Terretonin Biosynthesis Requires Methylation as Essential Step for Cyclization

Production of non-natural 5-methylorsellinate-derived meroterpenoids in Aspergillus oryzae

Fungal meroterpenoids are diverse structurally intriguing molecules with various biological properties. One large group within this compound class is derived from the aromatic precursor 3,5-dimethylorsellinic acid (DMOA). In this study, we constructed engineered metabolic pathways in the fungus Aspergillus oryzae to expand the molecular diversity of meroterpenoids. We employed the 5-methylorsellinic acid (5-MOA) synthase FncE and three additional biosynthetic enzymes for the formation of (6R,10'R)-epoxyfarnesyl-5-MOA methyl ester, which served as a non-native substrate for four terpene cyclases from DMOA-derived meroterpenoid pathways. As a result, we successfully generated six unnatural 5-MOA-derived meroterpenoid species, demonstrating the effectiveness of our approach in the generation of structural analogues of meroterpenoids.

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.

Recent developments in the engineered biosynthesis of fungal meroterpenoids

Meroterpenoids are hybrid compounds that are partially derived from terpenoids. This group of natural products displays large structural diversity, and many members exhibit beneficial biological activities. This mini-review highlights recent advances in the engineered biosynthesis of meroterpenoid compounds with C15 and C20 terpenoid moieties, with the reconstruction of fungal meroterpenoid biosynthetic pathways in heterologous expression hosts and the mutagenesis of key enzymes, including terpene cyclases and α-ketoglutarate (αKG)-dependent dioxygenases, that contribute to the structural diversity. Notable progress in genome sequencing has led to the discovery of many novel genes encoding these enzymes, while continued efforts in X-ray crystallographic analyses of these enzymes and the invention of AlphaFold2 have facilitated access to their structures. Structure-based mutagenesis combined with applications of unnatural substrates has further diversified the catalytic repertoire of these enzymes. The information in this review provides useful knowledge for the design of biosynthetic machineries to produce a variety of bioactive meroterpenoids.

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.

Functional analysis of a fungal P450 enzyme

Fungal cytochrome P450s participate in various physiological reactions, including the synthesis of internal cellular components, metabolic detoxification of xenobiotic compounds, and oxidative modification of natural products. Although functional analysis reports of fungal P450s continue to grow, there are still some difficulties as compared to prokaryotic P450s, because most of these fungal enzymes are transmembrane proteins. In this chapter, we will describe the methods for heterologous expression, in vivo analysis, enzyme preparation, and in vitro enzyme assays of the fungal P450 enzyme Trt6 and isomerase Trt14, which play important roles in the divergence of the biosynthetic pathway of terretonins, as a model for the functional analysis of fungal P450 enzymes.

Identification, Heterologous Expression, and Characterization of the Tolypodiol Biosynthetic Gene Cluster through an Integrated Approach

Cyanobacteria are tremendous producers of biologically active natural products, including the potent anti-inflammatory compound tolypodiol. However, linking biosynthetic gene clusters with compound production in cyanobacteria has lagged behind that in other bacterial genera. Tolypodiol is a meroterpenoid originally isolated from the cyanobacterium HT-58-2. Here we describe the identification of the tolypodiol biosynthetic gene cluster through heterologous expression in Anabaena and in vitro protein assays of a methyltransferase found in the tolypodiol biosynthetic gene cluster. We have also identified similar biosynthetic gene clusters in cyanobacterial and actinobacterial genomes, suggesting that meroterpenoids with structural similarity to the tolypodiols may be synthesized by other microbes. We also report the identification of two new analogs of tolypodiol that we have identified in both the original and heterologous producer. This work further illustrates the usefulness of Anabaena as a heterologous expression host for cyanobacterial compounds and how integrated approaches can help to link natural product compounds with their producing biosynthetic gene clusters.

