Late-Stage Terpene Cyclization by an Integral Membrane Cyclase in the Biosynthesis of Isoprenoid Epoxycyclohexenone Natural Products

Identification of a chlorosalicylic acid decarboxylase (CsaD) involved in decarboxylation of 3,6-DCSA from an anaerobic dicamba-degrading sludge

3,6-Dichlorosalicylic acid (3,6-DCSA) is the demethylation metabolite of herbicide 3,6-dichloro-2-methoxy benzoic acid (dicamba). Previous studies have shown that anaerobic sludge further transformed 3,6-DCSA through decarboxylation and dechlorination. However, the anaerobe, enzyme, and gene involved in the anaerobic degradation of 3,6-DCSA are still unknown. In this study, an anaerobic sludge that efficiently degraded dicamba was enriched, and a 3,6-DCSA decarboxylase, designated chlorosalicylic acid decarboxylase (CsaD), was partially purified and identified from the anaerobic sludge. Metagenomic analysis showed that the csaD gene was located in a gene cluster of metagenome-assembled genome 8 (MAG8). MAG8 belonged to an uncultured order, OPB41, in the class Coriobacteriia of the phylum Actinobacteria, and its abundance increased approximately once during the enrichment process. CsaD was a non-oxidative decarboxylase in the amidohydrolase 2 family catalyzing the decarboxylation of 3,6-DCSA and 6-chlorosalicylic acid (6-CSA). Its affinity and catalytic efficiency for 3,6-DCSA were significantly higher than those for 6-CSA. This study provides new insights into the anaerobic catabolism of herbicide dicamba.IMPORTANCEDicamba, an important hormone herbicide, easily migrates to anoxic habitats such as sediment, ground water, and deep soil. Thus, the anaerobic catabolism of dicamba is of importance. Anaerobic bacteria or sludge demethylated dicamba to 3,6-DCSA, and in a previous study, based on metabolite identification, it was proposed that 3,6-DCSA be further degraded via two pathways: decarboxylation to 2,5-dichlorophenol, then dechlorination to 3-chlorophenol (3-CP); or dechlorination to 6-CSA, then decarboxylation to 3-CP. However, there was no physiological and genetic validation for the pathway. In this study, CsaD catalyzed the decarboxylation of both 3,6-DCSA and 6-CSA, providing enzyme-level evidence for the anaerobic catabolism of 3,6-DCSA through the two pathways. CsaD was located in MAG8, which belonged to an uncultured anaerobic actinomycetes order, OPB41, indicating that anaerobic actinomycetes in OPB41 was involved in the decarboxylation of 3,6-DCSA. This study provides a basis for understanding the anaerobic catabolism of dicamba and the demethylation product, 3,6-DCSA.

Several secondary metabolite gene clusters in the genomes of ten Penicillium spp. raise the risk of multiple mycotoxin occurrence in chestnuts

Penicillium spp. produce a great variety of secondary metabolites, including several mycotoxins, on food substrates. Chestnuts represent a favorable substrate for Penicillium spp. development. In this study, the genomes of ten Penicillium species, virulent on chestnuts, were sequenced and annotated: P. bialowiezense. P. pancosmium, P. manginii, P. discolor, P. crustosum, P. palitans, P. viridicatum, P. glandicola, P. taurinense and P. terrarumae. Assembly size ranges from 27.5 to 36.8 Mb and the number of encoded genes ranges from 9,867 to 12,520. The total number of predicted biosynthetic gene clusters (BGCs) in the ten species is 551. The most represented families of BGCs are non ribosomal peptide synthase (191) and polyketide synthase (175), followed by terpene synthases (87). Genome-wide collections of gene phylogenies (phylomes) were reconstructed for each of the newly sequenced Penicillium species allowing for the prediction of orthologous relationships among our species, as well as other 20 annotated Penicillium species available in the public domain. We investigated in silico the presence of BGCs for 10 secondary metabolites, including 5 mycotoxins, whose production was validated in vivo through chemical analyses. Among the clusters present in this set of species we found andrastin A and its related cluster atlantinone A, mycophenolic acid, patulin, penitrem A and the cluster responsible for the synthesis of roquefortine C/glandicoline A/glandicoline B/meleagrin. We confirmed the presence of these clusters in several of the Penicillium species conforming our dataset and verified their capacity to synthesize them in a chestnut-based medium with chemical analysis. Interestingly, we identified mycotoxin clusters in some species for the first time, such as the andrastin A cluster in P. flavigenum and P. taurinense, and the roquefortine C cluster in P. nalgiovense and P. taurinense. Chestnuts proved to be an optimal substrate for species of Penicillium with different mycotoxigenic potential, opening the door to risks related to the occurrence of multiple mycotoxins in the same food matrix.

Macrophorins H and L, two new HMG-conjugate macrophorins from rihzospheric Penicillium sp. NX-05-G-3

Macrophorins H (4) and L (5), two rare HMG-conjugate macrophorins along with three known macrophorins (1-3), three DMOA-derived meroterpenoids (6-8) and two ergosterol derivates (9-10) were isolated from sterilized rice medium cultured Penicillium sp. NX-05-G-3. Their structures were elucidated by 1D and 2D NMR. The cytotoxicities of all compounds were evaluated, and compounds 1 and 2 showed extensive cytotoxicity against human cancer cell lines Hela, SCC15, MDA-MB-453 and A549, with IC50 values ranging from 17.6 to 32.8 µM.

