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

Ultraviolet-induced melanisation in lichens: physiological traits and transcriptome profile

Lichens are important components of high-latitude boreal and Arctic habitats. While stress tolerant, they are among the most sensitive ecosystem components to climate change, in particular, an increase in ultraviolet light (UV) arising from polar ozone depletion and deforestation. This study is the first to explore the effects of UV-B on gene expression in lichens to predict metabolic pathways involved in tolerance. Using transcriptome profiling and bioinformatic analyses, here we studied the effects of UV-B on gene expression in lichens using Lobaria pulmonaria (L.) Hoff. as a model species. UV-B exposure causes significant browning of the upper cortex of the thallus, which correlates to an increased expression of biosynthetic gene clusters involved in the synthesis of eu- and allomelanins and melanin precursors. Based on transcriptome analyses, we suggest that the biosynthesis of melanins and other secondary metabolites, such as naphthalene derivates, tropolones, anthraquinones, and xanthones, is a trade-off that lichens pay to protect essential metabolic processes such as photosynthesis and respiration. Expression profiles of general stress-associated genes, in particular, related to reactive oxygen species scavenging, protection of proteins, and DNA repair, clearly indicate that the mycobiont is the more UV-B-responsive and susceptible partner in lichen symbiosis. Our findings demonstrate that UV-B stress activates an intricate gene network involved in tolerance mechanisms of lichen symbionts. Knowledge obtained here may enable the prediction of likely effects on lichen biodiversity caused by climate change and pollution.

Discovery of a Fungal P450 with an Unusual Two-Step Mechanism for Constructing a Bicyclo[3.2.2]nonane Skeleton

The successful biomimetic or chemoenzymatic synthesis of target natural products (NPs) and their derivatives relies on enzyme discovery. Herein, we discover a fungal P450 BTG5 that can catalyze the formation of a bicyclo[3.2.2]nonane structure through an unusual two-step mechanism of dimerization and cyclization in the biosynthesis of beticolin 1, whose bicyclo[3.2.2]nonane skeleton connects an anthraquinone moiety and a xanthone moiety. Further investigation reveals that BTG5-T318 not only determines the substrate selectivity but also alters the catalytic reactions, which allows the separation of the reaction to two individual steps, thereby understanding its catalytic mechanism. It reveals that the first heterodimerization undergoes the common oxidation process for P450s, while the second uncommon formal redox-neutral cyclization step is proved as a redox-mediated reaction, which has never been reported. Therefore, this work advances our understanding of P450-catalyzed reactions and paves the way for expansion of the diversity of this class of NPs through synthetic biology.

Bioactive Compounds from an Endophytic Pezicula sp. Showing Antagonistic Effects against the Ash Dieback Pathogen

A fungal endophyte originating from the Canary Islands was identified as a potent antagonist against the fungal phytopathogen Hymenoscyphus fraxineus, which causes the devastating ash dieback disease. This endophyte was tentatively identified as Pezicula cf. ericae, using molecular barcoding. Isolation of secondary metabolites by preparative high-performance liquid chromatography (HPLC) yielded the known compounds CJ-17,572 (1), mycorrhizin A (3) and cryptosporioptides A-C (4-6), besides a new N-acetylated dihydroxyphenylalanin derivative 2, named peziculastatin. Planar structures were elucidated by NMR and HRMS data, while the relative stereochemistry of 2 was assigned by H,H and C,H coupling constants. The assignment of the unknown stereochemistry of CJ-17,572 (1) was hampered by the broadening of NMR signals. Nevertheless, after semisynthetic conversion of 1 into its methyl derivatives 7 and 8, presumably preventing tautomeric effects, the relative configuration could be assigned, whereas comparison of ECD data to those of related compounds determined the absolute configuration. Metabolites 1 and 3 showed significant antifungal effects in vitro against H. fraxineus. Furthermore, 4-6 exhibited significant dispersive effects on preformed biofilms of S. aureus at concentrations up to 2 µg/mL, while the biofilm formation of C. albicans was also inhibited. Thus, cryptosporioptides might constitute a potential source for the development of novel antibiofilm agents.

Recent advances in the identification of biosynthetic genes and gene clusters of the polyketide-derived pathways for anthraquinone biosynthesis and biotechnological applications

Natural anthraquinones are represented by a large group of compounds. Some of them are widespread across the kingdoms, especially in bacteria, fungi and plants, while the others are restricted to certain groups of organisms. Despite the significant pharmacological potential of several anthraquinones (hypericin, skyrin and emodin), their biosynthetic pathways and candidate genes coding for key enzymes have not been experimentally validated. Understanding the genetic and epigenetic regulation of the anthraquinone biosynthetic gene clusters in fungal endophytes would help not only understand their pathways in plants, which ensure their commercial availability, but also favor them as promising systems for prospective biotechnological production.