Isolation, (bio)synthetic studies and evaluation of antimicrobial properties of drimenol-type sesquiterpenes of Termitomyces fungi

Macrotermitinae termites have farmed fungi in the genus Termitomyces as a food source for millions of years. However, the biochemical mechanisms orchestrating this mutualistic relationship are largely unknown. To deduce fungal signals and ecological patterns that relate to the stability of this symbiosis, we explored the volatile organic compound (VOC) repertoire of Termitomyces from Macrotermes natalensis colonies. Results show that mushrooms emit a VOC pattern that differs from mycelium grown in fungal gardens and laboratory cultures. The abundance of sesquiterpenoids from mushrooms allowed targeted isolation of five drimane sesquiterpenes from plate cultivations. The total synthesis of one of these, drimenol, and related drimanes assisted in structural and comparative analysis of volatile organic compounds (VOCs) and antimicrobial activity testing. Enzyme candidates putatively involved in terpene biosynthesis were heterologously expressed and while these were not involved in the biosynthesis of the complete drimane skeleton, they catalyzed the formation of two structurally related monocyclic sesquiterpenes named nectrianolins.

Engineering fungal terpene biosynthesis

Covering: 2015 to 2022Fungal terpenoids are of large structural diversity and often exhibit interesting biological activities. Recent work has focused on two main aspects: (1) the discovery and understanding of unknown biosynthetic genes and pathways, and (2) the usage of already known biosynthetic genes in the construction of high yielding production strains. Both aspects will be covered in this review article that aims to summarise the most important work of the past few years.

Late-stage cascade of oxidation reactions during the biosynthesis of oxalicine B in Penicillium oxalicum

Oxalicine B (1) is an α-pyrone meroterpenoid with a unique bispirocyclic ring system derived from Penicillium oxalicum. The biosynthetic pathway of 15-deoxyoxalicine B (4) was preliminarily reported in Penicillium canescens, however, the genetic base and biochemical characterization of tailoring reactions for oxalicine B (1) has remained enigmatic. In this study, we characterized three oxygenases from the metabolic pathway of oxalicine B (1), including a cytochrome P450 hydroxylase OxaL, a hydroxylating Fe(II)/α-KG-dependent dioxygenase OxaK, and a multifunctional cytochrome P450 OxaB. Intriguingly, OxaK can catalyze various multicyclic intermediates or shunt products of oxalicines with impressive substrate promiscuity. OxaB was further proven via biochemical assays to have the ability to convert 15-hydroxdecaturin A (3) to 1 with a spiro-lactone core skeleton through oxidative rearrangement. We also solved the mystery of OxaL that controls C-15 hydroxylation. Chemical investigation of the wild-type strain and deletants enabled us to identify 10 metabolites including three new compounds, and the isolated compounds displayed potent anti-influenza A virus bioactivities exhibiting IC₅₀ values in the range of 4.0-19.9 μmol/L. Our studies have allowed us to propose a late-stage biosynthetic pathway for oxalicine B (1) and create downstream derivatizations of oxalicines by employing enzymatic strategies.

A Multifunctional Cytochrome P450 and a Meroterpenoid Cyclase in the Biosynthesis of Fungal Meroterpenoid Atlantinone B

The biosynthetic gene cluster of atlantinone B (10) was discovered in Penicillium chrysogenum MT-40. A multifunctional cytochrome P450 (AtlD) encoded by the cluster is responsible for the formation of the unique lactone-bridged ring and the 16β-hydroxyl of atlantinone B, and a new terpene cyclase (AtlC) can unprecedentedly accept the demethylated substrate epoxyfarnesyl-DMOA (4a) to generate three bicyclic meroterpenoids (5a-5c). This study paves the way for combinatorial synthesis of structurally diverse meroterpenoids for drug discovery.

Branching and converging pathways in fungal natural product biosynthesis

In nature, organic molecules with great structural diversity and complexity are synthesized by utilizing a relatively small number of starting materials. A synthetic strategy adopted by nature is pathway branching, in which a common biosynthetic intermediate is transformed into different end products. A natural product can also be synthesized by the fusion of two or more precursors generated from separate metabolic pathways. This review article summarizes several representative branching and converging pathways in fungal natural product biosynthesis to illuminate how fungi are capable of synthesizing a diverse array of natural products.