Fragment-Guided Genome Mining of Octacyclic Cyclophane Alkaloids from Fungi

Nature uses compact but functionalized biosynthetic fragments as building blocks to generate complex natural products. To leverage this strategy for the discovery of natural products with new scaffolds, we performed genome mining to identify biosynthetic gene clusters (BGCs) in fungi that embed genes that can synthesize targeted fragments. The three-enzyme pathway that biosynthesizes the strained dityrosine cyclophane in the herquline A pathway was used to identify a large number of potential BGCs that may use the cyclophane as a fragment. Characterization of a conserved BGC from fungal strains led to the isolation of octacyclin A, an octacyclic natural product with an unprecedented structure, including two hetero-[3.3.1]bicycles and a combination of fused, bridged, and macrocyclic rings. Biosynthetic steps leading to octacyclin A were fully elucidated using pathway reconstitution and enzymatic assays, unveiling intriguing chemical logic and new enzymatic reactions in building the octacyclic core. Our work demonstrates the potential utility of fragment-guided genome mining in expanding natural product chemical space.

New meroterpenoids and polyketides from the endophytic fungus Paraphaeosphaeria sp. C-XB-J-1 and their anti-inflammatory and SARS-CoV-2 Mpro inhibitory activities

Seven new meroterpenoids, paraphaeones A-G (1-7), and two new polyketides, paraphaeones H-I (8-9), along with eight known compounds (10-17), were isolated from the endophytic fungus Paraphaeosphaeria sp. C-XB-J-1. The structures of 1-9 were identified through the analysis of 1H, 13C, and 2D NMR spectra, assisted by HR-ESI-MS data. Compounds 1 and 7 exhibited a dose-dependent decrease in lactate dehydrogenase levels, with IC50 values of 1.78 μM and 1.54 μM, respectively. Moreover, they inhibited the secretion of IL-1β and CASP-1, resulting in a reduction in the activity levels of NLRP3 inflammasomes. Fluorescence microscopy results indicated that compound 7 concentration-dependently attenuated cell pyroptosis. Additionally, compounds 4 and 7 showed potential inhibitory effects on the severe acute respiratory syndrome coronavirus-2 main protease (SARS-CoV-2 Mpro), with IC50 values of 10.8 ± 0.9 μM and 12.9 ± 0.7 μM, respectively.

Genome Mining of a Deep-Sea-Derived Penicillium allii-sativi Revealed Polyketide-Terpenoid Hybrids with Antiosteoporosis Activity

Two novel meroterpenoids, alliisativins A and B (1, 2) were discovered through a genome-based exploration of the biosynthetic gene clusters of the deep-sea-derived fungus Penicillium allii-sativi MCCC entry 3A00580. Extensive spectroscopic analysis, quantum calculations, chemical derivatization, and biogenetic considerations were utilized to establish their structures. Alliisativins A and B (1, 2) possess a unique carbon skeleton featuring a drimane sesquiterpene with a highly oxidized polyketide. Noteworthily, alliisativin A (1) showed dual activity in promoting osteogenesis and inhibiting osteoclast, indicating an antiosteoporosis potential.

Unusual Sesquiterpenes from Streptomyces olindensis DAUFPE 5622

In nature, the vast majority of sesquiterpenes are produced by type I mechanisms, and glycosylated sesquiterpenes are rare in actinobacteria. Streptomyces olindensis DAUFPE 5622 produces the sesquiterpenes olindenones A-G, a new class of rearranged drimane sesquiterpenes. Olindenones B-D are oxygenated derivatives of olindenone A, while olindenones E-G are analogs glycosylated with dideoxysugars. 13C-isotope labeling studies demonstrated olindenone A biosynthesis occurs via the methylerythritol phosphate (MEP) pathway and suggested the rearrangement is only partially concerted. Based on the structures, one potential mechanism of olindenone A formation proceeds by cyclization of the linear terpenoid precursor, likely occurring via a terpene cyclase-mediated type II mechanism whereby the terminal alkene of the precursor is protonated, triggering carbocation-driven cyclization followed by rearrangement. Diphosphate hydrolysis may occur either before or after cyclization. Although a biosynthetic route is proposed, the terpene cyclase gene responsible for producing olindenones currently remains unidentified.

Class II terpene cyclases: structures, mechanisms, and engineering

Covering: up to July 2023Terpene cyclases (TCs) catalyze some of the most complicated reactions in nature and are responsible for creating the skeletons of more than 95 000 terpenoid natural products. The canonical TCs are divided into two classes according to their structures, functions, and mechanisms. The class II TCs mediate acid-base-initiated cyclization reactions of isoprenoid diphosphates, terpenes without diphosphates (e.g., squalene or oxidosqualene), and prenyl moieties on meroterpenes. The past twenty years witnessed the emergence of many class II TCs, their reactions and their roles in biosynthesis. Class II TCs often act as one of the first steps in the biosynthesis of biologically active natural products including the gibberellin family of phytohormones and fungal meroterpenoids. Due to their mechanisms and biocatalytic potential, TCs elicit fervent attention in the biosynthetic and organic communities and provide great enthusiasm for enzyme engineering to construct novel and bioactive molecules. To engineer and expand the structural diversities of terpenoids, it is imperative to fully understand how these enzymes generate, precisely control, and quench the reactive carbocation intermediates. In this review, we summarize class II TCs from nature, including sesquiterpene, diterpene, triterpene, and meroterpenoid cyclases as well as noncanonical class II TCs and inspect their sequences, structures, mechanisms, and structure-guided engineering studies.

