Cytochrome P450-Catalyzed Regio- and Stereoselective Phenol Coupling of Fungal Natural Products

Isocoumarins from Spegazzinia sp. MDCW-573 with Antibacterial and Proangiogenic Activities

Twelve new isocoumarins, spegazmarins A-L (1-12), including nine novel dimeric derivatives (1-9), three monomeric derivatives (10-12), as well as eight known ones (13-20), were isolated from the endophytic fungus Spegazzinia sp. MDCW-573. Their structures were elucidated by analysis of NMR, X-ray crystallography, and ECD data. Notably, the dimeric isocoumarins (1-9) possess a unique linkage, where the phenyl of one monomer is connected to the lactone of another. The methods for determining the configurations of both the monomeric and dimeric isocoumarins within this class were proposed, leading to the correction of the configurations of two previously reported isocoumarins. The isolated compounds inhibited Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, with MIC values of 1 to 64 μg/mL. Compounds 5, 6, and 12 significantly promoted the growth of zebrafish intersegmental vessels at concentrations of 10, 20, and 40 μM, respectively.

Concise Synthesis of Cyctetryptomycin A and B Enabled by Zr-Catalyzed Dimerization

A concise synthetic strategy utilizing a Zr catalyst for the construction of cyctetryptomycin A and B is reported. Cyctetryptomycin A and B are recently isolated, complex tetrameric natural products for which total synthesis has not been previously reported. This study presents a practical approach for the construction of two consecutive quaternary carbon centers with a Zr catalyst. Furthermore, the first total synthesis of cyctetryptomycin A and B was achieved by this Zr-catalyzed radical coupling. The radical dimerization reaction mediated by the Zr catalyst required 1,2-bis(diphenylphosphino)ethane (dppe) as an indispensable additive. Through both experimental and theoretical investigations into the mechanism of this Zr-catalyzed reaction, the specific role of dppe was elucidated. In addition, the synthetic approach was extended to enable the practical synthesis of other dimeric natural products, including tetratryptomycin A, dibrevianamide F, and ditryptophenaline. Finally, the synthetic mechanism of cyctetryptomycin A and B, through the oxidative macrocyclization of tetratryptomycin A by CttpC, was newly elucidated by both experimental and docking simulations.

Hybrid Type I and II Polyketide Synthases Yield Distinct Aromatic Polyketides

Bacterial aromatic polyketides are compounds with multiple aromatic rings synthesized by bacterial type II polyketide synthases (PKSs), some of which have been developed into clinical drugs. Compounds containing aromatic polyketides synthesized by hybrid type I and type II PKSs are extremely rare. Here, we report the discovery of a gene cluster encoding both modular type I and type II PKSs as well as KAS III through extensive bioinformatics analysis, leading to the characterization of the hybrid polyketide, spirocycline A. The structure of spirocycline A is rare among all aromatic polyketides, featuring a unique starter unit and four spirocycles and forming a dimer. Biosynthetic studies indicate that the starter unit of this molecule is synthesized by type I PKS in collaboration with two trans-acting ketoreductase (KR) and enoylreductase (ER). It is then transferred by KAS III to the type II PKS system, which synthesizes the tricyclic aromatic polyketide backbone. The subsequent formation of the spirocycle and dimerization are carried out by four redox enzymes encoded in the gene cluster. Overall, the discovery of spirocycline A provides a new approach for identifying novel aromatic polyketides and offers potential enzymatic tools for the bioengineering of these hybrid polyketides.

Expanding chemistry through in vitro and in vivo biocatalysis

Living systems contain a vast network of metabolic reactions, providing a wealth of enzymes and cells as potential biocatalysts for chemical processes. The properties of protein and cell biocatalysts-high selectivity, the ability to control reaction sequence and operation in environmentally benign conditions-offer approaches to produce molecules at high efficiency while lowering the cost and environmental impact of industrial chemistry. Furthermore, biocatalysis offers the opportunity to generate chemical structures and functions that may be inaccessible to chemical synthesis. Here we consider developments in enzymes, biosynthetic pathways and cellular engineering that enable their use in catalysis for new chemistry and beyond.

Exploring the Diverse Landscape of Fungal Cytochrome P450-Catalyzed Regio- and Stereoselective Dimerization of Diketopiperazines

Dimeric indole-containing diketopiperazines (di-DKPs) are a diverse group of natural products produced through cytochrome P450-catalyzed C-C or C-N coupling reactions. The regio- and stereoselectivity of these reactions plays a significant role in the structural diversity of di-DKPs. Despite their pivotal role, the mechanisms governing the selectivity in fungi are not fully understood. Employing bioinformatics analysis and heterologous expression experiments, five undescribed P450 enzymes (AmiP450, AcrP450, AtP450, AcP450, and AtuP450) responsible for the regio- and stereoselective dimerization of diketopiperazines (DKPs) in fungi are identified. The function of these P450s is consistent with phylogenetic analysis, highlighting their dominant role in controlling the dimerization modes. Combinatorial biosynthesis-based pathway reconstitution of non-native gene clusters expands the chemical space of fungal di-DKPs and reveals that the regioselectivity is influenced by the substrate. Furthermore, multiple sequence alignment and molecular docking of these enzymes demonstrate a C-terminal variable region near the substrate tunnel entrance in AtuP450 that is crucial for its regioselectivity. These findings not only reveal the secret of fungal di-DKPs diversity but also deepen understanding of the mechanisms and catalytic specificity involved in P450-catalyzed dimerization reactions.

