their metabolites. We illustrate how Bacillus spp. may react by modulating the production of secondary metabolites, such as cyclic lipopeptides or polyketides. On the other hand, some developmental processes, such as biofilm formation, motility, and sporulation may also be modified upon interaction, reflecting the adaptation of Bacillus multicellular communities to microbial competitors for preserving their ecological persistence. This review also points out the limited data available and a global lack of knowledge indicating that more research is needed in order to, not only better understand the ecology of bacilli in their natural soil niche, but also to better assess and improve their promising biocontrol potential.