Cyclo-diphenylalanine production in Aspergillus nidulans through stepwise metabolic engineering

Cyclo-diphenylalanine (cFF) is a symmetrical aromatic diketopiperazine (DKP) found wide-spread in microbes, plants, and resulting food products. As different bioactivities continue being discovered and relevant food and pharmaceutical applications gradually emerge for cFF, there is a growing need for establishing convenient and efficient methods to access this type of compound. Here, we present a robust cFF production system which entailed stepwise engineering of the filamentous fungal strain Aspergillus nidulans A1145 as a heterologous expression host. We first established a preliminary cFF producing strain by introducing the heterologous nonribosomal peptide synthetase (NRPS) gene penP1 to A. nidulans A1145. Key metabolic pathways involving shikimate and aromatic amino acid biosynthetic support were then engineered through a combination of gene deletions of competitive pathway steps, over-expressing feedback-insensitive enzymes in phenylalanine biosynthesis, and introducing a phosphoketolase-based pathway, which diverted glycolytic flux toward the formation of erythrose 4-phosphate (E4P). Through the stepwise engineering of A. nidulans A1145 outlined above, involving both heterologous pathway addition and native pathway metabolic engineering, we were able to produce cFF with titers reaching 611 mg/L in shake flask culture and 2.5 g/L in bench-scale fed-batch bioreactor culture. Our study establishes a production platform for cFF biosynthesis and successfully demonstrates engineering of phenylalanine derived diketopiperazines in a filamentous fungal host.

Functional analysis of transmembrane terpene cyclases involved in fungal meroterpenoid biosynthesis

Pyr4-family terpene cyclases are noncanonical transmembrane class II terpene cyclases that catalyze a variety of cyclization reactions in the biosynthesis of microbial terpenoids, such as meroterpenoids. However, although these cyclases are widely distributed in microorganisms, their three-dimensional structures have not been determined, possibly due to the transmembrane locations of these enzymes. In this chapter, we describe procedures for the functional analysis of transmembrane terpene cyclases based on their model structures generated using AlphaFold2. We used AdrI, the Pyr4-family terpene cyclase required for the biosynthesis of andrastin A and its homologs, as an example.

Global genome mining-driven discovery of an unusual biosynthetic logic for fungal polyketide-terpenoid hybrids

Genome mining has facilitated the efficient discovery of untapped natural products. We performed global genome mining in fungi and discovered a series of biosynthetic gene clusters (BGCs) that appeared to afford polyketide-terpenoid hybrids via a distinct biosynthetic mechanism from those adopted by known pathways. Characterization of one of the BGCs revealed that it yields the drimane-phthalide hybrid 1. During the biosynthesis of 1, the farnesyl group is unusually introduced by the dimethylallyltryptophan synthase-type prenyltransferase MfmD and is then cyclized by the Pyr4-family terpene cyclase MfmH. The replacement of MfmH with its homologue OcdTC gave another hybrid molecule with a monocyclic terpenoid moiety. Moreover, PsetPT, an MfmD homologue, was found to perform dimethylallylation and was then engineered to install a geranyl group. Our study unraveled an unusual biosynthetic mechanism for fungal phthalide-terpenoid hybrids and provided insights into how their structural diversification could be achieved.

Cyclohexenoneterpenes A-J: Cytotoxic meroterpenoids from mangrove-associated fungus Penicillium sp. N-5

Ten previously undescribed meroterpenoids, cyclohexenoneterpenes A-J (1-7, 18-20), together with 10 known analogues (8-17) were isolated from the mangrove-associated fungus Penicillium sp. N-5. Their structures were elucidated on the basis of extensive spectroscopic and mass spectrometric data. The absolute configurations of the undescribed compounds were assigned by electronic circular dichroism calculations, the modified Mosher's method, NMR calculations and DP4+ analysis. In the bioassay, compounds 10, 11, 15, and 20 exhibited cytotoxicities against SNB-19, MDA-MB-231, MDA-MB-435 and HCT-116 cell lines with IC50 values ranging from 1.4 to 19.1 μM.

Natural epoxyquinoids: isolation, biological activity and synthesis. An update

Epoxyquinoids are of continuing interest due to their wide natural distribution and diverse biological activities, including, but not limited to, antibacterial, antifungal, anticancer, enzyme inhibitory, and others. The last review on their total synthesis was published in 2017. Since then, almost 100 articles have been published on their isolation from nature and their biological profile. In addition, the review specifically considers synthesis, including total and enantioselective, as well as the development of shorter approaches for the construction of epoxyquinoids with complex chemical architecture. Thus, this review focuses on progress in this area in order to stimulate further research.