Study of phenoxy radical couplings using the enzymatic secretome of Botrytis cinerea

Phenoxy radical coupling reactions are widely used in nature for the synthesis of complex molecules such as lignin. Their use in the laboratory has great potential for the production of high value compounds from the polyphenol family. While the enzymes responsible for the generation of the radicals are well known, the behavior of the latter is still enigmatic and difficult to control in a reaction flask. Previous work in our laboratory using the enzymatic secretome of B. cinerea containing laccases has shown that incubation of stilbenes leads to dimers, while incubation of phenylpropanoids leads to dimers as well as larger coupling products. Building on these previous studies, this paper investigates the role of different structural features in phenoxy radical couplings. We first demonstrate that the presence of an exocyclic conjugated double bond plays a role in the generation of efficient reactions. In addition, we show that the formation of phenylpropanoid trimers and tetramers can proceed via a decarboxylation reaction that regenerates this reactive moiety. Lastly, this study investigates the reactivity of other phenolic compounds: stilbene dimers, a dihydro-stilbene, a 4-O-methyl-stilbene and a simple phenol with the enzymatic secretome of B. cinerea. The observed efficient dimerization reactions consistently correlate with the presence of a para-phenol conjugated to an exocyclic double bond. The absence of this structural feature leads to variable results, with some compounds showing low conversion or no reaction at all. This research has allowed the development of a controlled method for the synthesis of specific dimers and tetramers of phenylpropanoid derivatives and novel stilbene derivatives, as well as an understanding of features that can promote efficient radical coupling reactions.

A Cytochrome P450 Catalyzes Oxidative Coupling Formation of Insecticidal Dimeric Indole Piperazine Alkaloids

Cytochrome P450 (CYP450)-catalyzed oxidative coupling is an efficient strategy for using simple building blocks to construct complex structural scaffolds of natural products. Among them, heterodimeric coupling between two different monomers is relatively scarce, and the corresponding CYP450s are largely undiscovered. In this study, we discovered a fungal CYP450 (CpsD) and its associated cps cluster from 37208 CYP450s of Pfam PF00067 family member database and subsequently identified a group of new skeleton indole piperazine alkaloids (campesines A-G) by combination of genome mining and heterologous synthesis. Importantly, CYP450 CpsD mainly catalyzes intermolecular oxidative heterocoupling of two different indole piperazine monomers to generate an unexpected 6/5/6/6/6/6/5/6 eight-ring scaffold through the formation of one C-C bond and two C-N bonds, illuminating its first dimerase role in this family of natural products. The proposed catalytic mechanism of CpsD was deeply investigated by diversified substrate derivatization. Moreover, dimeric campesine G shows good insecticidal activity against the global honeybee pest Galleria mellonella. Our study shows a representative example of discovering new skeleton monomeric and dimeric indole piperazine alkaloids from microbial resources, expands our knowledge of bond formation by CYP450s and supports further development of the newly discovered and engineered campesine family compounds as potential biopesticides.

Atroposelective Synthesis of Aldehydes via Alcohol Dehydrogenase-Catalyzed Stereodivergent Desymmetrization

Axially chiral aldehydes have emerged recently as a unique class of motifs for drug design. However, few biocatalytic strategies have been reported to construct structurally diverse atropisomeric aldehydes. Herein, we describe the characterization of alcohol dehydrogenases to catalyze atroposelective desymmetrization of the biaryl dialdehydes. Investigations into the interactions between the substrate and key residues of the enzymes revealed the distinct origin of atroposelectivity. A panel of 13 atropisomeric monoaldehydes was synthesized with moderate to high enantioselectivity (up to >99% ee) and yields (up to 99%). Further derivatization allows enhancement of the diversity and application potential of the atropisomeric compounds. This study effectively expands the scope of enzymatic synthesis of atropisomeric aldehydes and provides insights into the binding modes and recognition mechanisms of such molecules.

Recent advances in oxidative phenol coupling for the total synthesis of natural products

Covering: 2008 to 2023This review will describe oxidative phenol coupling as applied in the total synthesis of natural products. This review covers catalytic and electrochemical methods with a brief comparison to stoichiometric and enzymatic systems assessing their practicality, atom economy, and other measures. Natural products forged by C-C and C-O oxidative phenol couplings as well as from alkenyl phenol couplings will be addressed. Additionally, exploration into catalytic oxidative coupling of phenols and other related species (carbazoles, indoles, aryl ethers, etc.) will be surveyed. Future directions of this particular area of research will also be assessed.

