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 biosynthesis3-6. [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 system8,9. However, although enzyme-catalysed [6+4] cycloadditions have been proposed10-12, 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 proposals11,12. 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.

P450-Modified Multicyclic Cyclophane-Containing Ribosomally Synthesized and Post-Translationally Modified Peptides.

Cyclic peptides with cyclophane linkers are an attractive compound type owing to the fine-tuned rigid three-dimensional structures and unusual biophysical features. Cytochrome P450 enzymes are capable of catalyzing not only the C-C and C-O oxidative coupling reactions found in vancomycin and other nonribosomal peptides (NRPs), but they also exhibit novel catalytic activities to generate cyclic ribosomally synthesized and post-translationally modified peptides (RiPPs) through cyclophane linkage. To discover more P450-modified multicyclic RiPPs, we set out to find cryptic and unknown P450-modified RiPP biosynthetic gene clusters (BGCs) through genome mining. Synergized bioinformatic analysis reveals that P450-modified RiPP BGCs are broadly distributed in bacteria and can be classified into 11 classes. Focusing on two classes of P450-modified RiPP BGCs where precursor peptides contain multiple conserved aromatic amino acid residues, we characterized 11 novel P450-modified multicyclic RiPPs with different cyclophane linkers through heterologous expression. Further mutation of the key ring-forming residues and combinatorial biosynthesis study revealed the order of bond formation and the specificity of P450s. This study reveals the functional diversity of P450 enzymes involved in the cyclophane-containing RiPPs and indicates that P450 enzymes are promising tools for rapidly obtaining structurally diverse cyclic peptide derivatives.

P450-Modified Ribosomally Synthesized Peptides with Aromatic Cross-Links.

Cyclization of linear peptides is an effective strategy to convert flexible molecules into rigid compounds, which is of great significance for enhancing the peptide stability and bioactivity. Despite significant advances in the past few decades, Nature and chemists' ability to macrocyclize linear peptides is still quite limited. P450 enzymes have been reported to catalyze macrocyclization of peptides through cross-linkers between aromatic amino acids with only three examples. Herein, we developed an efficient workflow for the identification of P450-modified RiPPs in bacterial genomes, resulting in the discovery of a large number of P450-modified RiPP gene clusters. Combined with subsequent expression and structural characterization of the products, we have identified 11 novel P450-modified RiPPs with different cross-linking patterns from four distinct classes. Our results greatly expand the structural diversity of P450-modified RiPPs and provide new insights and enzymatic tools for the production of cyclic peptides.

Pyridoxal-5'-phosphate-dependent bifunctional enzyme catalyzed biosynthesis of indolizidine alkaloids in fungi.

Indolizidine alkaloids such as anticancer drugs vinblastine and vincristine are exceptionally attractive due to their widespread occurrence, prominent bioactivity, complex structure, and sophisticated involvement in the chemical defense for the producing organisms. However, the versatility of the indolizidine alkaloid biosynthesis remains incompletely addressed since the knowledge about such biosynthetic machineries is only limited to several representatives. Herein, we describe the biosynthetic gene cluster (BGC) for the biosynthesis of curvulamine, a skeletally unprecedented antibacterial indolizidine alkaloid from Curvularia sp. IFB-Z10. The molecular architecture of curvulamine results from the functional collaboration of a highly reducing polyketide synthase (CuaA), a pyridoxal-5'-phosphate (PLP)-dependent aminotransferase (CuaB), an NADPH-dependent dehydrogenase (CuaC), and a FAD-dependent monooxygenase (CuaD), with its transportation and abundance regulated by a major facilitator superfamily permease (CuaE) and a Zn(II)Cys6 transcription factor (CuaF), respectively. In contrast to expectations, CuaB is bifunctional and capable of catalyzing the Claisen condensation to form a new C-C bond and the α-hydroxylation of the alanine moiety in exposure to dioxygen. Inspired and guided by the distinct function of CuaB, our genome mining effort discovers bipolamines A-I (bipolamine G is more antibacterial than curvulamine), which represent a collection of previously undescribed polyketide alkaloids from a silent BGC in Bipolaris maydis ATCC48331. The work provides insight into nature's arsenal for the indolizidine-coined skeletal formation and adds evidence in support of the functional versatility of PLP-dependent enzymes in fungi.

Total Biosynthesis of Mutaxanthene Unveils a Flavoprotein Monooxygenase Catalyzing Xanthene Ring Formation.