The Multifaceted Gene 275 Embedded in the PKS-PTS Gene Cluster Was Involved in the Regulation of Arthrobotrisin Biosynthesis, TCA Cycle, and Septa Formation in Nematode-Trapping Fungus Arthrobotrys oligospora

The predominant nematode-trapping fungus Arthrobotrys oligospora harbors a unique polyketide synthase-prenyltransferase (PKS-PTS) gene cluster AOL_s00215g responsible for the biosynthesis of sesquiterpenyl epoxy-cyclohexenoids (SECs) that are involved in the regulation of fungal growth, adhesive trap formation, antibacterial activity, and soil colonization. However, the function of one rare gene (AOL_s00215g275 (275)) embedded in the cluster has remained cryptic. Here, we constructed two mutants with the disruption of 275 and the overexpression of 275, respectively, and compared their fungal growth, morphology, resistance to chemical stress, nematicidal activity, transcriptomic and metabolic profiles, and infrastructures, together with binding affinity analysis. Both mutants displayed distinct differences in their TCA cycles, SEC biosynthesis, and endocytosis, combined with abnormal mitochondria, vacuoles, septa formation, and decreased nematicidal activity. Our results suggest that gene 275 might function as a separator and as an integrated gene with multiple potential functions related to three distinct genes encoding the retinoic acid induced-1, cortactin, and vacuolar iron transporter 1 proteins in this nematode-trapping fungus. Our unexpected findings provide insight into the intriguing organization and functions of a rare non-biosynthetic gene in a biosynthetic gene cluster.

The induced cryptic metabolites and antifungal activities from culture of Penicillium chrysogenum by supplementing with host Ziziphus jujuba extract

The productions of cryptic metabolites including three undescribed drimane sesquiterpenoids, penicichrins A-C, and three known compounds from Penicillium chrysogenum were activated by the host Ziziphus jujuba medium. The structures were established by comprehensive analysis of spectroscopic data. The spiro β-lactone, and gem-dimethyl dihydroxylation in induced penicichrins A-C were rare in natural products. Cryptic metabolites, monaspurpurone was first found in Penicillium. 4-Methoxy-3-methylgoniothalamin, and 2-hydroxy-l-phenyl-l,4-pentanedione were second example of isolation. Penicichrin A, monaspurpurone, 4-methoxy-3-methylgoniothalamin, physcion, ergosterol, and ergosta-7,22-dien-3β-ol had antifungal activities against phytopathogens, P. chrysogenum, Alternaria alternata and Aspergillus fumigatus with MICs ≤2 μg/mL, and 2-hydroxy-l-phenyl-l,4-pentanedione had flowering activity. So the chemical constituents from Z. jujuba could induce the productions of cryptic metabolites with plant growth-promoting activity from endophyte P. chrysogenum.

Modulating Activity Evaluation of Gut Microbiota with Versatile Toluquinol

Gut microbiota have important implications for health by affecting the metabolism of diet and drugs. However, the specific microbial mediators and their mechanisms in modulating specific key intermediate metabolites from fungal origins still remain largely unclear. Toluquinol, as a key versatile precursor metabolite, is commonly distributed in many fungi, including Penicillium species and their strains for food production. The common 17 gut microbes were cultivated and fed with and without toluquinol. Metabolic analysis revealed that four strains, including the predominant Enterococcus species, could metabolize toluquinol and produce different metabolites. Chemical investigation on large-scale cultures led to isolation of four targeted metabolites and their structures were characterized with NMR, MS, and X-ray diffraction analysis, as four toluquinol derivatives (1–4) through O1/O4-acetyl and C5/C6-methylsulfonyl substitutions, respectively. The four metabolites were first synthesized in living organisms. Further experiments suggested that the rare methylsulfonyl groups in 3–4 were donated from solvent DMSO through Fenton’s reaction. Metabolite 1 displayed the strongest inhibitory effect on cancer cells A549, A2780, and G401 with IC₅₀ values at 0.224, 0.204, and 0.597 μM, respectively, while metabolite 3 displayed no effect. Our results suggest that the dominant Enterococcus species could modulate potential precursors of fungal origin and change their biological activity.

Harnessing Fe(II)/α-ketoglutarate-dependent oxygenases for structural diversification of fungal meroterpenoids

Fungal meroterpenoids are structurally diverse natural products with important biological activities. During their biosynthesis, α-ketoglutarate-dependent oxygenases (αKG-DOs) catalyze a wide range of chemically challenging transformation reactions, including desaturation, epoxidation, oxidative rearrangement, and endoperoxide formation, by selective C-H bond activation, to produce molecules with more complex and divergent structures. Investigations on the structure-function relationships of αKG-DO enzymes have revealed the intimate molecular bases of their catalytic versatility and reaction mechanisms. Notably, the catalytic repertoire of αKG-DOs is further expanded by only subtle changes in their active site and lid-like loop-region architectures. Owing to their remarkable biocatalytic potential, αKG-DOs are ideal candidates for future chemoenzymatic synthesis and enzyme engineering for the generation of terpenoids with diverse structures and biological activities.