Novel Biocatalysts from Specialized Metabolism

Enzymes are increasingly recognized as valuable (bio)catalysts that complement existing synthetic methods. However, the range of biotransformations used in the laboratory is limited. Here we give an overview on the biosynthesis-inspired discovery of novel biocatalysts that address various synthetic challenges. Prominent examples from this dynamic field highlight remarkable enzymes for protecting-group-free amide formation and modification, control of pericyclic reactions, stereoselective hetero- and polycyclizations, atroposelective aryl couplings, site-selective C-H activations, introduction of ring strain, and N-N bond formation. We also explore unusual functions of cytochrome P450 monooxygenases, radical SAM-dependent enzymes, flavoproteins, and enzymes recruited from primary metabolism, which offer opportunities for synthetic biology, enzyme engineering, directed evolution, and catalyst design.

Enzymatic Benzofuranoindoline Formation in the Biosynthesis of the Strained Bridgehead Bicyclic Dipeptide (+)-Azonazine A

We uncovered and reconstituted a concise biosynthetic pathway of the strained dipeptide (+)-azonazine A from marine-derived Aspergillus insulicola. Formation of the hexacyclic benzofuranoindoline ring system from cyclo-(l-Trp-N-methyl-l-Tyr) is catalyzed by a P450 enzyme through an oxidative cyclization. Supplementing the producing strain with various indole-substituted tryptophan derivatives resulted in the generation of a series of azonazine A analogs.

A Ferric-Superoxide Intermediate Initiates P450-Catalyzed Cyclic Dipeptide Dimerization

The cytochrome P450 (CYP) AspB is involved in the biosynthesis of the diketopiperazine (DKP) aspergilazine A. Tryptophan-linked dimeric DKP alkaloids are a large family of natural products that are found in numerous species and exhibit broad and often potent bioactivity. The proposed mechanisms for C-N bond formation by AspB, and similar C-C bond formations by related CYPs, have invoked the use of a ferryl-intermediate as an oxidant to promote substrate dimerization. Here, the parallel application of steady-state and transient kinetic approaches reveals a very different mechanism that involves a ferric-superoxide species as a primary oxidant to initiate DKP-assembly. Single turnover kinetic isotope effects and a substrate analog suggest the probable nature and site for abstraction. The direct observation of CYP-superoxide reactivity rationalizes the atypical outcome of AspB and reveals a new reaction manifold in heme enzymes.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Molecular identification of a laccase that catalyzes the oxidative coupling of a hydroxycinnamic acid amide for hordatine biosynthesis in barley

Plants produce dimerized phenolic compounds as secondary metabolites. Hordatine A (HA), a dehydrodimer of p-coumaroylagmatine (pCA), is an antifungal compound accumulated at high levels in young barley (Hordeum vulgare) seedlings. The enzyme responsible for the oxidative dimerization of pCA, which is the final step of the hordatine biosynthetic pathway, has not been identified. In this study, we first verified the presence of this enzyme activity in the crude extract of barley seedlings. Because the enzyme activity was not dependent on H2 O2 , the responsible enzyme was not peroxidase, which was previously implicated in HA biosynthesis. The analysis of the dissection lines of wheat (Triticum aestivum) carrying aberrant barley 2H chromosomes detected HA in the wheat lines carrying the distal part of the 2H short arm. This chromosomal region contains two laccase genes (HvLAC1 and HvLAC2) that are highly expressed at the seedling stage and may encode enzymes that oxidize pCA during the formation of HA. Changes in the HvLAC transcript levels coincided with the changes in the HA biosynthesis-related enzyme activities in the crude extract and the HA content in barley seedlings. Moreover, HvLAC genes were heterologously expressed in Nicotiana benthamiana leaves and in bamboo (Phyllostachys nigra) suspension cells and HA biosynthetic activities were detected in the crude extract of transformed N. benthamiana leaves and bamboo suspension cells. The HA formed by the enzymatic reaction had the same stereo-configuration as the naturally occurring HA. These results demonstrate that HvLAC enzymes mediate the oxidative coupling of pCA during HA biosynthesis.

Assessing the Biosynthetic Inventory of the Biocontrol Fungus Trichoderma afroharzianum T22

Natural products biosynthesized from biocontrol fungi in the rhizosphere can have both beneficial and deleterious effects on plants. Herein, we performed a comprehensive analysis of natural product biosynthetic gene clusters (BGCs) from the widely used biocontrol fungus Trichoderma afroharzianum T22 (ThT22). This fungus encodes at least 64 BGCs, yet only seven compounds and four BGCs were previously characterized or mined. We correlated 21 BGCs of ThT22 with known primary and secondary metabolites through homologous BGC comparison and characterized one unknown BGC involved in the biosynthesis of eujavanicol A using heterologous expression. In addition, we performed untargeted transcriptomics and metabolic analysis to demonstrate the activation of silent ThT22 BGCs via the "one strain many compound" (OSMAC) approach. Collectively, our analysis showcases the biosynthetic capacity of ThT22 and paves the way for fully exploring the roles of natural products of ThT22.