Biosynthetic Pathways of Dimeric Natural Products Containing Bisanthraquinone and Related Xanthones

Many dimeric natural products containing bisanthraquinone and related xanthones with diverse structures and versatile bioactivities have been isolated over the years. However, the complicated biosynthetic pathways of such natural products, which have remained elusive until recently, negatively impact their mass bioproduction and biosynthetic structural modification for drug discovery. In this concept, we summarize the recent progress in gene cluster mining and biosynthetic pathway elucidation of natural products containing bisanthraquinone and related xanthones. These pioneering works may pave the way for further biosynthetic pathway elucidation and structure modification of dimeric natural products through gene and protein engineering.

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.

Structural analysis of an anthrol reductase inspires enantioselective synthesis of enantiopure hydroxycycloketones and β-halohydrins

Asymmetric reduction of prochiral ketones, particularly, reductive desymmetrization of 2,2-disubstituted prochiral 1,3-cyclodiketones to produce enantiopure chiral alcohols is challenging. Herein, an anthrol reductase CbAR with the ability to accommodate diverse bulky substrates, like emodin, for asymmetric reduction is identified. We firstly solve crystal structures of CbAR and CbAR-Emodin complex. It reveals that Tyr210 is critical for emodin recognition and binding, as it forms a hydrogen-bond interaction with His162 and π-π stacking interactions with emodin. This ensures the correct orientation for the stereoselectivity. Then, through structure-guided engineering, variant CbAR-H162F can convert various 2,2-disubstituted 1,3-cyclodiketones and α-haloacetophenones to optically pure (2S, 3S)-ketols and (R)-β-halohydrins, respectively. More importantly, their stereoselectivity mechanisms are also well explained by the respective crystal structures of CbAR-H162F-substrate complex. Therefore, this study demonstrates that an in-depth understanding of catalytic mechanism is valuable for exploiting the promiscuity of anthrol reductases to prepare diverse enantiopure chiral alcohols.

Biochemical and structural insights of multifunctional flavin-dependent monooxygenase FlsO1-catalyzed unexpected xanthone formation

Xanthone-containing natural products display diverse pharmacological properties. The biosynthetic mechanisms of the xanthone formation have not been well documented. Here we show that the flavoprotein monooxygenase FlsO1 in the biosynthesis of fluostatins not only functionally compensates for the monooxygenase FlsO2 in converting prejadomycin to dehydrorabelomycin, but also unexpectedly converts prejadomycin to xanthone-containing products by catalyzing three successive oxidations including hydroxylation, epoxidation and Baeyer-Villiger oxidation. We also provide biochemical evidence to support the physiological role of FlsO1 as the benzo[b]-fluorene C5-hydrolase by using nenestatin C as a substrate mimic. Finally, we resolve the crystal structure of FlsO1 in complex with the cofactor flavin adenine dinucleotide close to the “in” conformation to enable the construction of reactive substrate-docking models to understand the basis of a single enzyme-catalyzed multiple oxidations. This study highlights a mechanistic perspective for the enzymatic xanthone formation in actinomycetes and sets an example for the versatile functions of flavoproteins.

The biosynthesis of xanthones has not been well documented. Here, the authors report that monooxygenase FlsO1 catalyzes three successive oxidations – hydroxylation, epoxidation and Baeyer–Villiger oxidation—to form the xanthone scaffold in actinomycetes.

Chemistry and biosynthesis of bacterial polycyclic xanthone natural products

Covering: up to the end of 2021Bacterial polycyclic xanthone natural products (BPXNPs) are a growing family of natural xanthones featuring a pentangular architecture with various modifications to the tricyclic xanthone chromophore. Their structural diversities and various activities have fueled biosynthetic and chemical synthetic studies. Moreover, their more potent activities than the clinically used drugs make them potential candidates for the treatment of diseases. Future unraveling of structure activity relationships (SARs) will provide new options for the (bio)-synthesis of drug analogues with higher activities. This review summarizes the isolation, structural elucidation and biological activities and more importantly, the recent strategies for the microbial biosynthesis and chemical synthesis of BPXNPs. Regarding their biosynthesis, we discuss the recent progress in enzymes that synthesize tricyclic xanthone, the protein candidates for structural moieties (methylene dioxygen bridge and nitrogen heterocycle), tailoring enzymes for methylation and halogenation. The chemical synthesis part summarizes the recent methodology for the division synthesis and coupling construction of achiral molecular skeletons. Ultimately, perspectives on the biosynthetic study of BPXNPs are discussed.