Chemical Diversity and Biosynthesis of Drimane-Type Sesquiterpenes in the Fungal Kingdom

Drimane-type sesquiterpenes are a class of compounds produced by a wide range of organisms, initially isolated and characterized in plants. Meanwhile, in the past 20-30 years, a large number of novel structures from many divergent fungi have been elucidated. Recently, the biosynthesis of drimane-type sesquiter-penes and their esters has been explained in two filamentous fungi, namely Aspergillus oryzae and Aspergillus calidoustus, disclosing the basic biosynthetic principles needed to identify similar pathways in the fungal kingdom.

New Trends and Future Opportunities in the Enzymatic Formation of C-C, C-N, and C-O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021, 1, 1-21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021, 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C-C, C-N and C-O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.

Polyketide Synthase-Terpenoid Synthase Hybrid Pathway Regulation of Trap Formation through Ammonia Metabolism Controls Soil Colonization of Predominant Nematode-Trapping Fungus

Polyketide synthase-terpenoid synthase (PKS-TPS) hybrid pathways for biosynthesis of unique sesquiterpenyl epoxy-cyclohexenoids (SECs) have been found to be widely distributed in plant pathogenic fungi. However, the natural and ecological functions of these pathways and their metabolites still remain cryptic. In this study, the whole PKS-TPS hybrid pathway in the predominant nematode-trapping fungus Arthrobotrys oligospora was first proposed according to all the intermediates and their derivatives from all the A. oligospora mutants with a deficiency in each gene involved in SEC biosynthesis. Most mutants displayed significantly increased trap formation which was correlated with alteration of the ammonia level. Further analysis revealed that the main metabolites involved in ammonia metabolism were largely increased in most mutants. However, significantly retarded colonization in soil were observed in most mutants compared to the wild-type strain due to significantly decreased antibacterial activities. Our results suggested that A. oligospora used the PKS-TPS hybrid pathway for fungal soil colonization via decreasing fungal nematode-capturing ability. This also provided solid evidence that boosting fungal colonization in soil was the secondary metabolite whose biosynthesis depended on a PKS-TPS hybrid pathway.

CRISPR-based transcriptional activation tool for silent genes in filamentous fungi

Filamentous fungi are historically known to be a rich reservoir of bioactive compounds that are applied in a myriad of fields ranging from crop protection to medicine. The surge of genomic data available shows that fungi remain an excellent source for new pharmaceuticals. However, most of the responsible biosynthetic gene clusters are transcriptionally silent under laboratory growth conditions. Therefore, generic strategies for activation of these clusters are required. Here, we present a genome-editing-free, transcriptional regulation tool for filamentous fungi, based on the CRISPR activation (CRISPRa) methodology. Herein, a nuclease-defective mutant of Cas9 (dCas9) was fused to a highly active tripartite activator VP64-p65-Rta (VPR) to allow for sgRNA directed targeted gene regulation. dCas9-VPR was introduced, together with an easy to use sgRNA "plug-and-play" module, into a non-integrative AMA1-vector, which is compatible with several filamentous fungal species. To demonstrate its potential, this vector was used to transcriptionally activate a fluorescent reporter gene under the control of the penDE core promoter in Penicillium rubens. Subsequently, we activated the transcriptionally silent, native P. rubens macrophorin biosynthetic gene cluster by targeting dCas9-VPR to the promoter region of the transcription factor macR. This resulted in the production of antimicrobial macrophorins. This CRISPRa technology can be used for the rapid and convenient activation of silent fungal biosynthetic gene clusters, and thereby aid in the identification of novel compounds such as antimicrobials.

Overexpression of Global Regulator Talae1 Leads to the Discovery of New Antifungal Polyketides From Endophytic Fungus Trichoderma afroharzianum

Transcription regulation caused by global regulators exerts important effects on fungal secondary metabolism. By overexpression of the global regulator Talae1 in a Ficus elastica-associated fungus Trichoderma afroharzianum, two structurally new polyketides (1 and 2) that were newly produced in the transformant were isolated and identified. Their structures, including the absolute configurations, were elucidated through a combination of high resolution mass spectrometer (HRMS), NMR, and electronic circular dichroism (ECD) calculations. The growth inhibitory activities of compounds 1 and 2 were evaluated against four bacteria and six plant-pathogenic fungi. Compound 1 showed the highest antifungal activity against Botrytis cinerea and Fusarium oxysporum f. sp. nicotianae with MIC of 8 μg/ml. To the best of our knowledge, this is the first study to report on the application of the global regulator in T. afroharzianum to activate the biosynthesis of bioactive secondary metabolites.

Exploiting the Potential of Meroterpenoid Cyclases to Expand the Chemical Space of Fungal Meroterpenoids

Fungal meroterpenoids are a diverse group of hybrid natural products with impressive structural complexity and high potential as drug candidates. In this work, we evaluate the promiscuity of the early structure diversity-generating step in fungal meroterpenoid biosynthetic pathways: the multibond-forming polyene cyclizations catalyzed by the yet poorly understood family of fungal meroterpenoid cyclases. In total, 12 unnatural meroterpenoids were accessed chemoenzymatically using synthetic substrates. Their complex structures were determined by 2D NMR studies as well as crystalline-sponge-based X-ray diffraction analyses. The results obtained revealed a high degree of enzyme promiscuity and experimental results which together with quantum chemical calculations provided a deeper insight into the catalytic activity of this new family of non-canonical, terpene cyclases. The knowledge obtained paves the way to design and engineer artificial pathways towards second generation meroterpenoids with valuable bioactivities based on combinatorial biosynthetic strategies.