Conversion and synthesis of chemicals catalyzed by fungal cytochrome P450 monooxygenases: A review

Cytochrome P450s (also called CYPs or P450s) are a superfamily of heme-containing monooxygenases. They are distributed in all biological kingdoms. Most fungi have at least two P450-encoding genes, CYP51 and CYP61, which are housekeeping genes that play important roles in the synthesis of sterols. However, the kingdom fungi is an interesting source of numerous P450s. Here, we review reports on fungal P450s and their applications in the bioconversion and biosynthesis of chemicals. We highlight their history, availability, and versatility. We describe their involvement in hydroxylation, dealkylation, oxygenation, C═C epoxidation, C-C cleavage, C-C ring formation and expansion, C-C ring contraction, and uncommon reactions in bioconversion and/or biosynthesis pathways. The ability of P450s to catalyze these reactions makes them promising enzymes for many applications. Thus, we also discuss future prospects in this field. We hope that this review will stimulate further study and exploitation of fungal P450s for specific reactions and applications.

Current status of secondary metabolite pathways linked to their related biosynthetic gene clusters in Aspergillus section Nigri

Covering: up to the end of 2021Aspergilli are biosynthetically 'talented' micro-organisms and therefore the natural products community has continually been interested in the wealth of biosynthetic gene clusters (BGCs) encoding numerous secondary metabolites related to these fungi. With the rapid increase in sequenced fungal genomes combined with the continuous development of bioinformatics tools such as antiSMASH, linking new structures to unknown BGCs has become much easier when taking retro-biosynthetic considerations into account. On the other hand, in most cases it is not as straightforward to prove proposed biosynthetic pathways due to the lack of implemented genetic tools in a given fungal species. As a result, very few secondary metabolite biosynthetic pathways have been characterized even amongst some of the most well studied Aspergillus spp., section Nigri (black aspergilli). This review will cover all known biosynthetic compound families and their structural diversity known from black aspergilli. We have logically divided this into sub-sections describing major biosynthetic classes (polyketides, non-ribosomal peptides, terpenoids, meroterpenoids and hybrid biosynthesis). Importantly, we will focus the review on metabolites which have been firmly linked to their corresponding BGCs.

Development of a P450 Fusion Enzyme for Biaryl Coupling in Yeast

Despite the diverse and potent bioactivities displayed by axially chiral biaryl natural products, their application in drug discovery is limited by restricted access to these complex molecular scaffolds. In particular, fundamental challenges remain in controlling the site- and atroposelectivity in biaryl coupling reactions. In contrast, Nature has a wealth of biosynthetic enzymes that catalyze biaryl coupling reactions with catalyst-controlled selectivity. In particular, a growing subset of fungal P450s have been identified to catalyze site- and atroposelective biaryl couplings. Herein, we optimize a whole-cell biocatalytic platform in Pichia pastoris to synthesize biaryl molecules through the recombinant production of the fungal P450 KtnC. Moreover, engineering redox self-sufficient fusion enzymes further improves the efficiency of the system. Altogether, this work provides a platform for biaryl coupling reactions in yeast that can be applied to engineering a currently underexplored pool of fungal P450s into selective biocatalysts for the synthesis of complex biaryl compounds.

Finding Superior Biocatalysts via Homolog Screening

Many enzymes possess high catalytic efficiency and selectivity that far surpass classical organic or organometallic catalysts. However, the initial starting enzyme for a given transformation does not always possess the right properties needed for broad utilization. Searching in genome/protein sequence libraries for homologs, aided with powerful bioinformatic tools developed in recent years, provides an avenue to identify superior biocatalysts. Herein, we highlight several case studies to illustrate the power of this concept. A brief discussion on its complementarity with contemporary approaches in protein engineering (such as directed evolution) and possible future developments is also provided.

Directed Evolution of Flavin-Dependent Halogenases for Site- and Atroposelective Halogenation of 3-Aryl-4(3H)-Quinazolinones via Kinetic or Dynamic Kinetic Resolution

In this study, we engineer a variant of the flavin-dependent halogenase RebH that catalyzes site- and atroposelective halogenation of 3-aryl-4(3H)-quinazolinones via kinetic or dynamic kinetic resolution. The required directed evolution uses a combination of random and site-saturation mutagenesis, substrate walking using two probe substrates, and a two-tiered screening approach involving the analysis of variant conversion and then enantioselectivity of improved variants. The resulting variant, 3-T, provides >99:1 e.r. for the (M)-atropisomer of the major brominated product, 25-fold improved conversion, and 91-fold improved site selectivity relative to the parent enzyme on the probe substrate used in the final rounds of evolution. This high activity and selectivity translate well to several additional substrates with varied steric and electronic properties. Computational modeling and docking simulations are used to rationalize the effects of key mutations on substrate binding. Given the range of substrates that have been used for atroposelective synthesis via electrophilic halogenation in the literature, these results suggest that flavin-dependent halogenases (FDHs) could find many additional applications for atroposelective catalysis. More broadly, this study highlights how RebH can be engineered to accept structurally diverse substrates that enable its use for enantioselective catalysis.

