Engineered Biosynthesis of 5/5/6 Type Polycyclic Tetramate Macrolactams in an Ikarugamycin (5/6/5 Type)-Producing Chassis

Achieving Regioselectivity for Remote C-H Activation by Substructure Conformations: an Approach of Paralogous Cytochrome P450 Enzymes

For advanced synthetic intermediates or natural products with multiple unactivated and energetically similar C(sp3)-H bonds, controlling regioselectivity for the C-H activation is particularly challenging. The use of cytochrome P450 enzymes (CYPs) is a promising solution to the 'regioelectivity' challenge in remote C-H activation. Notably, CYPs and organic catalysts share a fundamental principle: they strive to control the distance and geometry between the metal reaction center and the target C-H site. Most structural analyses of the regioselectivity of CYPs are limited to the active pocket, particularly when explaining why regioselectivity could be altered by enzyme engineering through mutagenesis. However, the substructures responsible for forming the active pocket in CYPs are well known to display complex dynamic changes and substrate-induced plasticity. In this context, we highlight a comparative study of the recently reported paralogous CYPs, IkaD and CftA, which achieve different regioselectivity towards the same substrate ikarugamycin by distinct substructure conformations. We propose that substructural conformation-controlled regioselectivity might also be present in CYPs of other natural product biosynthesis pathways, which should be considered when engineering CYPs for regioselective modifications.

Critical analysis of polycyclic tetramate macrolactam biosynthetic gene cluster phylogeny and functional diversity

Polycyclic tetramate macrolactams (PTMs) are bioactive natural products commonly associated with certain actinobacterial and proteobacterial lineages. These molecules have been the subject of numerous structure-activity investigations since the 1970s. New members continue to be pursued in wild and engineered bacterial strains, and advances in PTM biosynthesis suggest their outwardly simplistic biosynthetic gene clusters (BGCs) belie unexpected product complexity. To address the origins of this complexity and understand its influence on PTM discovery, we engaged in a combination of bioinformatics to systematically classify PTM BGCs and PTM-targeted metabolomics to compare the products of select BGC types. By comparing groups of producers and BGC mutants, we exposed knowledge gaps that complicate bioinformatics-driven product predictions. In sum, we provide new insights into the evolution of PTM BGCs while systematically accounting for the PTMs discovered thus far. The combined computational and metabologenomic findings presented here should prove useful for guiding future discovery.IMPORTANCEPolycyclic tetramate macrolactam (PTM) pathways are frequently found within the genomes of biotechnologically important bacteria, including Streptomyces and Lysobacter spp. Their molecular products are typically bioactive, having substantial agricultural and therapeutic interest. Leveraging bacterial genomics for the discovery of new related molecules is thus desirable, but drawing accurate structural predictions from bioinformatics alone remains challenging. This difficulty stems from a combination of previously underappreciated biosynthetic complexity and remaining knowledge gaps, compounded by a stream of yet-uncharacterized PTM biosynthetic loci gleaned from recently sequenced bacterial genomes. We engaged in the following study to create a useful framework for cataloging historic PTM clusters, identifying new cluster variations, and tracing evolutionary paths for these molecules. Our data suggest new PTM chemistry remains discoverable in nature. However, our metabolomic and mutational analyses emphasize the practical limitations of genomics-based discovery by exposing hidden complexity.

A new polycyclic tetramate macrolactam from Allostreptomyces RD068384: stereochemistry and antifungal potential

A new polycyclic tetramate macrolactam designated allostreptamide (1), together with four known congeners, were isolated from the culture extract of Allostreptomyces RD068384. The planar structure of the new compound was elucidated through interpretation of NMR and MS data. The absolute configuration was determined through ROESY and ECD analyses. The isolated compounds revealed antifungal potential against fourteen Candida albicans isolates with minimum inhibitory concentrations (MICs) ranging from 64 to 2048 µg ml-1. Compound 3 showed antibiofilm action and considerably reduced the viability of five isolates (36%) in the formed biofilm. The qRT-PCR revealed that 3 downregulated the BCR1, PLB2, ALS1, and SAP5 biofilm related gene expression. Therefore, 3 could be a promising antifungal therapy for C. albicans infections.