Regulation of Secondary Metabolism in the Penicillium Genus

Penicillium, one of the most common fungi occurring in a diverse range of habitats, has a worldwide distribution and a large economic impact on human health. Hundreds of the species belonging to this genus cause disastrous decay in food crops and are able to produce a varied range of secondary metabolites, from which we can distinguish harmful mycotoxins. Some Penicillium species are considered to be important producers of patulin and ochratoxin A, two well-known mycotoxins. The production of these mycotoxins and other secondary metabolites is controlled and regulated by different mechanisms. The aim of this review is to highlight the different levels of regulation of secondary metabolites in the Penicillium genus.

Marine-Derived Penicillium purpurogenum Reduces Tumor Size and Ameliorates Inflammation in an Erlich Mice Model

Background: This study addresses the antitumoral properties of Penicillium purpurogenum isolated from a polluted lagoon in Northeastern Brazil. Methods: Ethyl Acetate Extracellular Extract (EAE) was used. The metabolites were studied using direct infusion mass spectrometry. The solid Ehrlich tumor model was used for antitumor activity. Female Swiss mice were divided into groups (n = 10/group) as follows: The negative control (CTL−), treated with a phosphate buffered solution; the positive control (CTL+), treated with cyclophosphamide (25 mg/kg); extract treatments at doses of 4, 20, and 100 mg/kg; animals without tumors or treatments (Sham); and animals without tumors treated with an intermediate dose (EAE20). All treatments were performed intraperitoneally, daily, for 15 days. Subsequently, the animals were euthanized, and the tumor, lymphoid organs, and serum were used for immunological, histological, and biochemical parameter evaluations. Results: The extract was rich in meroterpenoids. All doses significantly reduced tumor size, and the 20 and 100 mg/kg doses reduced tumor-associated inflammation and tumor necrosis. The extract also reduced the cellular infiltration of lymphoid organs and circulating TNF-α levels. The extract did not induce weight loss or renal and hepatic toxic changes. Conclusions: These results indicate that P. purpurogenum exhibits immunomodulatory and antitumor properties in vivo. Thus, fungal fermentation is a valid biotechnological approach to the production of antitumor agents.

Novel Polyketide-Terpenoid Hybrid Metabolites from a Potent Nematicidal Arthrobotrys oligospora Mutant ΔAOL_s00215g278

Here, we reported that detailed investigation on trace targeted metabolites from nematode-trapping fungus Arthrobotrys oligospora mutant with deletion of P450 gene AOL_s00215g278 led to isolation of 9 new polyketide-terpenoid hybrid derivatives, including four new glycosides of the key precursor farnesyl hydrotoluquinol (1) and, surprisingly, four new sesquiterpenyl epoxy-cyclohexenoids (SECs) analogues. Among them, two major target metabolites 1 and 14 displayed moderate nematode inhibitory ability. Moreover, the mutant lacking AOL_s00215g278 could form far more nematode-capturing traps within 6 h in contact with nematodes and show rapid potent nematicidal activity with killing 93.7% preys, though deletion of the P450 gene resulted in dramatic decrease in fungal colony growth and failure to produce fungal conidia. The results unequivocally revealed that gene AOL_s00215g278 should be involved in not only the SEC biosynthetic pathway in the nematode-trapping fungus A. oligospora but also fungal conidiation and nematicidal activity.

Chemistry of fungal meroterpenoid cyclases

Covering: up to July 2020Fungal meroterpenoid cyclases are a recently discovered emerging family of membrane-integrated, non-canonical terpene cyclases. They catalyze the conversion of hybrid isoprenic precursors towards complex scaffolds and are therefore of great importance in the structure diversification in meroterpenoid biosynthesis. The products of these pathways exhibit intriguing molecular scaffolds and highly potent bioactivities, making them privileged structures from Nature and attractive candidates for drug development or industrial applications. This review will provide a comprehensive and comparative view on fungal meroterpenoid cyclases, their intriguing chemistries and importance for the scaffold formation step towards polycyclic meroterpenoid natural products.

Deletion of the Histone Deacetylase HdaA in Endophytic Fungus Penicillium chrysogenum Fes1701 Induces the Complex Response of Multiple Bioactive Secondary Metabolite Production and Relevant Gene Cluster Expression