A Streptomyces P450 enzyme dimerizes isoflavones from plants

Dimerization is a widespread natural strategy that enables rapid structural diversification of natural products. However, our understanding of the dimerization enzymes involved in this biotransformation is still limited compared to the numerous reported dimeric natural products. Here, we report the characterization of three new isoflavone dimers from Streptomyces cattleya cultured on an isoflavone-containing agar plate. We further identified a cytochrome P450 monooxygenase, CYP158C1, which is able to catalyze the dimerization of isoflavones. CYP158C1 can also dimerize plant-derived polyketides, such as flavonoids and stilbenes. Our work represents a unique bacterial P450 that can dimerize plant polyphenols, which extends the insights into P450-mediated biaryl coupling reactions in biosynthesis.

An Enantioselective Suzuki-Miyaura Coupling To Form Axially Chiral Biphenols

Axial chirality features prominently in molecules of biological interest as well as chiral catalyst designs, and atropisomeric 2,2′-biphenols are particularly prevalent. Atroposelective metal-catalyzed cross-coupling is an attractive and modular approach to access enantioenriched biphenols, and yet existing protocols cannot achieve this directly. We address this challenge through the use of enantiopure, sulfonated SPhos (sSPhos), an existing ligand that has until now been used only in racemic form and that derives its chirality from an atropisomeric axis that is introduced through sulfonation. We believe that attractive noncovalent interactions involving the ligand sulfonate group are responsible for the high levels of asymmetric induction that we obtain in the 2,2′-biphenol products of Suzuki–Miyaura coupling, and we have developed a highly practical resolution of sSPhos via diastereomeric salt recrystallization.

Engineered Cytochrome P450-Catalyzed Oxidative Biaryl Coupling Reaction Provides a Scalable Entry into Arylomycin Antibiotics

We report herein the first example of a cytochrome P450-catalyzed oxidative carbon-carbon coupling process for a scalable entry into arylomycin antibiotic cores. Starting from wild-type hydroxylating cytochrome P450 enzymes and engineered Escherichia coli, a combination of enzyme engineering, random mutagenesis, and optimization of reaction conditions generated a P450 variant that affords the desired arylomycin core 2d in 84% assay yield. Furthermore, this process was demonstrated as a viable route for the production of the arylomycin antibiotic core on the gram scale. Finally, this new entry affords a viable, scalable, and practical route for the synthesis of novel Gram-negative antibiotics.

Phytophenol Dimerization Reaction: From Basic Rules to Diastereoselectivity and Beyond

Phytophenol dimerization, which is a radical-mediated coupling reaction, plays a critical role in many fields, including lignin biosynthesis. To understand the reaction, 2,2-diphenyl-1-picrylhydrazyl radical was used to initiate a series of phytophenol dimerization reactions in methanol. The products were identified using ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UHPLC-ESI-Q-TOF-MS/MS) analysis in situ. The identified products mainly included biphenols, magnolol, honokiol, gingerol 6,6′-dimers, 3,6-dimethoxylcatechol β,β′ dimer, euphorbetin, bis-eugenol, dehydrodiisoeugenol, trans-ε-viniferin, (+) pinoresinol, and (−) pinoresinol. Structure–function relationship analysis allowed four basic rules to be defined: meta-excluded, C–C bonding domination, ortho-diOH co-activation, and exocyclic C=C involvement. The exocyclic C=C involvement, however, required conjugation with the phenolic core and the para-site of the -OH group, to yield a furan-fused dimer with two chiral centers. Computational chemistry indicated that the entire process was completed via a radical coupling reaction and an intramolecular conjugate addition reaction. Similar results were also found for the horseradish peroxidase (HRP)-catalyzed coniferyl alcohol dimerization, which produced (+) and (−) pinoresinols (but no (−) epipinoresinol), suggesting that the HRP-catalyzed process was essentially an exocyclic C=C-involved phytophenol dimerization reaction. The reaction was highly diastereoselective. This was attributed to the intramolecular reaction, which prohibited Re-attack. The four basic rules and diastereoselectivity can explain and even predict the main products in various chemical and biological events, especially oxidase-catalyzed lignin cyclization.

Sclerotiamides C-H, Notoamides from a Marine Gorgonian-Derived Fungus with Cytotoxic Activities

Bioassay-guided fractionation in association with LC-MS and NMR detection led to the isolation of six new alkaloids, sclerotiamides C-H (1-6), from the marine gorgonian-derived fungus Aspergillus sclerotiorum LZDX-33-4. Their structures were determined from extensive spectroscopic data, including ECD data and single-crystal X-ray diffraction analysis for configurational assignments. Sclerotiamides C (1) and D (2) are notoamide-type alkaloids with the incorporation of a unique 2,2-diaminopropane unit, and sclerotiamides E (3) and F (4) are unprecedented notoamide hybrids with a new coumarin unit. Sclerotiamide H (6) represents a new highly oxidized notoamide scaffold. Sclerotiamides C and F showed significant inhibition against a panel of tumor cell lines with IC₅₀ values ranging from 1.6 to 7.9 μM. Sclerotiamide C induces apoptosis in HeLa cells by arresting the cell cycle, activating ROS production, and regulating apoptosis-related proteins in the MAPK signaling pathway. The present study extends the scaffold diversity of the notoamides and provides a potential lead for the development of a cytotoxic agent.