Discovery of Aburatubolactams Reveals Biosynthetic Logic for Distinct 5/5-Type Polycyclic Tetramate Macrolactams

A known polycyclic tetramate macrolactam (aburatubolactam C, 3) and three new ones (aburatubolactams D-F, 4-6, respectively) were isolated from the marine-derived Streptomyces sp. SCSIO 40070. The absolute configuration of 3 was established by X-ray analysis. A combinatorial biosynthetic approach unveiled biosynthetic enzymes dictating the formation of distinct 5/5-type ring systems (such as C7-C14 cyclization by AtlB1 in 5 and C6-C13 cyclization by AtlB2 in 6) in aburatubolactams.

A Mechanistic Understanding of the Distinct Regio- and Chemoselectivity of Multifunctional P450s by Structural Comparison of IkaD and CftA Complexed with Common Substrates

Regio- and chemoselective C-H activation at multi-positions of a single molecule is fascinating but chemically challenging. The homologous cytochrome P450 enzymes IkaD and CftA catalyze multiple C-H oxidations on the same polycyclic tetramate macrolactam (PoTeM) ikarugamycin, with distinct regio- and chemoselectivity. Herein we provide mechanistic understanding of their functional differences by solving crystal structures of IkaD and CftA in complex with ikarugamycin and unnatural substrates. Distinct conformations of the F/G region in IkaD and CftA are found to differentiate the orientation of PoTeM substrates, by causing different binding patterns with polar moieties to determine site selection, oxidation order, and chemoselectivity. Fine-tuning the polar subpocket altered the regioselectivity of IkaD, indicating that substrate re-orientation by mutating residues distal to the oxidation site could serve as an important method in future engineering of P450 enzymes.

Discovery of Oxidized Polycyclic Tetramate Macrolactams Bearing One or Two Rings through Combinatorial Pathway Reassembly

The structural diversity of polycyclic tetramate macrolactams (PoTeMs) are mainly generated by the cyclases and cytochrome P450s (CYPs). The PoTeM cluster sah in Saccharopolyspora hirsuta harboring two CYP genes was combinatorially reassembled and heterologously expressed in Streptomyces. As a result, six new cytotoxic PoTeMs, sahamides A-F (1-6), were discovered, and 1-3 are the first examples of oxidized one-ring PoTeMs. Remarkably, SahE represents the first CYP performing oxidative modification on the ornithine moiety of PoTeMs.

Natural products from mangrove sediments-derived microbes: Structural diversity, bioactivities, biosynthesis, and total synthesis

The mangrove forests are a complex ecosystem, and the microbial communities in mangrove sediments play a critical role in the biogeochemical cycles of mangrove ecosystems. Mangrove sediments-derived microbes (MSM), as a rich reservoir of natural product diversity, could be utilized in the exploration of new antibiotics or drugs. To understand the structural diversity and bioactivities of the metabolites of MSM, this review for the first time provides a comprehensive overview of 519 natural products isolated from MSM with their bioactivities, up to 2021. Most of the structural types of these compounds are alkaloids, lactones, xanthones, quinones, terpenoids, and steroids. Among them, 210 compounds are obtained from bacteria, most of which are from Streptomyces, while 309 compounds are from fungus, especially genus Aspergillus and Penicillium. The pharmacological mechanisms of some representative lead compounds are well studied, revealing that they have important medicinal potentials, such as piericidins with anti-renal cell cancer effects, azalomycins with anti-MRSA activities, and ophiobolins as antineoplastic agents. The biosynthetic pathways of representative natural products from MSM have also been summarized, especially ikarugamycin, piericidins, divergolides, and azalomycins. In addition, the total synthetic strategies of representative secondary metabolites from MSM are also reviewed, such as piericidin A and borrelidin. This review provides an important reference for the research status of natural products isolated from MSM and the lead compounds worthy of further development, and reveals that MSM have important medicinal values and are worthy of further development.

Biosynthesis, regulation, and engineering of natural products from Lysobacter

Covering: up to August 2021Lysobacter is a genus of Gram-negative bacteria that was classified in 1987. Several Lysobacter species are emerging as new biocontrol agents for crop protection in agriculture. Lysobacter are prolific producers of new bioactive natural products that are largely underexplored. So far, several classes of structurally interesting and biologically active natural products have been isolated from Lysobacter. This article reviews the progress in Lysobacter natural product research over the past ten years, including molecular mechanisms for biosynthesis, regulation and mode of action, genome mining of cryptic biosynthetic gene clusters, and metabolic engineering using synthetic biology tools.