Epigenetic regulation plays a critical role in controlling fungal secondary metabolism. Here, we report the pleiotropic effects of the epigenetic regulator HdaA (histone deacetylase) on secondary metabolite production and the associated biosynthetic gene clusters (BGCs) expression in the plant endophytic fungus Penicillium chrysogenum Fes1701. Deletion of the hdaA gene in strain Fes1701 induced a significant change of the secondary metabolite profile with the emergence of the bioactive indole alkaloid meleagrin. Simultaneously, more meleagrin/roquefortine-related compounds and less chrysogine were synthesized in the ΔhdaA strain. Transcriptional analysis of relevant gene clusters in ΔhdaA and wild strains indicated that disruption of hdaA had different effects on the expression levels of two BGCs: the meleagrin/roquefortine BGC was upregulated, while the chrysogine BGC was downregulated. Interestingly, transcriptional analysis demonstrated that different functional genes in the same BGC had different responses to the disruption of hdaA. Thereinto, the roqO gene, which encodes a key catalyzing enzyme in meleagrin biosynthesis, showed the highest upregulation in the ΔhdaA strain (84.8-fold). To our knowledge, this is the first report of the upregulation of HdaA inactivation on meleagrin/roquefortine alkaloid production in the endophytic fungus P. chrysogenum. Our results suggest that genetic manipulation based on the epigenetic regulator HdaA is an important strategy for regulating the productions of secondary metabolites and expanding bioactive natural product resources in endophytic fungi.

Overexpression of Global Regulator PbrlaeA Leads to the Discovery of New Polyketide in Fungus Penicillium Brocae HDN-12-143

Overexpression of the PbrlaeA gene of the fungus Penicillium brocae HDN-12-143 resulted in the isolation of four compounds including fumigatin chlorohydrin (1), whose configuration has not been reported before, and one new compound iso-fumigatin chlorohydrin (2). All structures including absolute configurations were elucidated on the basis of comprehensive spectroscopic data, ¹³C NMR calculations, and ECD calculations. Compounds 1 and 2 exhibited cytotoxic activity against HL-60 with IC₅₀ of 18.63 and 24.83 μM.

Alternarin A, a Drimane Meroterpenoid, Suppresses Neuronal Excitability from the Coral-Associated Fungi Alternaria sp. ZH-15

Alternarin A (1), a rearranged drimane meroterpenoid characterized by a thioglycerate moiety, was isolated together with two known analogues from the coral-associated fungi Alternaria sp. ZH-15. Its structure was determined based on spectroscopic analysis, modified Mosher's method, and TDDFT/ECD calculations. In a primary cultured cortical neuronal network, compound 1 effectively inhibited the activity of spontaneous synchronous Ca²⁺ oscillations and 4-aminopyridine induced epileptic discharges in the low micromolar concentration range.

Terpene synthases in disguise: enzymology, structure, and opportunities of non-canonical terpene synthases

Covering: up to July 2019 Terpene synthases (TSs) are responsible for generating much of the structural diversity found in the superfamily of terpenoid natural products. These elegant enzymes mediate complex carbocation-based cyclization and rearrangement cascades with a variety of electron-rich linear and cyclic substrates. For decades, two main classes of TSs, divided by how they generate the reaction-triggering initial carbocation, have dominated the field of terpene enzymology. Recently, several novel and unconventional TSs that perform TS-like reactions but do not resemble canonical TSs in sequence or structure have been discovered. In this review, we identify 12 families of non-canonical TSs and examine their sequences, structures, functions, and proposed mechanisms. Nature provides a wide diversity of enzymes, including prenyltransferases, methyltransferases, P450s, and NAD+-dependent dehydrogenases, as well as completely new enzymes, that utilize distinctive reaction mechanisms for TS chemistry. These unique non-canonical TSs provide immense opportunities to understand how nature evolved different tools for terpene biosynthesis by structural and mechanistic characterization while affording new probes for the discovery of novel terpenoid natural products and gene clusters via genome mining. With every new discovery, the dualistic paradigm of TSs is contradicted and the field of terpene chemistry and enzymology continues to expand.

Roquefornine A, a sesterterpenoid with a 5/6/5/5/6-fused ring system from the fungus Penicillium roqueforti YJ-14

Roquefornine A, a sesterterpenoid with an unprecedented 5/6/5/5/6-membered pentacyclic system, was characterized from Penicillium roqueforti YJ-14.

Synthetic Protein Scaffolding at Biological Membranes

Protein scaffolding is a natural phenomenon whereby proteins colocalize into macromolecular complexes via specific protein-protein interactions. In the case of metabolic enzymes, protein scaffolding drives metabolic flux through specific pathways by colocalizing enzyme active sites. Synthetic protein scaffolding is increasingly used as a mechanism to improve product specificity and yields in metabolic engineering projects. To date, synthetic scaffolding has focused primarily on soluble enzyme systems, but many metabolic pathways for high-value secondary metabolites depend on membrane-bound enzymes. The compositional diversity of biological membranes and general challenges associated with modifying membrane proteins complicate scaffolding with membrane-requiring enzymes. Several recent studies have introduced new approaches to protein scaffolding at membrane surfaces, with notable success in improving product yields from specific metabolic pathways.

Immunosuppressant mycophenolic acid biosynthesis employs a new globin-like enzyme for prenyl side chain cleavage

Mycophenolic acid (MPA, 1) and its derivatives are first-line immunosuppressants used in organ transplantation and for treating autoimmune diseases. Despite chemical synthetic achievements, the biosynthetic formation of a seven-carbon carboxylic acid pharmacophore side chain of 1, especially the processes involving the cleavage of the prenyl side chain between DHMP (4) and DMMPA (5), remains unknown. In this work, we identified a membrane-bound prenyltransferase, PgMpaA, that transfers FPP to 4 to yield FDHMP (6). Compound 6 undergoes the first cleavage step via a new globin-like enzyme PgMpaB to form a cryptic intermediate 12. Heterologous expression of PgMpa genes in Aspergillus nidulans demonstrates that the second cleavage step (from 12 to 5) of 1 is a PgMpa cluster-independent process in vivo. Our results, especially the discovery of the broad tolerance of substrates recognized by PgMpaB, set up a strategy for the formation of "pseudo-isopentenyl" natural products using fungal globin-like enzymes.