Chemical diversity, medicinal potentialities, biosynthesis, and pharmacokinetics of anthraquinones and their congeners derived from marine fungi: a comprehensive update

Marine fungi receive excessive attention as prolific producers of structurally unique secondary metabolites, offering promising potential as substitutes or conjugates for current therapeutics, whereas existing research has only scratched the surface in terms of secondary metabolite diversity and potential industrial applications as only a small share of bioactive natural products have been identified from marine-derived fungi thus far. Anthraquinones derived from filamentous fungi are a distinct large group of polyketides containing compounds which feature a common 9,10-dioxoanthracene core, while their derivatives are generated through enzymatic reactions such as methylation, oxidation, or dimerization to produce a large variety of anthraquinone derivatives. A considerable number of reported anthraquinones and their derivatives have shown significant biological activities as well as highly economical, commercial, and biomedical potentialities such as anticancer, antimicrobial, antioxidant, and anti-inflammatory activities. Accordingly, and in this context, this review comprehensively covers the state-of-art over 20 years of about 208 structurally diverse anthraquinones and their derivatives isolated from different species of marine-derived fungal genera along with their reported bioactivity wherever applicable. Also, in this manuscript, we will present in brief recent insights centred on their experimentally proved biosynthetic routes. Moreover, all reported compounds were extensively investigated for their in-silico drug-likeness and pharmacokinetics properties which intriguingly highlighted a list of 20 anthraquinone-containing compounds that could be considered as potential drug lead scaffolds.

Synthetic and Computational Study of the Enantioselective [3+2]-Cycloaddition of Chromones with MBH Carbonates

Highly efficient and straightforward access to enantioenriched five-membered ring-fused chromanones is developed via [3+2]-cycloaddition of 3-cyanochromones with Morita-Baylis-Hillman carbonates. Densely functionalized chiral cyclopenta[b]chromanones with three continuous quaternary and tertiary stereogenic carbon centers were obtained in high yields with high ee and dr (≤97% yield, 97% ee, and >20:1 dr). Moreover, density functional theory calculations have been carried out to investigate the mechanism and regio- and diastereoselectivity of the reaction.

Zelkovamycins F and G, Cyclopeptides with Cα-Methyl-threonine Residues, from an Endophytic Kitasatospora sp

Zelkovamycins F and G (1 and 2), two new natural cyclic octapeptides possessing the unprecedented nonproteinogenic amino acid residues l-α-methyl-threonine and l-α-methyl-allo-threonine, respectively, along with four new analogues, zelkovamycins H-K (3-6), were identified from the endophytic Kitasatospora sp. CPCC 204717. Their structures were elucidated by detailed analysis of NMR and HRESIMS/MS spectroscopic data. The configurations of amino acid residues were determined by Marfey's analysis combined with NMR calculations. Compounds 1, 2, and 4 showed potent antibacterial activity against methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis. The structure-activity relationship study revealed that the 2-methyl-3-oxobutyrine and sarcosine residues played important roles in their antibacterial activities. Zelkovamycin (7) and zelkovamycin E (8) exhibited significant antiviral activity against the hepatitis C virus.

Elucidation of the Complete Biosynthetic Pathway of Phomoxanthone A and Identification of a Para-Para Selective Phenol Coupling Dimerase

Fungal cytochrome P450 enzymes have been shown to catalyze regio- and stereoselective oxidative intermolecular phenol coupling. However, an enzyme capable of catalyzing undirected para-para (C4-4') coupling has not been reported. Here, we revealed the biosynthetic gene cluster (BGC) of phomoxanthone A from the marine fungus Diaporthe sp. SYSU-MS4722. We heterologously expressed 14 biosynthetic genes in Aspergillus oryzae NSAR1 and found that PhoCDEFGHK is involved in the early stage of phomoxanthone A biosynthesis to give chrysophanol and that chrysophanol is then processed by PhoBJKLMNP to yield penexanthone B. A feeding experiment suggested that PhoO, a cytochrome P450 enzyme, catalyzed the regioselective oxidative para-para coupling of penexanthone B to give phomoxanthone A. The mechanism of PhoO represents a novel enzymatic 4,4'-linkage dimerization method for tetrahydroxanthone formations, which would facilitate biosynthetic engineering of structurally diverse 4,4'-linked dimeric tetrahydroxanthones.