In silico analyses of maleidride biosynthetic gene clusters

Maleidrides are a family of structurally related fungal natural products, many of which possess diverse, potent bioactivities. Previous identification of several maleidride biosynthetic gene clusters, and subsequent experimental work, has determined the 'core' set of genes required to construct the characteristic medium-sized alicyclic ring with maleic anhydride moieties. Through genome mining, this work has used these core genes to discover ten entirely novel putative maleidride biosynthetic gene clusters, amongst both publicly available genomes, and encoded within the genome of the previously un-sequenced epiheveadride producer Wicklowia aquatica CBS 125634. We have undertaken phylogenetic analyses and comparative bioinformatics on all known and putative maleidride biosynthetic gene clusters to gain further insights regarding these unique biosynthetic pathways.

Discovery of branching meroterpenoid biosynthetic pathways in Aspergillus insuetus: involvement of two terpene cyclases with distinct cyclization modes

Branching meroterpenoid biosynthetic pathways were discovered in the fungus Aspergillus insuetus CBS 107.25, in which two terpene cyclases, InsA7 and InsB2, accept the same substrate but generate distinctly cyclized products.

Biocatalytic and chemical derivatization of fungal meroditerpenoid chevalone E

Fungal meroditerpenoids include diverse molecules with structural complexity and a broad range of biological activities. We have previously obtained meroditerpenoid chevalone E (1) and its oxidized analogues by heterologously expressing...

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.

Novel Cyclohexyl Meroterpenes Produced by Combinatorial Biosynthesis

Structurally diverse fungal meroterpenoids are promising drug seed compounds. To obtain unnatural, novel meroterpene scaffolds, we tested combinatorial biosynthesis by co-expressing functionally distinct terpene cyclase (TPC) genes, pyr4, ascF, andB, or cdmG, with the biosynthetic genes for the production of a TPC substrate, (10'R)-epoxyfarnesyl-dimethylorsellinic acid-3,5-methyl ester, in Aspergillus oryzae NSAR1 as a heterologous host. As a result, all of the tested TPCs afforded the same two novel mono-cyclization products. This study provides important information on the substrate scope of the TPCs, and will contribute to the production of unnatural, novel molecules for future drug discovery.

Linking a Gene Cluster to Atranorin, a Major Cortical Substance of Lichens, through Genetic Dereplication and Heterologous Expression

The depside and depsidone series compounds of polyketide origin accumulate in the cortical or medullary layers of lichen thalli. Despite the taxonomic and ecological significance of lichen chemistry and its pharmaceutical potentials, there has been no single piece of genetic evidence linking biosynthetic genes to lichen substances. Thus, we systematically analyzed lichen polyketide synthases (PKSs) for categorization and identification of the biosynthetic gene cluster (BGC) involved in depside/depsidone production. Our in-depth analysis of the interspecies PKS diversity in the genus Cladonia and a related Antarctic lichen, Stereocaulon alpinum, identified 45 BGC families, linking lichen PKSs to 15 previously characterized PKSs in nonlichenized fungi. Among these, we identified highly syntenic BGCs found exclusively in lichens producing atranorin (a depside). Heterologous expression of the putative atranorin PKS gene (coined atr1) yielded 4-O-demethylbarbatic acid, found in many lichens as a precursor compound, indicating an intermolecular cross-linking activity of Atr1 for depside formation. Subsequent introductions of tailoring enzymes into the heterologous host yielded atranorin, one of the most common cortical substances of macrolichens. Phylogenetic analysis of fungal PKS revealed that the Atr1 is in a novel PKS clade that included two conserved lichen-specific PKS families likely involved in biosynthesis of depsides and depsidones. Here, we provide a comprehensive catalog of PKS families of the genus Cladonia and functionally characterize a biosynthetic gene cluster from lichens, establishing a cornerstone for studying the genetics and chemical evolution of diverse lichen substances. IMPORTANCE Lichens play significant roles in ecosystem function and comprise about 20% of all known fungi. Polyketide-derived natural products accumulate in the cortical and medullary layers of lichen thalli, some of which play key roles in protection from biotic and abiotic stresses (e.g., herbivore attacks and UV irradiation). To date, however, no single lichen product has been linked to respective biosynthetic genes with genetic evidence. Here, we identified a gene cluster family responsible for biosynthesis of atranorin, a cortical substance found in diverse lichen species, by categorizing lichen polyketide synthase and reconstructing the atranorin biosynthetic pathway in a heterologous host. This study will help elucidate lichen secondary metabolism, harnessing the lichen's chemical diversity, hitherto obscured due to limited genetic information on lichens.