Unexpected Biosynthesis of Fluorescein-Like Arthrocolins against Resistant Strains in an Engineered Escherichia coli

Here we provide an unprecedented biofactory where fluorescent dye-like complex xanthenes could be produced in an engineered Escherichia coli. Feeding the strain with toluquinol or hydroquinones resulted in production of novel "unnatural" natural products including four arthrocolins embedded with indolyltriphenyl quaternary carbons. Arthrocolins A-C potently inhibited various human cancer cell lines including paclitaxel-resistant cell line A549/Taxol and methicillin-resistant Staphylococcus aureus and immensely restored the sensitivity of intractable fluconazole-resistant human pathogen Candida albicans to fluconazole.

Discovery of Two New Sorbicillinoids by Overexpression of the Global Regulator LaeA in a Marine-Derived Fungus Penicillium dipodomyis YJ-11

Overexpression of the global regulator LaeA in a marine-derived fungal strain of Penicillium dipodomyis YJ-11 induced obvious morphological changes and metabolic variations. Further chemical investigation of the mutant strain afforded a series of sorbicillinoids including two new ones named 10,11-dihydrobislongiquinolide (1) and 10,11,16,17-tetrahydrobislongiquinolide (2), as well as four known analogues, bislongiquinolide (3), 16,17-dihydrobislongiquinolide (4), sohirnone A (5), and 2′,3′-dihydrosorbicillin (6). The results support that the global regulator LaeA is a useful tool in activating silent gene clusters in Penicillium strains to obtain previously undiscovered compounds.

Expanstines A–D: four unusual isoprenoid epoxycyclohexenones generated byPenicillium expansumYJ-15 fermentation and photopromotion

Expanstines A–D fromP. expansumYJ-15 were derived under UV and visible light from 4′-oxomacrophoin A (5) and its acetate (6), which plausibly underwent a rare intramolecular photoene reaction and a [2 + 2] Paternò–Büchi photoaddition respectively.

Genome mining of cyclodipeptide synthases unravels unusual tRNA-dependent diketopiperazine-terpene biosynthetic machinery

Cyclodipeptide synthases (CDPSs) can catalyze the formation of two successive peptide bonds by hijacking aminoacyl-tRNAs from the ribosomal machinery resulting in diketopiperazines (DKPs). Here, three CDPS-containing loci (dmt1–3) are discovered by genome mining and comparative genome analysis of Streptomyces strains. Among them, CDPS DmtB1, encoded by the gene of dmt1 locus, can synthesize cyclo(L-Trp-L-Xaa) (with Xaa being Val, Pro, Leu, Ile, or Ala). Systematic mutagenesis experiments demonstrate the importance of the residues constituting substrate-binding pocket P1 for the incorporation of the second aa-tRNA in DmtB1. Characterization of dmt1–3 unravels that CDPS-dependent machinery is involved in CDPS-synthesized DKP formation followed by tailoring steps of prenylation and cyclization to afford terpenylated DKP compounds drimentines. A phytoene-synthase-like family prenyltransferase (DmtC1) and a membrane terpene cyclase (DmtA1) are required for drimentines biosynthesis. These results set the foundation for further increasing the natural diversity of complex DKP derivatives.

Diketopiperazine derivatives are bioactive molecules with scaffold formed by the condensation of two amino acids. Here, Yao et al. mine the genomes of Streptomyces strains and identify new biosynthetic machinery for drimentines biosynthesis, which includes cyclodipeptide synthase, prenyltransferase, and terpene cyclase.

Three previously unrecognised classes of biosynthetic enzymes revealed during the production of xenovulene A

Xenovulene A is a complex fungal meroterpenoid, produced by the organism hitherto known as Acremonium strictum IMI 501407, for which limited biosynthetic evidence exists. Here, we generate a draft genome and show that the producing organism is previously unknown and should be renamed as Sarocladium schorii. A biosynthetic gene cluster is discovered which bears resemblance to those involved in the biosynthesis of fungal tropolones, with additional genes of unknown function. Heterologous reconstruction of the entire pathway in Aspergillus oryzae allows the chemical steps of biosynthesis to be dissected. The pathway shows very limited similarity to the biosynthesis of other fungal meroterpenoids. The pathway features: the initial formation of tropolone intermediates; the likely involvement of a hetero Diels–Alder enzyme; a terpene cyclase with no significant sequence homology to any known terpene cyclase and two enzymes catalysing oxidative-ring contractions.

Xenovulene A is a fungal compound that has the potential to be used as an antidepressant. Here, the authors unravel the pathway leading to its formation in fungi and discover a new class of enzymes, which accounts for some unusual chemistry in the synthesis of xenovulene.

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 tryptorubin A isolated from a Streptomyces species.