Combinatorial Biosynthesis of Oxidized Combamides Using Cytochrome P450 Enzymes from Different Polycyclic Tetramate Macrolactam Pathways

Polycyclic tetramate macrolactams (PoTeMs) are a family of natural products containing a tetramic acid moiety and a polycyclic system. Due to the valuable biological activities of different PoTeMs and the genetic simplicity of their biosynthetic genes, it is highly desirable to manipulate the biosynthesis of PoTeMs by swapping modification genes between different pathways. Herein, by combining the cytochrome P450 (CYP) enzymes from different PoTeM pathways with the combamides' biosynthetic genes, the new combamides G (3), I (5), and J (6) along with the known combamides B (1), D (2), and H (4) were identified from the recombinant strains. Combamides G (3), H (4), and J (6) displayed cytotoxic activity against human cancer cell lines. Furthermore, our results demonstrated for the first time the substrate specificity of the PoTeM-related CYPs in vivo, which will facilitate the engineered biosynthesis of other PoTeMs in the future.

Genome Mining and Metabolic Profiling Uncover Polycyclic Tetramate Macrolactams from Streptomyces koyangensis SCSIO 5802

We have previously shown deep-sea-derived Streptomyces koyangensis SCSIO 5802 to produce two types of active secondary metabolites, abyssomicins and candicidins. Here, we report the complete genome sequence of S. koyangensis SCSIO 5802 employing bioinformatics to highlight its potential to produce at least 21 categories of natural products. In order to mine novel natural products, the production of two polycyclic tetramate macrolactams (PTMs), the known 10-epi-HSAF (1) and a new compound, koyanamide A (2), was stimulated via inactivation of the abyssomicin and candicidin biosynthetic machineries. Detailed bioinformatics analyses revealed a PKS/NRPS gene cluster, containing 6 open reading frames (ORFs) and spanning ~16 kb of contiguous genomic DNA, as the putative PTM biosynthetic gene cluster (BGC) (termed herein sko). We furthermore demonstrate, via gene disruption experiments, that the sko cluster encodes the biosynthesis of 10-epi-HSAF and koyanamide A. Finally, we propose a plausible biosynthetic pathway to 10-epi-HSAF and koyanamide A. In total, this study demonstrates an effective approach to cryptic BGC activation enabling the discovery of new bioactive metabolites; genome mining and metabolic profiling methods play key roles in this strategy.

Expression of the Clifednamide Biosynthetic Pathway in Streptomyces Generates 27,28-seco-Derivatives

Polycyclic tetramate macrolactams (PoTeMs) are a group of hybrid PK-NRP natural products having a variable set of carbocyclic rings, a conserved assembly pathway, and diverse bioactivities. We report here the identification of seven new PoTeMs, clifednamides D-J (3-9), along with the known clifednamides A (1) and B (2) through rational pathway refactoring and heterologous expression. Remarkably, clifednamides D (3), G (6), and H (7) feature an unprecedented 27,28-seco skeleton. The cytotoxic activities of compounds 1-9 indicated that the hydroxy group of C-25, the methyl group of C-30, the inner five-membered ring, and the intact macrocycle are all critical for the activities. Meanwhile, the cytochrome P450 enzyme Cft_S023A and the hydroxylase Cft_S023E involved in oxidative tailoring of clifednamides were found to decorate the fused 5-6 bicyclic intermediates. Accordingly, the biosynthetic pathway for clifednamides was proposed.

Targeted isolation of new polycyclic tetramate macrolactams from the deepsea-derived Streptomyces somaliensis SCSIO ZH66

With a combined strategy of bioinformatics analysis, gene manipulation coupled with variation of growth conditions, the targeted activation of polycyclic tetramate macrolactams (PTMs) in the deepsea-derived Streptomyces somaliensis SCSIO ZH66 was conducted, which afforded a new (1) PTM, named somamycin A, along with three enol-type tetramic acid tautomers (2-4, somamycins B-D) of 10-epi-hydroxymaltophilin, 10-epi-maltophilin and 10-epi-HSAF, respectively. The structures of compounds 1-4 were elucidated by extensive spectroscopic analyses together with ECD calculations. Compound 1 exhibited notable growth inhibition against plant pathogenic fungi Fusariumoxysporum MHKW and Alternariabrassicae BCHB with the MIC values of 1.6 and 3.1 μg/mL, respectively, which were more potent than those of the positive control nystatin; and compounds 3 and 4 displayed moderate antifungal activities. Moreover, compounds 1-4 exhibited moderate cytotoxicity against the human cancer cell lines of HCT116 and K562.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as flavipeside A from Aspergillus flavipes.