Biosynthesis of fungal polyketides by collaborating and trans-acting enzymes

Covering: 1999 up to 2021Fungal polyketides encompass a range of structurally diverse molecules with a wide variety of biological activities. The giant multifunctional enzymes that synthesize polyketide backbones remain enigmatic, as do many of the tailoring enzymes involved in functional modifications. Recent advances in elucidating biosynthetic gene clusters (BGCs) have revealed numerous examples of fungal polyketide synthases that require the action of collaborating enzymes to synthesize the carbon backbone. This review will discuss collaborating and trans-acting enzymes involved in loading, extending, and releasing polyketide intermediates from fungal polyketide synthases, and additional modifications introduced by trans-acting enzymes demonstrating the complexity encountered when investigating natural product biosynthesis in fungi.

Biosynthetic machineries of anthraquinones and bisanthraquinones in Talaromyces islandicus

Talaromyces islandicus is a unique fungus that produces more than 20 numbers of anthraquinones (AQs) and their dimeric natural products, bisanthraquinones (BQs). These compounds share a 9,10-anthracenedione core derived from emodin. The biosynthetic pathway of emodin has been firmly established, while that of other AQs and BQs is still unclear. In this study, we identified the biosynthetic gene clusters for chrysophanol and skyrin. The function of key modification enzymes was examined by performing biotransformation experiments and in vitro enzymatic reactions with emodin and its derivatives, allowing us to propose a mechanism for the modification reactions. The present study provides insight into the biosynthesis of AQs and BQs in T. islandicus.

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.

Anti-inflammatory Dimeric Tetrahydroxanthones from an Endophytic Muyocopron laterale

Twelve new dimeric tetrahydroxanthones, muyocoxanthones A-L (1-12), were isolated from the endophytic fungus, Muyocopron laterale. Their structures were characterized on the basis of the interpretation of NMR and HRESIMS data. The absolute configurations of 1-10 and 12 were unambiguously determined by ECD spectrum data and single-crystal X-ray diffraction analysis. Compounds 2, 6, and 11 showed inhibitory activity against the LPS-induced production of nitric oxide (NO) in RAW 264.7 cells with IC₅₀ values of 5.2, 1.3, and 5.1 μM, respectively.

Bienzyme-Catalytic and Dioxygenation-Mediated Anthraquinone Ring Opening

The C-10-C-4a bond cleavage of anthraquinone is believed to be a crucial step in fungal seco-anthraquinone biosynthesis and has long been proposed as a classic Baeyer-Villiger oxidation. Nonetheless, genetic, enzymatic, and chemical information on ring opening remains elusive. Here, a revised questin ring-opening mechanism was elucidated by in vivo gene disruption, in vitro enzymatic analysis, and 18O chasing experiments. It has been confirmed that the reductase GedF is responsible for the reduction of the keto group at C-10 in questin to a hydroxyl group with the aid of NADPH. The C-10-C-4a bond of the resultant questin hydroquinone is subsequently cleaved by the atypical cofactor-free dioxygenase GedK, giving rise to desmethylsulochrin. This proposed bienzyme-catalytic and dioxygenation-mediated anthraquinone ring-opening reaction shows universality.

New Xanthones with Antiagricultural Fungal Pathogen Activities from the Endophytic Fungus Diaporthe goulteri L17

Six new xanthone dimers, diaporxanthones A-F (1-6), and an unusual xanthone monomer diaporxanthone G (7), in addition to seven known analogues (8-14), were isolated and identified from endophytic Diaporthe goulteri L17 harbored in the fruits of the salt-tolerant plant Vitex trifolia. The chemical structures of these metabolites were elucidated on the basis of nuclear magnetic resonance, high-resolution electrospray ionization mass spectrometry, and reported data in the literature. Their absolute configurations were established by single-crystal X-ray diffraction analysis together with time-dependent density functional theory electronic circular dichroism calculations. Among these compounds, compounds 1 and 6 exhibited moderate antifungal activities against Nectria sp. and Colletotrichum musae and compound 4 showed significant cytotoxicity against all selected five cancer cell lines.

