Molecular basis for the unusual ring reconstruction in fungal meroterpenoid biogenesis

Three-dimensional structural alignment based discovery and molecular basis of AtoB, catalyzing linear tetracyclic formation

Enzymes from the nuclear transport factor 2-like (NTF2-like) superfamily represent a rare group of biocatalysts with diverse catalytic functions facilitating intriguing skeleton formations. However, most proteins of this family remain enigmatic and await further elucidation. In this study, a combination of protein structural alignment with clustering analysis uncovers a new aldolase, AtoB, belonging to the NTF2-like superfamily. AtoB catalyzes the key intramolecular aldol reaction in linear tetracyclic meroterpenoid biosynthesis. The X-ray crystal structures of AtoB and AtoB-ligand complex are established at 1.9 Å and 1.6 Å resolution, respectively, revealing the rotation of the α4 helix and key residues in the active site for substrate binding. Molecular docking and site-directed mutagenesis demonstrate an acid-base pair involved in the AtoB-catalyzed aldol reaction, of which Arg59 is responsible for stereocontrol of hydroxylated C-10a during condensation. These findings provide valuable information for understanding the catalytic mechanisms of the AtoB-catalyzed aldol reaction. Additionally, a branching biosynthetic pathway of aspertetranones is elucidated during the exploration of the natural substrate of AtoB.

Isolation, structural elucidation and biosynthetic pathway of bioactive prenyl quinone compounds from Panus lecomtei based on untargeted metabolomics combined with molecular networking

Panus lecomtei is a relatively unfamiliar and undeveloped mushroom. This study generated ethyl acetate extracts of P. lecomtei intracellular (I), extracellular (E) and total fermentation broth (T). Both E and T extracts demonstrated antioxidant and antibacterial activities at 100 to 200 μg/mL. The composition differences of metabolites of these extracts were further studied based on comparative metabolomics by LS/MS and molecular network analysis. The results revealed that there were over 2000 significantly distinct metabolites among the three extracts, with abundant prenyl quinone compounds. Furthermore, the molecular network clarified the conversion relationship of P. lecomtei metabolites. Seven known prenyl quinone derivatives (1-7) were isolated from the E extract. Among them, compound 3 displayed excellent antioxidant activity and modest antibacterial activity. Compound 5 was discovered in fungi for the first time. Finally, a potential biosynthetic route for prenyl quinone in P. lecomtei was suggested.

An Enzymatic Carbon-Carbon Bond Cleavage and Aldol Reaction Cascade Converts an Angular Scaffold into the Linear Tetracyclic Core of Ochraceopones

Meroterpenoids of the ochraceopones family featuring a linear tetracyclic scaffold exhibit exceptional antiviral and anti-inflammatory activities. The biosynthetic pathway and chemical logic to generate this linear tetracycle, however, remain unknown. In this study, we identified and characterized all biosynthetic enzymes to afford ochraceopones and elucidated the complete biosynthetic pathway. We demonstrated that the linear tetracyclic scaffold of ochraceopones was derived from an angular tetracyclic precursor. A multifunctional cytochrome P450 OchH was validated to catalyze the free-radical-initiated carbon-carbon bond cleavage of the angular tetracycle. Then, a new carbon-carbon bond was verified to be constructed using a new aldolase OchL, which catalyzes an intramolecular aldol reaction to form the linear tetracycle. This carbon-carbon bond fragmentation and aldol reaction cascade features an unprecedented strategy for converting a common angular tetracycle to a distinctive linear tetracyclic scaffold in meroterpenoid biosynthesis.

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 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.

Peniandrastins A-H: Andrastin-type meroterpenoids with immunosuppressive activity from a Penicillium sp