Enantioselective Total Synthesis of Berkeleyone A and Preaustinoids

Herein we report the first enantioselective total synthesis of 3,5-dimethylorsellinic acid-derived meroterpenoids (-)-berkeleyone A and its five congeners ((-)-preaustinoids A, A1, B, B1, and B2) in 12-15 steps, starting from commercially available 2,4,6-trihydroxybenzoic acid hydrate. Based upon the recognition of latent symmetry within D-ring, our convergent synthesis features two critical reactions: 1) a symmetry-breaking, diastereoselective dearomative alkylation to assemble the entire carbon core, and 2) a Sc(OTf)₃ -mediated sequential Krapcho dealkoxycarbonylation/carbonyl α-tert-alkylation to forge the intricate bicyclo[3.3.1]nonane framework. We also conducted our preliminary biomimetic investigations and uncovered a series of rearrangements (α-ketol, α-hydroxyl-β-diketone, etc.) responsible for the biomimetic diversification of (-)-berkeleyone A into its five preaustinoid congeners.

Reconstitution of Polyketide-Derived Meroterpenoid Biosynthetic Pathway in Aspergillus oryzae

The heterologous gene expression system with Aspergillus oryzae as the host is an effective method to investigate fungal secondary metabolite biosynthetic pathways for reconstruction to produce un-natural molecules due to its high productivity and genetic tractability. In this review, we focus on biosynthetic studies of fungal polyketide-derived meroterpenoids, a group of bioactive natural products, by means of the A. oryzae heterologous expression system. The heterologous expression methods and the biosynthetic reactions are described in detail for future prospects to create un-natural molecules via biosynthetic re-design.

Peculiarities of meroterpenoids and their bioproduction

Meroterpenoids are a class of terpenoid-containing hybrid natural products with impressive structural architectures and remarkable pharmacological activities. Remarkable advances in enzymology and synthetic biology have greatly contributed to the elucidation of the molecular basis for their biosynthesis. Here, we review structurally unique meroterpenoids catalyzed by novel enzymes and unusual enzymatic reactions over the period of last 5 years. We also discuss recent progress on the biomimetic synthesis of chrome meroterpenoids and synthetic biology-driven biomanufacturing of tropolone sesquiterpenoids, merochlorins, and plant-derived meroterpenoid cannabinoids. In particular, we focus on the novel enzymes involved in the biosynthesis of polyketide-terpenoids, nonribosomal peptide-terpenoids, terpenoid alkaloids, and meroterpenoid with unique structures. The biological activities of these meroterpenoids are also discussed. The information reviewed here might provide useful clues and lay the foundation for developing new meroterpenoid-derived drugs. KEY POINTS: • Meroterpenoids possess intriguing structural features and relevant biological activities. • Novel enzymes are involved in the biosynthesis of meroterpenoids with unique structures. • Biomimetic synthesis and synthetic biology enable the construction and manufacturing of complex meroterpenoids.

A new antibacterial 3,5-dimethylorsellinic acid-based meroterpene from the marine fungus Aspergillus sp. CSYZ-1

Chemical investigation of the extracts of Aspergillus sp. CSYZ-1 resulted in the identification of compound 1, aspergillactone, a new 3,5-dimethylorsellinic acid-based meroterpenoid, together with four known metabolites (2-5). The structure and relative configuration of 1 were unambiguously determined by nuclear magnetic resonance (NMR), mass spectrometry. The absolute configuration of 1 was defined by quantum chemical TDDFT calculated and the experimental ECD spectra. The possible biosynthetic pathway of compound 1 was also proposed. The new compound exhibited potent antimicrobial activity against Helicobacter pylori and Staphylococcus aureus with MIC values of around 1-4 and 2-16 μg/mL, respectively.

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.

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.