Transcription Factor Repurposing Offers Insights into Evolution of Biosynthetic Gene Cluster Regulation

The fungal kingdom has provided advances in our ability to identify biosynthetic gene clusters (BGCs) and to examine how gene composition of BGCs evolves across species and genera. However, little is known about the evolution of specific BGC regulators that mediate how BGCs produce secondary metabolites (SMs). A bioinformatics search for conservation of the Aspergillus fumigatus xanthocillin BGC revealed an evolutionary trail of xan-like BGCs across Eurotiales species. Although the critical regulatory and enzymatic genes were conserved in Penicillium expansum, overexpression (OE) of the conserved xan BGC transcription factor (TF) gene, PexanC, failed to activate the putative xan BGC transcription or xanthocillin production in P. expansum, in contrast to the role of AfXanC in A. fumigatus. Surprisingly, OE::PexanC was instead found to promote citrinin synthesis in P. expansum via trans induction of the cit pathway-specific TF, ctnA, as determined by cit BGC expression and chemical profiling of ctnA deletion and OE::PexanC single and double mutants. OE::AfxanC results in significant increases of xan gene expression and metabolite synthesis in A. fumigatus but had no effect on either xanthocillin or citrinin production in P. expansum. Bioinformatics and promoter mutation analysis led to the identification of an AfXanC binding site, 5'-AGTCAGCA-3', in promoter regions of the A. fumigatus xan BGC genes. This motif was not in the ctnA promoter, suggesting a different binding site of PeXanC. A compilation of a bioinformatics examination of XanC orthologs and the presence/absence of the 5'-AGTCAGCA-3' binding motif in xan BGCs in multiple Aspergillus and Penicillium spp. supports an evolutionary divergence of XanC regulatory targets that we speculate reflects an exaptation event in the Eurotiales. IMPORTANCE Fungal secondary metabolites (SMs) are an important source of pharmaceuticals on one hand and toxins on the other. Efforts to identify the biosynthetic gene clusters (BGCs) that synthesize SMs have yielded significant insights into how variation in the genes that compose BGCs may impact subsequent metabolite production within and between species. However, the role of regulatory genes in BGC activation is less well understood. Our finding that the bZIP transcription factor XanC, located in the xanthocillin BGC of both Aspergillus fumigatus and Penicillium expansum, has functionally diverged to regulate different BGCs in these two species emphasizes that the diversification of BGC regulatory elements may sometimes occur through exaptation, which is the co-option of a gene that evolved for one function to a novel function. Furthermore, this work suggests that the loss/gain of transcription factor binding site targets may be an important mediator in the evolution of secondary-metabolism regulatory elements.

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.

Heterologous Biosynthesis of Tetrahydroxanthone Dimers: Determination of Key Factors for Selective or Divergent Synthesis

Tetrahydroxanthone dimers are fungal products, among which secalonic acid D (1) is one of the most studied compounds because of its potent biological activity. Because the biosynthetic gene cluster of 1 has been previously identified, we sought to heterologously produce 1 in Aspergillus oryzae by expressing the relevant biosynthetic genes. However, our initial attempt of the total biosynthesis of 1 failed; instead, it produced four isomers of 1 due to the activity of an endogenous enzyme of A. oryzae. Subsequent overexpression of the Baeyer-Villiger monooxygenase, AacuH, which competes with the endogenous enzyme, altered the product profile and successfully generated 1. Characterization of the key biosynthetic enzymes revealed the surprising substrate promiscuity of the dimerizing enzyme, AacuE, and indicated that efficient synthesis of 1 requires highly selective preparation of the tetrahydroxanthone monomer, which is apparently controlled by AacuH. This study facilitates engineered biosynthesis of tetrahydroxanthone dimers both in a selective and divergent manner.

Enzymatic dimerization in the biosynthetic pathway of microbial natural products

Covering: up to August 2020The dramatic increase in the identification of dimeric natural products generated by microorganisms and plants has played a significant role in drug discovery. The biosynthetic pathways of these products feature inherent dimerization reactions, which are valuable for biosynthetic applications and chemical transformations. The extraordinary mechanisms of the dimerization of secondary metabolites should advance our understanding of the uncommon chemical rules for natural product biosynthesis, which will, in turn, accelerate the discovery of dimeric reactions and molecules in nature and provide promising strategies for the total synthesis of natural products through dimerization. This review focuses on the enzymes involved in the dimerization in the biosynthetic pathway of microbial natural products, with an emphasis on cytochrome P450s, laccases, and intermolecular [4 + 2] cyclases, along with other atypical enzymes. The identification, characterization, and catalytic landscapes of these enzymes are also introduced.