Eight unreported andrastin-type meroterpenoids, namely peniandrastins A-H (1-8), along with six known analogues (9-14), were isolated from the fermentation of a soil-derived fungus Penicillium sp.sb62. Their structures with absolute configurations were elucidated by detailed analyses of the spectroscopic data and single-crystal X-ray diffraction. Compounds 1-4 belong to a rare class of 21-nor-andrastin meroterpenoids, of which 1 bears a 10-hydroperoxyl group, and 2 and 3 feature a 6/6/6/5/5 and a 6/6/6/5/6 pentacyclic systems, respectively. Compounds 5-8 are C25 andrastin-type meroterpenoids, wherein 5 features an unprecedented cyclopentan-1-keton-3-hemiacetal moiety. Additionally, the absolute configuration of compound 9 was corroborated by single-crystal X-ray crystallography for the first time. All isolates were evaluated for their immunosuppressive activities. As a result, compounds 1, 3, 4, 7-9 and 12-14 inhibited concanavalin A-induced T cell proliferation with IC50 values ranging from 7.49 to 36.52 μM, and 1-4, 6-9 and 12-14 inhibited lipopolysaccharide-induced B cell proliferation with IC50 values ranging from 6.73 to 26.27 μM. The preliminary structure-activity relationships (SARs) of those isolates were also discussed.

Biosynthesis of Polycyclic Natural Products from Conjugated Polyenes via Tandem Isomerization and Pericyclic Reactions

We report biosynthetic pathways that can synthesize and transform conjugated octaenes and nonaenes to complex natural products. The biosynthesis of (-)-PF1018 involves an enzyme PfB that can control the regio-, stereo-, and periselectivity of multiple reactions starting from a conjugated octaene. Using PfB as a lead, we discovered a homologous enzyme, BruB, that facilitates diene isomerization, tandem 8π-6π-electrocyclization, and a 1,2-divinylcyclobutane Cope rearrangement to generate a new-to-nature compound.

Coordination of Polyketide Release and Multiple Detoxification Pathways for Tolerable Production of Fungal Mycotoxins

Efficient biosynthesis of microbial bioactive natural products (NPs) is beneficial for the survival of producers, while self-protection is necessary to avoid self-harm resulting from over-accumulation of NPs. The underlying mechanisms for the effective but tolerable production of bioactive NPs are not well understood. Herein, in the biosynthesis of two fungal polyketide mycotoxins aurovertin E (1) and asteltoxin, we show that the cyclases in the gene clusters promote the release of the polyketide backbone, and reveal that a signal peptide is crucial for their subcellular localization and full activity. Meanwhile, the fungus adopts enzymatic acetylation as the major detoxification pathway of 1. If intermediates are over-produced, the non-enzymatic shunt pathways work as salvage pathways to avoid excessive accumulation of the toxic metabolites for self-protection. These findings provided new insight into the interplay of efficient backbone release and multiple detoxification strategies for the production of fungal bioactive NPs.

The 'emodin family' of fungal natural products-amalgamating a century of research with recent genomics-based advances

Covering: up to 2022A very large group of biosynthetically linked fungal secondary metabolites are formed via the key intermediate emodin and its corresponding anthrone. The group includes anthraquinones such as chrysophanol and cladofulvin, the grisandienes geodin and trypacidin, the diphenyl ether pestheic acid, benzophenones such as monodictyphenone and various xanthones including the prenylated shamixanthones, the agnestins and dimeric xanthones such as the ergochromes, cryptosporioptides and neosartorin. Such compounds exhibit a wide range of bioactivities and as such have been utilised in traditional medicine for centuries, as well as garnering more recent interest from the pharmaceutical sector. Additional interest comes from industries such as textiles and cosmetics due to their use as natural colourants. A variety of biosynthetic routes and mechanisms have been proposed for this family of compounds, being altered and updated as new biosynthetic methods develop and new results emerge. After nearly 100 years of such research, this review aims to provide a comprehensive overview of what is currently known about the biosynthesis of this important family, amalgamating the early chemical and biosynthetic studies with the more recent genetics-based advances and comparative bioinformatics.

Structure-based engineering of α-ketoglutarate dependent oxygenases in fungal meroterpenoid biosynthesis

Non-heme iron- and α-ketoglutarate-dependent oxygenases (αKG OXs) are key enzymes that play a major role in diversifying the structure of fungal meroterpenoids. They activate a specific C-H bond of the substrate to first generate radical species, which is usually followed by oxygen rebound to produce cannonical hydroxylated products. However, in some cases remarkable chemistry induces dramatic structural changes in the molecular scaffolds, depending on the stereoelectronic characters of the substrate/intermediates and the resulting conformational changes/movements of the active site of the enzyme. Their molecular bases have been extensively investigated by crystallographic structural analyses and structure-based mutagenesis, which revealed intimate structural details of the enzyme reactions. This information facilitates the manipulation of the enzyme reactions to create unnatural, novel molecules for drug discovery. This review summarizes recent progress in the structure-based engineering of αKG OX enzymes, involved in the biosynthesis of polyketide-derived fungal meroterpenoids. The literature published from 2016 through February 2022 is reviewed.