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.

Biocatalytic oxidative cross-coupling reactions for biaryl bond formation

Biaryl compounds, with two connected aromatic rings, are found across medicine, materials science and asymmetric catalysis1,2. The necessity of joining arene building blocks to access these valuable compounds has inspired several approaches for biaryl bond formation and challenged chemists to develop increasingly concise and robust methods for this task3. Oxidative coupling of two C-H bonds offers an efficient strategy for the formation of a biaryl C-C bond; however, fundamental challenges remain in controlling the reactivity and selectivity for uniting a given pair of substrates4,5. Biocatalytic oxidative cross-coupling reactions have the potential to overcome limitations inherent to numerous small-molecule-mediated methods by providing a paradigm with catalyst-controlled selectivity6. Here we disclose a strategy for biocatalytic cross-coupling through oxidative C-C bond formation using cytochrome P450 enzymes. We demonstrate the ability to catalyse cross-coupling reactions on a panel of phenolic substrates using natural P450 catalysts. Moreover, we engineer a P450 to possess the desired reactivity, site selectivity and atroposelectivity by transforming a low-yielding, unselective reaction into a highly efficient and selective process. This streamlined method for constructing sterically hindered biaryl bonds provides a programmable platform for assembling molecules with catalyst-controlled reactivity and selectivity.

Recent advances in catalytic oxidative reactions of phenols and naphthalenols

This critical review aims to provide an overview of oxidative phenol and naphthalenol transformations in nature and synthetic chemistry.

Engineered and Artificial Metalloenzymes for Selective C-H Functionalization

The direct functionalization of C-H bonds constitutes a powerful strategy to construct and diversify organic molecules. However, controlling the chemo- and site-selectivity of this transformation in particularly complex molecular settings represents a significant challenge. Metalloenzymes are ideal platforms for achieving catalyst-controlled selective C-H bond functionalization as their reactivities can be tuned by protein engineering and/or redesign of their cofactor environment. In this review, we highlight recent progress in the development of engineered and artificial metalloenzymes for C-H functionalization, with a focus on biocatalytic strategies for selective C-H oxyfunctionalization and halogenation as well as C-H amination and C-H carbene insertion via abiological nitrene and carbene transfer chemistries. Engineered heme- and non-heme iron dependent enzymes have emerged as promising scaffolds for executing these transformations with high chemo-, regio- and stereocontrol as well as tunable selectivity. These emerging systems and methodologies have expanded the toolbox of sustainable strategies for organic synthesis and created new opportunities for the generation of chiral building blocks, the late-stage C-H functionalization of complex molecules, and the total synthesis of natural products.

Monapinone Coupling Enzyme Produces Non-Natural Heterodimers

The monapinone coupling enzyme (MCE), a fungal multicopper oxidase, catalyzes the regioselective C–C coupling between tricyclic monapinone A (the primary substrate) and other monapinones (secondary substrates) to produce atropisomeric biaryl homo- or heterodimers. In this study, mono-, bi- and tricyclic compounds were tested to determine whether they worked as secondary substrates for MCE. Among 14 cyclic compounds, MCE utilized semivioxanthin, YWA1, 1,3-naphthalenediol and flaviolin as secondary substrates to produce non-natural heterodimers. The atropisomeric biaryl heterodimers produced by MCE from monapinone A and semivioxanthin were isolated, and their structures were elucidated by NMR and MS. These findings indicate that MCE recognizes bi- and tricyclic compounds with a 1,3-dihydroxy or 1-hydroxy-3-methoxy benzene ring as a secondary substrate.

Enzymatic strategies for asymmetric synthesis

Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure chiral compounds are presented, with a focus on the reactions developed within the past decade.

State-of-the-Art Biocatalysis

The use of enzyme-mediated reactions has transcended ancient food production to the laboratory synthesis of complex molecules. This evolution has been accelerated by developments in sequencing and DNA synthesis technology, bioinformatic and protein engineering tools, and the increasingly interdisciplinary nature of scientific research. Biocatalysis has become an indispensable tool applied in academic and industrial spheres, enabling synthetic strategies that leverage the exquisite selectivity of enzymes to access target molecules. In this Outlook, we outline the technological advances that have led to the field's current state. Integration of biocatalysis into mainstream synthetic chemistry hinges on increased access to well-characterized enzymes and the permeation of biocatalysis into retrosynthetic logic. Ultimately, we anticipate that biocatalysis is poised to enable the synthesis of increasingly complex molecules at new levels of efficiency and throughput.

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products

The mycotoxin terrein is derived from the C₁₀‐precursor 6‐hydroxymellein (6‐HM) via an oxidative ring contraction. Although the corresponding biosynthetic gene cluster (BGC) has been identified, details of the enzymatic oxidative transformations are lacking. Combining heterologous expression and in vitro studies we show that the flavin‐dependent monooxygenase (FMO) TerC catalyzes the initial oxidative decarboxylation of 6‐HM. The reactive intermediate is further hydroxylated by the second FMO TerD to yield a highly oxygenated aromatic species, but further reconstitution of the pathway was hampered. A related BGC was identified in the marine‐derived Roussoella sp. DLM33 and confirmed by heterologous expression. These studies demonstrate that the biosynthetic pathways of terrein and related (polychlorinated) congeners diverge after oxidative decarboxylation of the lactone precursor that is catalyzed by a conserved FMO and further indicate that early dehydration of the side chain is an essential step.