Flavoprotein monooxygenases (FPMOs) play important roles in generating structural complexity and diversity in natural products biosynthesized by type II polyketide synthases (PKSs). In this study, we used genome mining to discover novel mutaxanthene analogues and investigated the biosynthesis of these aromatic polyketides and their unusual xanthene framework. We determined the complete biosynthetic pathway of mutaxathene through in vivo gene deletion and in vitro biochemical experiments. We show that a multifunctional FPMO, MtxO4, catalyzes ring rearrangement and generates the required xanthene ring through a multistep transformation. In addition, we successfully obtained all necessary enzymes for in vitro reconstitution and completed the total biosynthesis of mutaxanthene in a stepwise manner. Our results revealed the formation of a rare xanthene ring in type II polyketide biosynthesis, and demonstrate the potential of using total biosynthesis for the discovery of natural products synthesized by type II PKSs.

Cytochrome†P450 Catalyzes Benzene Ring Formation in the Biosynthesis of Trialkyl-Substituted Aromatic Polyketides.

Lorneic acid and related natural products are characterized by a trialkyl-substituted benzene ring. The formation of the aromatic core in the middle of the polyketide chain is unusual. We characterized a cytochrome P450 enzyme that can catalyze the hallmark benzene ring formation from an acyclic polyene substrate through genetic and biochemical analysis. Using this P450 as a beacon for genome mining, we obtained 12 homologous type I polyketide synthase (PKS) gene clusters, among which two gene clusters are activated and able to produce trialkyl-substituted aromatic polyketides. Quantum chemical calculations were performed to elucidate the plausible mechanism for P450-catalyzed benzene ring formation. Our work expands our knowledge of the catalytic diversity of cytochrome P450.

In Vitro Reconstitution of Cinnamoyl Moiety Reveals Two Distinct Cyclases for Benzene Ring Formation.

Cinnamoyl-containing natural products (CCNPs) are a small class of bacterial metabolites with notable bioactivities. The biosynthesis of cinnamoyl moiety has been proposed to be assembled by an unusual highly reducing (HR) type II polyketide synthases (PKS). However, the biosynthetic route, especially the cyclization step for the benzene ring formation, remains unclear. In this work, we successfully reconstituted the pathway of cinnamoyl moiety in kitacinnamycin biosynthesis through a step-wise approach in vitro and demonstrated that a three-protein complex, Kcn17-Kcn18-Kcn19, can catalyze 6π-electrocyclization followed by dehydrogenation to form the benzene ring. We found that the three-protein homologues were widely distributed among 207 HR type II PKS biosynthetic gene clusters including five known CCNPs. In contrast, in the biosynthesis of youssoufene, a cinnamoyl-containing polyene, we identified that the benzene ring formation was accomplished by a distinct orphan protein. Thus, our work resolved the long-standing mystery in cinnamoyl biosynthesis and revealed two distinct enzymes that can synthesize benzene rings via polyene precursors.

Spirocitromycetin, a Fungal Polyketide with an Antiosteoporotic Pharmacophore.

Spirocitromycetin, an antiosteoporotic polyketide bearing a unique spirocycle, was characterized from a human mucus sputum-derived Penicillium velutinum. Its structure and absolute configuration were elucidated spectrally, with its biosynthetic pathway likely mediated via polivione, a reported heptaketide. Spirocitromycetin was shown to be antiosteoporotic at 0.1 µM in the prednisolone-induced osteoporotic zebrafish model. A combination of spirocitromycetin variant synthesis and bioassay has identified 5'-methyl-3'H-spiro[chromane-3,2'-furan]-3',4-dione as an unreported antiosteroporotic pharmacophore. Collectively, this work offers new starting (sub)structures that may be of significance for antiosteoporotic drug discovery.

Alternatones A and B, two polyketides possessing novel skeletons from entophyte Alternaria alternate L-10.

On our ongoing searching for bioactive natural products derived from entophytes, two polyketides possessing novel skeletons, alternatones A-B (1-2), were identified from the culture of Alternaria alternate L-10. Their structures were established by a combination of spectroscopic and single-crystal X-ray diffraction with Cu Ka radiation. Alternatone A (1) exhibited cytotoxic activity against human hepatoma carcinoma HepG-2 cell line. The putative biosynthetic pathways for compounds 1-2 were also proposed.

Biosynthesis of Sordarin Revealing a Diels-Alderase for the Formation of the Norbornene Skeleton.