Nonheme Iron- and 2-Oxoglutarate-Dependent Dioxygenases in Fungal Meroterpenoid Biosynthesis

This review summarizes the recent progress in research on the non-heme Fe(II)- and 2-oxoglutarate-dependent dioxygenases, which are involved in the biosynthesis of pharmaceutically important fungal meroterpenoids. This enzyme class activates a selective C-H bond of the substrate and catalyzes a wide range of chemical reactions, from simple hydroxylation to dynamic carbon skeletal rearrangements, thereby significantly contributing to the structural diversification and complexification of the molecules. Structure-function studies of these enzymes provide an excellent platform for the development of useful biocatalysts for synthetic biology to create novel molecules for future drug discovery.

Terreuspyridine: An Unexpected Pyridine-Fused Meroterpenoid Alkaloid with a Tetracyclic 6/6/6/6 Skeleton from Aspergillus terreus

Terreuspyridine (1), the first 3,5-demethylorsellinic acid (DMOA) derived meroterpenoid alkaloid, was isolated from the fungus Aspergillus terreus, which represents a new type of meroterpenoid possessing an unexpected tetracyclic 6/6/6/6 architecture. The structure of 1 with absolute configuration was determined by X-ray diffraction analysis. Biogenetically, it was proposed to be derived from the fusion of a DMOA-meroterpenoid and a glutamate. Terreuspyridine (1) exhibited moderate inhibitory activity against the BChE with an IC₅₀ value of 16.4 μM.

Penicimeroterpenoids A-C, Meroterpenoids with Rearrangement Skeletons from the Marine-Derived Fungus Penicillium sp. SCSIO 41512

Three novel andrastin-type meroterpenoids, penicimeroterpenoids A-C (1-3), possessing two unprecedented skeletons consisting of fused 6/5/6/6/7 and 6/5/6/6/4 polycyclic systems, were obtained from the marine-derived fungus Penicillium sp. SCSIO 41512. Their structures were determined by spectroscopic methods, and the absolute configurations were further determined by single-crystal X-ray diffraction analysis for 1 and quantum chemical calculations of ECD spectra for 2 and 3, respectively. A plausible biosynthetic pathway for 1-3 was proposed.

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.

Biomimetic Synthesis of Meroterpenoids by Dearomatization-Driven Polycyclization

A biomimetic route to farnesyl pyrophosphate and dimethyl orsellinic acid (DMOA)-derived meroterpenoid scaffolds has yet to be reported despite great interest from the chemistry and biomedical research communities. A concise synthetic route with the potential to access DMOA-derived meroterpenoids is highly desirable to create a library of related compounds. Herein, we report novel dearomatization methodology followed by polyene cyclization to access DMOA-derived meroterpenoid frameworks in six steps from commercially available starting materials. Furthermore, several farnesyl alkene substrates were used to generate structurally novel, DMOA-derived meroterpenoid derivatives. DFT calculations combined with experimentation provided a rationale for the observed thermodynamic distribution of polycyclization products.

Genome mining for fungal polyketide-diterpenoid hybrids: discovery of key terpene cyclases and multifunctional P450s for structural diversification

The biosynthesis of the fungal meroterpenoid chevalone E and its derivatives has been successfully elucidated and reconstituted.

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

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

Terretonin N: A New Meroterpenoid from Nocardiopsis sp

Terretonin N (1), a new highly oxygenated and unique tetracyclic 6-hydroxymeroterpenoid, was isolated together with seven known compounds from the ethyl acetate extract of a solid-state fermented culture of Nocardiopsis sp. Their structures were elucidated by spectroscopic analysis. The structure and absolute configuration of 1 were unambiguously determined by X-ray crystallography. The isolation and taxonomic characterization of Nocardiopsis sp. is reported. The antimicrobial activity and cytotoxicity of the strain extract and compound 1 were studied using different microorganisms and a cervix carcinoma cell line, respectively.