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.

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.

Structural Basis for Isomerization Reactions in Fungal Tetrahydroxanthone Biosynthesis and Diversification

The novel isomerase NsrQ, from Aspergillus novofumigatus, is a key enzyme in the biosynthesis of fungal tetrahydroxanthones and is responsible for dearomatizing cyclization to provide a tetrahydroxanthone scaffold. NsrQ catalyzes a two-step isomerization reaction, involving the isomerization of allylic alcohol and subsequent inversion of configuration at the methyl group. We report on the biochemical and structural characterizations of NsrQ, and its homologue Dcr3, from Diaporthe longicolla. The crystal structures of NsrQ and Dcr3 revealed their similar overall structures, with a cone-shaped α+β barrel fold, to those of the nuclear transport factor 2-like superfamily enzymes. Furthermore, the structures of Dcr3 and NsrQ variants complexed with substrate analogues and the site-directed mutagenesis studies identified the catalytic residues and the important hydrophobic residues in shaping the active site pocket for substrate binding. These enzymes thus utilize Glu and His residues as acid-base catalysts. Based on these observations, we proposed a detailed reaction mechanism for NsrQ-catalyzed isomerization reactions.

Furanaspermeroterpenes A and B, two unusual meroterpenoids with a unique 6/6/6/5/5 pentacyclic skeleton from the Marine-derived fungus Aspergillus terreus GZU-31-1

Furanaspermeroterpenes A (1) and B (2), with a unique 6/6/6/5/5 pentacyclic skeleton, and five new congeners aspermeroterpenes D-H (3-7) were co-isolated from the marine-derived fungus Aspergillus terreus GZU-31-1. Among them, compounds 1 and 2 with rare five-membered D/E coupling rings were the first example of DMOA-derived meroterpenoids. Moreover, compound 3 was the first reported 6/6/6/6/5 pentacyclic meroterpenoid featuring an unusual cis-fused A/B ring. In the bioassays, all of the isolates were evaluated on the inhibitory activities against lipopolysaccharide-induced nitric oxide production in RAW 264.7 cells, and compounds 3-7 exhibited significant anti-inflammatory activity with IC₅₀ values ranging from 6.74 to 29.59 μM than positive control (Indomethacin, IC₅₀ 30.98 μM).

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.

Anti-cryptococcal activity of preussolides A and B, phosphoethanolamine-substituted 24-membered macrolides, and leptosin C from coprophilous isolates of Preussia typharum

Cryptococcus neoformans is a serious human pathogen with limited options for treatment. We have interrogated extracts from fungal fermentations to find Cryptococcus-inhibiting natural products using assays for growth inhibition and differential thermosensitivity. Extracts from fermentations of four fungal strains from wild and domestic animal dung from Arkansas and West Virginia, USA were identified as Preussia typharum. The extracts exhibited two antifungal regions. Purification of one region yielded new 24-carbon macrolides incorporating both a phosphoethanolamine unit and a bridging tetrahydrofuran ring. The structures of these metabolites were established mainly by analysis of high-resolution mass spectrometry and 2D NMR data. Relative configurations were assigned using NOESY data, and the structure assignments were supported by NMR comparison with similar compounds. These new metabolites are designated preussolides A and B. The second active region was caused by the cytotoxin, leptosin C. Genome sequencing of the four strains revealed biosynthetic gene clusters consistent with those known to encode phosphoethanolamine-bearing polyketide macrolides and the biosynthesis of dimeric epipolythiodioxopiperazines. All three compounds showed moderate to potent and selective antifungal activity toward the pathogenic yeast C. neoformans.