Sordarin (1) is a fungal diterpene glycoside that displays potent antifungal bioactivity through inhibition of elongation factor 2. The structures of sordarin and related compounds feature a highly rearranged tetracyclic diterpene core. In this study, we identified a concise pathway in the biosynthesis of sordarin. A diterpene cyclase (SdnA) generates the 5/8/5 cycloaraneosene framework, which is decorated by a set of P450s that catalyze a series of oxidation reactions, including hydroxylation, desaturation, and C-C bond oxidative cleavage, to give a carboxylate intermediate with a terminal alkene and a cyclopentadiene moiety. A novel Diels-Alderase SdnG catalyzes an intramolecular Diels-Alder (IMDA) reaction on this intermediate to forge the sordarin core structure. Subsequent methyl hydroxylation and glycosylation complete the biosynthesis of sordarin. Our work discloses a new strategy used by nature for the formation of the rearranged diterpene skeleton.

An NADPH-Dependent Ketoreductase Catalyses the Tetracyclic to Pentacyclic Skeletal Rearrangement in Chartreusin Biosynthesis.

Redox tailoring enzymes play key roles in generating structural complexity and diversity in type II polyketides. In chartreusin biosynthesis, the early 13 C-labeling experiments and bioinformatic analysis suggest the unusual aglycone is originated from a tetracyclic anthracyclic polyketide. Here, we demonstrated that the carbon skeleton rearrangement from a linear anthracyclic polyketide to an angular pentacyclic biosynthetic intermediate requires two redox enzymes. The flavin-dependent monooxygenase ChaZ catalyses a Baeyer-Villiger oxidation on resomycin C to form a seven-membered lactone. Subsequently, a ketoreductase ChaE rearranges the carbon skeleton and affords the α-pyrone containing pentacyclic intermediate in an NADPH-dependent manner via tandem reactions including the reduction of the lactone carbonyl group, Aldol-type reaction, followed by a spontaneous γ-lactone ring formation, oxidation and aromatization. Our work reveals an unprecedented function of a ketoreductase that contributes to generate structural complexity of aromatic polyketide.

Cytotoxic Xanthone Derivatives from the Mangrove-Derived Endophytic Fungus Peniophora incarnata Z4.

The mangrove-derived endophytic fungus Peniophora incarnata Z4 produced seven new xanthone derivatives, including four new tetrahydroxanthones (1-4), one new chromone (5), one new xanthone (6), and one new xanthone dimer (7), together with one known compound, globosuxanthone B (8). Their structures were determined by an extensive analysis of 1D and 2D NMR, HRESIMS, ECD, and single-crystal X-ray diffraction data. In cytotoxic activity assays, compound 2 showed cytotoxicity against three carcinoma cell lines with IC50 values less than 10 µM.

Cytotoxic Trichothecene Macrolides Produced by the Endophytic Myrothecium roridum.

Six new macrolides named myrothecines D-G (1-4), 16-hydroxymytoxin B (5), and 14'-dehydrovertisporin (6), including four 10,13-cyclotrichothecane derivatives, in addition to 12 known compounds (7-18), were isolated from three endophytic Myrothecium roridum, IFB-E008, IFB-E009, and IFB-E012. The isolated compounds were characterized by MS, NMR, CD, and single-crystal X-ray crystallography. The isolated macrolides exhibited an antiproliferation effect against chronic myeloid leukemia K562 and colorectal carcinoma SW1116 cell lines. Compounds 1-6 were cytotoxic, with IC50 values ranging between 56 nM and 16 µM. Since slight structural changes led to obvious activity differences, the CoMFA (comparative molecular field analysis) and CoMSIA (comparative molecular similarity indices analysis) methods were then used to explore the 3D QSAR (three-dimensional quantitative structure-activity relationship) of these macrolides. The result showed that the steric, electrostatic, hydrophobic, and H-bond acceptor factors were involved in their cytotoxicity and provided an in-depth understanding of the structure-activity relationships of these metabolites.

Bialternacins A-F, Aromatic Polyketide Dimers from an Endophytic Alternaria sp.

Six novel aromatic polyketide dimers, bialternacins A-F (1-6), were isolated from a plant endophytic Alternaria sp. The structures of compounds 1-6 were elucidated on the basis of extensive spectroscopic analysis, single-crystal X-ray diffraction, and electronic circular dichroism analysis. Compounds 1, 2, 5, and 6 were characterized as four pairs of racemic mixtures. Compound (+)-5 was demonstrated to show acetylcholinesterase inhibitory activity with an IC50 value of 15.5 µM. A putative biosynthetic pathway for these compounds was proposed.

Molecular Basis for the Final Oxidative Rearrangement Steps in Chartreusin Biosynthesis.