Terretonin M: A new meroterpenoid from the thermophilic Aspergillus terreus TM8 and revision of the absolute configuration of penisimplicins

Terretonin M (1), a new highly oxygenated tetracyclic meroterpenoid, was isolated from the thermophilic fungus Aspergillus terreus TM8 together with 10 known metabolites: terrelumamide A, asterrelenin, 7-prenyl-indolyl-3-carbaldehyde, (3β,5α,6β)-3,5,6-trihydroxy-ergosta-7,22-diene, sitostenone, linoleic acid, ergosterol, uracil, p-hydroxy-benzoic acid, and indole-3-carboxylic acid. The chemical structure of the new compound was elucidated by extensive 1D, 2D NMR, and ESI HR mass measurements, and by comparison with literature data. The absolute configuration of 1 was resolved by analysis of its NOESY spectrum and comparison of its experimental ECD spectrum with DFT calculations. In parallel to this work, revision of the absolute configuration of penisimplicins 3a and 3b is proposed on the basis of their ECD and ORD data. The isolation and taxonomic characterisation of A. terreus TM8 is reported, and the antimicrobial activity of the crude extract and the isolated compounds was studied as well.

Traversing Biosynthetic Carbocation Landscapes in the Total Synthesis of Andrastin and Terretonin Meroterpenes

Meroterpenes derived from dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate have attracted much biosynthetic attention, yet only recently have synthetic solutions to any family members appeared. A key point of divergence in DMOA-derived meroterpene biosynthesis is the protoaustinoid A carbocation, which can be diverted to either the berkeleyone, andrastin, or terretonin structural classes by cyclase-controlled rearrangement pathways. Shown herein is that the protoaustinoid bicyclo[3.3.1]nonane nucleus can be reverted to either andrastin or terretonin ring systems under abiotic reaction conditions. The first total syntheses of members of these natural product families are reported as their racemates.

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

Macrophorins are representative examples of isoprenoid epoxycyclohexenones containing cyclized drimane moieties. We located and characterized the biosynthetic gene cluster of macrophorin from Penicillium terrestris. MacJ encoded by this cluster was characterized to be the first example of a membrane-bound type-II terpene cyclase catalyzing the cyclization of meroterpenoids via direct protonation of the terminal olefinic bond in acyclic yanuthones. The late-stage functionalization and substrate promiscuity of MacJ make it a potential biocatalyst for the synthesis of macrophorin analogues.

Iodinated Meroditerpenes from a Red Alga Callophycus sp

Unique iodine-containing meroditerpenes iodocallophycoic acid A (1) and iodocallophycols A-D (2-5) were discovered from the Fijian red alga Callophycus sp. Because flexibility of the molecular skeleton impaired full characterization of relative stereochemistries by NMR spectroscopy, a DFT-based theoretical model was developed to derive relevant interproton distances which were compared to those calculated from NOE measurements, yielding the relative stereochemistries. The correct 2S,6S,7S,10S,14S enantiomers were then identified by comparison of theoretical and experimental ECD spectra. Biological activities of these iodinated and brominated meroditerpenes and additional new, related bromophycoic acid F (6) and bromophycoic acid A methyl ester (7), were evaluated for relevant human disease targets. Iodocallophycoic acid A (1) showed moderate antibiotic activity against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecium (VREF) with MIC values of 1.4 and 2.2 μg mL-1, respectively. It also potentiated the anti-MRSA activity of oxacillin in a synergistic fashion, resulting in an 8-fold increase in oxacillin potency, for a MIC of 16 μg mL-1.

Total Synthesis of (±)-Berkeleyone A

A 13-step total synthesis of the fungal meroterpenoid berkeleyone A is reported. The molecular skeleton is formed using the first examples of two critical construction reactions: (1) an epoxide-initiated, β-ketoester-terminated polycyclization, and (2) an isomerization-cyclization cascade to generate the remaining bicyclo[3.3.1]nonane framework. The resulting 6-step synthesis of the carbocyclic core of the berkeleyone natural products has been used to access protoaustinoid A and berkeleyone A, and will aid future biosynthetic investigations into the origin of related natural products.

Annulative Methods Enable a Total Synthesis of the Complex Meroterpene Berkeleyone A

Synthetic pathways to complex meroterpenes derived from 3,5-dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate have not been reported despite heavy biosynthetic and medicinal interest. Herein we report the first total synthesis of berkeleyone A, a potential gateway compound to a plethora of fungal-derived meroterpenes, in 13 steps. In addition, we have further developed a novel annulation reaction for the synthesis of hydroxylated 1,3-cyclohexadiones in a single step.