Fusarochromene, a novel tryptophan-derived metabolite from Fusarium sacchari

Fusarochromene isolated from the plant pathogenic fungus, Fusarium sacchari is closely related to a group of mycotoxins including fusarochromanone previously isolated from various Fusaria spp. Despite their assumed polyketide biogenesis, incorporation studies with 13C-labelled acetate, glycerol and tryptophans show that fusarochromene is unexpectedly derived via oxidative cleavage of the aromatic amino acid tryptophan. A putative biosynthetic gene cluster has been identified.

Promiscuity of an unrelated anthrol reductase of Talaromyces islandicus WF-38-12

A new anthrol reductase from Talaromyces islandicus (ARti-2).

Regio- and stereoselective intermolecular phenol coupling enzymes in secondary metabolite biosynthesis

Covering: 2005 to 2020Phenol coupling is a key reaction in the biosynthesis of important biopolymers such as lignin and melanin and of a plethora of biarylic secondary metabolites. The reaction usually leads to several different regioisomeric products due to the delocalization of a radical in the reaction intermediates. If axial chirality is involved, stereoisomeric products are obtained provided no external factor influences the selectivity. Hence, in non-enzymatic organic synthesis it is notoriously difficult to control the selectivity of the reaction, in particular if the coupling is intermolecular. From biosynthesis, it is known that especially fungi, plants, and bacteria produce biarylic compounds regio- and stereoselectively. Nonetheless, the involved enzymes long evaded discovery. First progress was made in the late 1990s; however, the breakthrough came only with the genomic era and, in particular, in the last few years the number of relevant publications has dramatically increased. The discoveries reviewed in this article reveal a remarkable diversity of enzymes that catalyze oxidative intermolecular phenol coupling, including various classes of laccases, cytochrome P450 enzymes, and heme peroxidases. Particularly in the case of laccases, the catalytic systems are often complex and additional proteins, substrates, or reaction conditions have a strong influence on activity and regio- and atroposelectivity. Although the field of (selective) enzymatic phenol coupling is still in its infancy, the diversity of enzymes identified recently could make it easier to select suitable candidates for biotechnological development and to approach this challenging reaction through biocatalysis.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

Formicamycin biosynthesis involves a unique reductive ring contraction

Using a combination of biomimetic chemistry and molecular genetics we demonstrate that formicamycin biosynthesis proceeds via reductive Favorskii-like reaction.

Fasamycin natural products are biosynthetic precursors of the formicamycins. Both groups of compounds are polyketide natural products that exhibit potent antibacterial activity despite displaying different three-dimensional topologies. We show here that transformation of fasamycin into formicamycin metabolites requires two gene products and occurs via a novel two-step ring expansion-ring contraction pathway. Deletion of forX, encoding a flavin dependent monooxygenase, abolished formicamycin production and leads to accumulation of fasamycin E. Deletion of the adjacent gene forY, encoding a flavin dependent oxidoreductase, also abolished formicamycin biosynthesis and led to the accumulation of new lactone metabolites that represent Baeyer–Villiger oxidation products of the fasamycins. These results identify ForX as a Baeyer–Villiger monooxygenase capable of dearomatizing ring C of the fasamycins. Through in vivo cross feeding and biomimetic semi-synthesis experiments we showed that these lactone products represent biosynthetic intermediates that are reduced to formicamycins in a unique reductive ring contraction reaction catalyzed by ForY.

Bioactive Polyhydroxanthones from Penicillium purpurogenum

Eight new polyhydroxanthones, penicixanthones A-H (1-8), including four monomers (1-4) and four dimers (5-8), were isolated from solid cultures of Penicillium purpurogenum SC0070. Their structures were elucidated by extensive spectroscopic analysis, X-ray single-crystal diffraction, and theoretical computations of ECD spectra. Penicixanthone B (2) has a hexahydroxanthone structure featuring an unusual oxygen bridge between C-6 and C-8a. Penicixanthone D (4) is distinct from other penicixanthones in stereochemistry, and its biosynthetic mechanism was proposed based on theoretical simulations for the reaction pathway of C-10a epimerization. Penicixanthone G (6) exhibited the most potent cytotoxicity (IC₅₀: 0.3-0.6 μM) when tested against human carcinoma A549, HeLa, and HepG2 cells, whereas it was nontoxic to the normal Vero cells (IC₅₀ > 50 μM). It also displayed the strongest antibacterial activity (MIC: 0.4 μg/mL) against both Staphylococcus aureus and the methicillin-resistant strain MRSA.

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 niduterpenoid A from Aspergillus nidulans.