Discovery of andrastones from the deep-sea-derived Penicillium allii-sativi MCCC 3A00580 by OSMAC strategy

Andrastones are unusual 6,6,6,5-tetracyclic meroterpenoids that are rarely found in nature. Previously, three andrastones were obtained from the rice static fermentation extract of the deep-sea-derived fungus Penicillium allii-sativi MCCC 3A00580. Inspired by one strain many compounds (OSMAC) approach, the oat static fermentation on P. allii-sativi was conducted. As a result, 14 andrastones were isolated by UV-guided isolation. The chemical structures of the nine new compounds (1-9) was established by comprehensive analysis of the NMR, MS, ECD, and X-ray crystallography and the five known ones (10-14) were assigned by comparing their NMR, MS, and OR data with those reported in literature. Compound 1 bears a novel hemiketal moiety while 2 is the first example to possess a novel tetrahydrofuran moiety via C-7 and C-15. All isolates were tested for anti-allergic bioactivity. Compound 10, 3-deacetylcitreohybridonol, significantly decreased degranulation with the IC₅₀ value of 14.8 μM, compared to that of 92.5 μM for the positive control, loratadine. Mechanism study indicated 10 could decrease the generation of histamine and TNF-α by reducing the accumulation of Ca²⁺ in RBL-2H3 cells. These findings indicate andrastones could be potential to discover new anti-allergic candidate drugs.

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.

Fungal-derived brevianamide assembly by a stereoselective semipinacolase

Fungal bicyclo[2.2.2]diazaoctane indole alkaloids represent an important family of natural products with a wide-spectrum of biological activities. Although biomimetic total syntheses of representative compounds have been reported, the details of their biogenesis, especially the mechanisms for assembly of diastereomerically distinct and enantiomerically antipodal metabolites, have remained largely uncharacterized. Brevianamide A represents a basic form of the sub-family bearing a dioxopiperazine core and a rare 3-spiro-ψ-indoxyl skeleton. Here, we identified the Brevianamide A biosynthetic gene cluster from Penicillium brevicompactum NRRL 864 and elucidated the metabolic pathway. BvnE was revealed to be an essential isomerase/semi-pinacolase that specifies selective production of the natural product. Structural elucidation, molecular modeling, and mutational analysis of BvnE, and quantum chemical calculations provided mechanistic insights into the diastereoselective formation of the 3-spiro-ψ-indoxyl moiety in Brevianamide A. This occurs through a BvnE-controlled semi-pinacol rearrangement and a subsequent spontaneous intramolecular [4+2] hetero-Diels-Alder cycloaddition.

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.

Highly oxygenated meroterpenoids from the Antarctic fungus Aspergillus terreus

Eleven highly oxygenated meroterpenoids, named terreustoxins A-K, along with five known analogues, were isolated from the Antarctic fungus Aspergillus terreus. The structures and absolute configurations of these undescribed compounds were characterized by NMR spectroscopy, single-crystal X-ray crystallography, and ECD experiments. Terreustoxins A-D are the first examples of meroterpenoids with two ortho-hydroxy groups at C-6 and C-7 in the terretonins family. Terreustoxin C and terretonin inhibited the proliferation of Con A-induced murine T cells at the concentration of 10 μM.

Compartmentalized biosynthesis of mycophenolic acid

Mycophenolic acid (MPA) from filamentous fungi is the first natural product antibiotic to be isolated and crystallized, and a first-line immunosuppressive drug for organ transplantations and autoimmune diseases. However, some key biosynthetic mechanisms of such an old and important molecule have remained unclear. Here, we elucidate the MPA biosynthetic pathway that features both compartmentalized enzymatic steps and unique cooperation between biosynthetic and β-oxidation catabolism machineries based on targeted gene inactivation, feeding experiments in heterologous expression hosts, enzyme functional characterization and kinetic analysis, and microscopic observation of protein subcellular localization. Besides identification of the oxygenase MpaB' as the long-sought key enzyme responsible for the oxidative cleavage of the farnesyl side chain, we reveal the intriguing pattern of compartmentalization for the MPA biosynthetic enzymes, including the cytosolic polyketide synthase MpaC' and O-methyltransferase MpaG', the Golgi apparatus-associated prenyltransferase MpaA', the endoplasmic reticulum-bound oxygenase MpaB' and P450-hydrolase fusion enzyme MpaDE', and the peroxisomal acyl-coenzyme A (CoA) hydrolase MpaH'. The whole pathway is elegantly comediated by these compartmentalized enzymes, together with the peroxisomal β-oxidation machinery. Beyond characterizing the remaining outstanding steps of the MPA biosynthetic steps, our study highlights the importance of considering subcellular contexts and the broader cellular metabolism in natural product biosynthesis.