Identify of Fast-Growing Related Genes Especially in Height Growth by Combining QTL Analysis and Transcriptome in Salix matsudana (Koidz)

The study on the fast-growing traits of trees, mainly valued by tree height (TH) and diameter at breast height (DBH), is of great significance to promote the development of the forest industry. Quantitative trait locus (QTL) mapping based on high-density genetic maps is an efficient approach to identify genetic regions for fast-growing traits. In our study, a high-density genetic map for the F₁ population was constructed. The genetic map had a total size of 5,484.07 centimorgan (cM), containing 5,956 single nucleotide polymorphisms (SNPs) based on Specific Length Amplified Fragment sequencing. Six fast-growing related stable QTL were identified on six chromosomes, and five stable QTL were identified by a principal component analysis (PCA). By combining the RNA-seq analysis for the two parents and two progenies with the qRT-PCR analysis, four candidate genes, annotated as DnaJ, 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1), Caffeic acid 3-O-methyltransferase 1 (COMT1), and Dirigent protein 6 (DIR6), that may regulate height growth were identified. Several lignin biosynthesis-related genes that may take part in height growth were detected. In addition, 21 hotspots in this population were found. The results of this study will provide an important foundation for further studies on the molecular and genetic regulation of TH and DBH.

(+/-)-Dievodialetins A-G: Seven pairs of enantiomeric coumarin dimers with anti-acetylcholinesterase activity from the roots of Evodia lepta Merr

Seven pairs of undescribed enantiomeric bis-coumarins, (±)-dievodialetins A-G, were separated from the roots of Evodia lepta Merr. Two coumarin nuclei were linked via a 1,4-dimethyl4-vinylcyclohexene moiety in (±)-dievodialetins C-G. The structures of the undescribed compounds, including their absolute configurations were elucidated by spectroscopic analyses, X-ray diffraction, and computational calculations. In the biosynthetic pathways, these bis-coumarins were presumably derived from the precursors demethylsuberosin and 3-(3-methylbut-2-enyl)umbelliferone via a [4 + 2] Diels-Alder reaction. Besides, all compounds exhibited neuroprotective effects by inhibiting acetylcholinesterase (AChE) activity with IC₅₀ values ranging from 7.3 to 12.1 nM and they also suppressed oxidative stress (MDA and SOD) and neuroinflammation (IL-1β and IL-6).

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.

Cytochrome P450 enzymes in fungal natural product biosynthesis

Covering: 2015 to the end of 2020 Fungal-derived polyketides, non-ribosomal peptides, terpenoids and their hybrids contribute significantly to the chemical space of total natural products. Cytochrome P450 enzymes play essential roles in fungal natural product biosynthesis with their broad substrate scope, great catalytic versatility and high frequency of involvement. Due to the membrane-bound nature, the functional and mechanistic understandings for fungal P450s have been limited for quite a long time. However, recent technical advances, such as the efficient and precise genome editing techniques and the development of several filamentous fungal strains as heterologous P450 expression hosts, have led to remarkable achievements in fungal P450 studies. Here, we provide a comprehensive review to cover the most recent progresses from 2015 to 2020 on catalytic functions and mechanisms, research methodologies and remaining challenges in the fast-growing field of fungal natural product biosynthetic P450s.

Antioxidant product analysis of Hulu Tea (Tadehagi triquetrum)

Phytophenols from Hulu Tea can produce not only homodimers but also a heterodimer through the antioxidant activity.

Research progress on the source, production, and anti-cancer mechanisms of paclitaxel

Paclitaxel, a tetracyclic diterpenoid compounds, was firstly isolated from the bark of the Pacific yew trees. Currently, as a low toxicity, high efficiency, and broad-spectrum natural anti-cancer drug, paclitaxel has been widely used against ovarian cancer, breast cancer, uterine cancer, and other cancers. As the matter of fact, natural paclitaxel from Taxus species has been proved to be environmentally unsustainable and economically unfeasible. For this reason, researchers from all over the world are devoted to searching for new ways of obtaining paclitaxel. At present, other methods, including artificial cultivation of Taxus plants, microbial fermentation, chemical synthesis, tissue and cell culture have been sought and developed subsequently. Meanwhile, the biosynthesis of paclitaxel is also an extremely attractive method. Unlike other anti-cancer drugs, paclitaxel has its unique anti-cancer mechanisms. Here, the source, production, and anti-cancer mechanisms of paclitaxel were summarized and reviewed, which can provide theoretical basis and reference for further research on the production, anti-cancer mechanisms and utilization of paclitaxel.

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.