Oxidative rearrangements play key roles in introducing structural complexity and biological activities of natural products biosynthesized by type II polyketide synthases (PKSs). Chartreusin (1) is a potent antitumor polyketide that contains a unique rearranged pentacyclic aromatic bilactone aglycone derived from a type II PKS. Herein, we report an unprecedented dioxygenase, ChaP, that catalyzes the final α-pyrone ring formation in 1 biosynthesis using flavin-activated oxygen as an oxidant. The X-ray crystal structures of ChaP and two homologues, docking studies, and site-directed mutagenesis provided insights into the molecular basis of the oxidative rearrangement that involves two successive C-C bond cleavage steps followed by lactonization. ChaP is the first example of a dioxygenase that requires a flavin-activated oxygen as a substrate despite lacking flavin binding sites, and represents a new class in the vicinal oxygen chelate enzyme superfamily.

New Antibacterial Phenone Derivatives Asperphenone A-C from Mangrove-Derived Fungus Aspergillus sp. YHZ-1.

Marine fungi are a promising source of novel bioactive natural products with diverse structure. In our search for new bioactive natural products from marine fungi, three new phenone derivatives, asperphenone A-C (1-3), have been isolated from the ethyl acetate extract of the fermentation broth of the mangrove-derived fungus, Aspergillus sp. YHZ-1. The chemical structures of these natural products were elucidated on the basis of mass spectrometry, one- and two-dimensional NMR spectroscopic analysis and asperphenone A and B were confirmed by single-crystal X-ray crystallography. Compounds 1 and 2 exhibited weak antibacterial activity against four Gram-positive bacteria, Staphylococcus aureus CMCC(B) 26003, Streptococcus pyogenes ATCC19615, Bacillus subtilis CICC 10283 and Micrococcus luteus, with the MIC values higher than 32.0 µM.

Novel chaetospirolactone and orsellide F from an endophytic fungus Chaetomium sp.

Chaetospirolactone (1), a novel spiro-lactone bearing a rare 1-oxaspiro [4.4] non-7-ene-2,6-dione skeleton, and orsellide F (2), together with six known compounds (3-8), were isolated from an endophytic fungus Chaetomium sp. NF00754. Their structures were determined by interpretation of spectroscopic data. The absolute configurations of 1 and 2 were established by analysis of single X-ray crystallographic data and CD spectra. Compounds 3, 4, and 6 showed moderate acetylcholinesterase inhibitory activity with IC50 values of 7.34, 5.19, and 7.67 µM, respectively.

New p-terphenyls from the endophytic fungus Aspergillus sp. YXf3.

Five new p-terphenyls named prenylterphenyllin D (1), prenylterphenyllin E (2), 2'-O-methylprenylterphenyllin (3), 4-O-methylprenylterphenyllin (4) and 3'-O-methylterphenyllin (5) together with seven known compounds (6-12), were isolated from cultures of Aspergillus sp. YXf3. The structures of the new compounds were elucidated by extensive MS and NMR analyses. The NMR and MS data of 5 is reported for the first time, as its structure was listed in SciFinder Scholar with no associated reference. Compounds 6 and 7 were distinguished from each other on the basis of 2D NMR experiments. Compounds 1, 2, 3 and 8 showed antibacterial activities against X. oryzae pv. oryzicola Swings and E. amylovora with the same MIC values of 20µg/mL while 10 exhibited activities against E. amylovora with an MIC value of 10µg/mL.

Rifamorpholines A-E, potential antibiotics from locust-associated actinobacteria Amycolatopsis sp. Hca4.

Cultivation of locust associated rare actinobacteria, Amycolatopsis sp. HCa4, has provided five unusual macrolactams rifamorpholines A-E. Their structures were determined by interpretation of spectroscopic and crystallographic data. Rifamorpholines A-E possess an unprecedented 5/6/6/6 ring chromophore, representing a new subclass of rifamycin antibiotics. The biosynthetic pathway for compounds 1-5 involves a key 1,6-cyclization for the formation of the morpholine ring. Compounds 2 and 4 showed potent activities against methicillin-resistant Staphylococcus aureus (MRSA) with MICs of 4.0 and 8.0 µM, respectively.

Strepchazolins A and B: Two New Alkaloids from a Marine Streptomyces chartreusis NA02069.

Two new alkaloids, strepchazolins A (1) and B (2), together with a previously reported compound, streptazolin (3), were isolated from a marine actinomycete, Streptomyces chartreusis NA02069, collected in the Coast of Hainan Island, China. The structures of new compounds were determined by extensive NMR, mass spectroscopic and X-ray crystallographic analysis, as well as modified Mosher's method. Compound 1 showed weak anti-Bacillus subtilis activity with the MIC value of 64.0 µM, and weak inhibitory activity against acetylcholinesterase (AChE) in vitro with IC50 value of 50.6 µM, while its diastereoisomer, Compound 2, is almost inactive.