Structure revision of cryptosporioptides and determination of the genetic basis for dimeric xanthone biosynthesis in fungi

Three novel dimeric xanthones, cryptosporioptides A-C were isolated from Cryptosporiopsis sp. 8999 and their structures elucidated. Methylation of cryptosporioptide A gave a methyl ester with identical NMR data to cryptosporioptide, a compound previously reported to have been isolated from the same fungus. However, HRMS analysis revealed that cryptosporioptide is a symmetrical dimer, not a monomer as previously proposed, and the revised structure was elucidated by extensive NMR analysis. The genome of Cryptosporiopsis sp. 8999 was sequenced and the dimeric xanthone (dmx) biosynthetic gene cluster responsible for the production of the cryptosporioptides was identified. Gene disruption experiments identified a gene (dmxR5) encoding a cytochrome P450 oxygenase as being responsible for the dimerisation step late in the biosynthetic pathway. Disruption of dmxR5 led to the isolation of novel monomeric xanthones. Cryptosporioptide B and C feature an unusual ethylmalonate subunit: a hrPKS and acyl CoA carboxylase are responsible for its formation. Bioinformatic analysis of the genomes of several fungi producing related xanthones, e.g. the widely occurring ergochromes, and related metabolites allows detailed annotation of the biosynthetic genes, and a rational overall biosynthetic scheme for the production of fungal dimeric xanthones to be proposed.

Enzyme-catalysed [6+4] cycloadditions in the biosynthesis of natural products

Pericyclic reactions are powerful transformations for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. Their role in biosynthesis is increasingly apparent, and mechanisms by which pericyclases can catalyse reactions are of major interest¹. [4+2] cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis² for the formation of six-membered rings and are now well-established in biosynthesis^3-6. [6+4] and other 'higher-order' cycloadditions were predicted⁷ in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring system^8,9. However, although enzyme-catalysed [6+4] cycloadditions have been proposed^10-12, they have not been proven to occur. Here we demonstrate a group of enzymes that catalyse a pericyclic [6+4] cycloaddition, which is a crucial step in the biosynthesis of streptoseomycin-type natural products. This type of pericyclase catalyses [6+4] and [4+2] cycloadditions through a single ambimodal transition state, which is consistent with previous proposals^11,12. The [6+4] product is transformed to a less stable [4+2] adduct via a facile Cope rearrangement, and the [4+2] adduct is converted into the natural product enzymatically. Crystal structures of three pericyclases, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis establish the mechanism of this transformation. This work shows how enzymes are able to catalyse concerted pericyclic reactions involving ambimodal transition states.

Marine natural products

Covering: January to December 2017This review covers the literature published in 2017 for marine natural products (MNPs), with 740 citations (723 for the period January to December 2017) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 477 papers for 2017), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Geographic distributions of MNPs at a phylogenetic level are reported.

Brasilianoids A-F, New Meroterpenoids From the Sponge-Associated Fungus Penicillium brasilianum

3,5-Dimethylorsellinic acid (DMOA) derived meroterpenoids comprise an unique class of natural products with diverse scaffolds and with a broad spectrum of bioactivities. Bioinformatics analysis of the gene clusters in association with the qRT-PCR detection of the amplification of two key genes led to speculate that the sponge associated fungus Penicillium brasilianum WZXY-m122-9 is a potential producer of meroterpenoids. Chromatographic separation of the EtOAc extract of this fungal strain on a large-scale fermentation resulted in the isolation of six new DMOA-related meroterpenoids with trivial names of brasilianoids A–F (1-6), together with preaustinoid D and preaustinoid A2. The structures were determined by extensive analyses of spectroscopic data, including the X-ray diffraction and the ECD data for configurational assignment. Brasilianoids A and F showed an unprecedented skeleton with a γ-lactone in ring A, while brasilianoids B–C featured a 7/6/6/5/5 pentacyclic ring system finding in nature for the first time. The biosynthetic relationship among the isolated compounds was postulated. Compound 1 significantly stimulated the expression of filaggrin and caspase-14 in HaCaT cells in dose-dependent manner, while compounds 2 and 3 showed moderate inhibition against NO production in LPS-induced RAW 264.7 macrophages.

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.