Chemical repertoire and biosynthetic machinery of the Aspergillus flavus secondary metabolome: A review

Filamentous fungi represent a rich source of extrolites, including secondary metabolites (SMs) comprising a great variety of astonishing structures and interesting bioactivities. State-of-the-art techniques in genome mining, genetic manipulation, and secondary metabolomics have enabled the scientific community to better elucidate and more deeply appreciate the genetic and biosynthetic chemical arsenal of these microorganisms. Aspergillus flavus is best known as a contaminant of food and feed commodities and a producer of the carcinogenic family of SMs, aflatoxins. This fungus produces many SMs including polyketides, ribosomal and nonribosomal peptides, terpenoids, and other hybrid molecules. This review will discuss the chemical diversity, biosynthetic pathways, and biological/ecological role of A. flavus SMs, as well as their significance concerning food safety and security.

Biscoumarin-pyrimidine conjugates as potent anticancer agents and binding mechanism of hit candidate with human serum albumin

In our continuing efforts to develop therapeutically active coumarin-based compounds, a series of new C4-C4' biscoumarin-pyrimidine conjugates (1a-l) was synthesized via S_N 2 reaction of substituted 4-bromomethyl coumarin with thymine. All compounds were characterized using spectroscopic techniques, that is, attenuated total reflection infrared (ATR-IR), CHN elemental analysis, and ¹ H and ¹³ C NMR (nuclear magnetic resonance). In addition, the structure of compound 1d (1,3-bis[(7-chloro-2-oxo-2H-chromen-4-yl)methyl]-5-methylpyrimidine-2,4(1H,3H)-dione) was established through X-ray crystallography. Compounds 1a-l were screened for in vitro anticancer activity against C6 rat glioma cells. Among the screened compounds, 1,3-bis[(6-chloro-2-oxo-2H-chromen-4-yl)methyl]-5-methylpyrimidine-2,4(1H,3H)-dione (1c) was identified as the best antiproliferative candidate, exhibiting an IC₅₀ value of 4.85 μM. All the compounds (1a-l) were found to be nontoxic toward healthy human embryonic kidney cells (HEK293), indicating their selective nature. In addition, the most active compound (1c) displayed strong binding interactions with the drug carrier protein, human serum albumin, and exhibited good solution stability at biological pH conditions. Fluorescence, UV-visible spectrophotometry and molecular modeling methodologies were employed for studying the interaction mechanism of compound 1c with protein.

Structure-activity relationship and mechanism of four monostilbenes with respect to ferroptosis inhibition

Erastin-treated bone marrow-derived mesenchymal stem cells (bmMSCs) were prepared and used to compare the ferroptosis inhibitory bioactivities of four monostilbenes, including rhapontigenin (1a), isorhapontigenin (1b), piceatannol-3'-O-glucoside (1c), and rhapontin (1d). Their relative levels were 1c ≈ 1b > 1a ≈ 1d in 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (C11-BODIPY), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and flow cytometric assays. The comparison highlighted two 4'-OH-containing monostilbenes (1c and 1b) in ferroptosis inhibitory bioactivity. Similar structure-activity relationships were also observed in antioxidant assays, including 1,1-diphenyl-2-picryl-hydrazl radical (DPPH˙)-trapping, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical (PTIO˙)-trapping, and Fe3+-reducing assays. UPLC-ESI-Q-TOF-MS analysis of the DPPH˙-trapping reaction of the monostilbenes revealed that they can inhibit ferroptosis in erastin-treated bmMSCs through a hydrogen donation-based antioxidant pathway. After hydrogen donation, these monostilbenes usually produce the corresponding stable dimers; additionally, the hydrogen donation potential was enhanced by the 4'-OH. The enhancement by 4'-OH can be attributed to the transannular resonance effect. This effect can be used to predict the inhibition potential of other π-π conjugative phenolics.

Broadening the scope of biocatalytic C-C bond formation

The impeccable control over chemo-, site-, and stereoselectivity possible in enzymatic reactions has led to a surge in the development of new biocatalytic methods. Despite carbon-carbon (C-C) bonds providing the central framework for organic molecules, development of biocatalytic methods for their formation has been largely confined to the use of a select few lyases over the last several decades, limiting the types of C-C bond-forming transformations possible through biocatalytic methods. This Review provides an update on the suite of enzymes available for highly selective biocatalytic C-C bond formation. Examples will be discussed in reference to the (1) native activity of enzymes, (2) alteration of activity through protein or substrate engineering for broader applicability, and (3) utility of the biocatalyst for abiotic synthesis.

Synthesis of quinazoin-4-ones through an acid ion exchange resin mediated cascade reaction

An interesting cascade reaction of N-(2-(4,5-dihydrooxazol-2-yl)phenyl)benzamide in the presence of an acid ion exchange resin is described. In this reaction, a range of substrates bearing various substituent groups are well compatible. This work provides a green and atom-economical alternative approach for the synthesis of quinazolin-4-ones in good yields.

Enzymatic Formation of Rufoschweinitzin, a Binaphthalene from the Basidiomycete Cortinarius rufoolivaceus

Dimeric polyketides are widespread fungal secondary metabolites. They occur in both ascomycetes and basidiomycetes and, therefore, across fungal phyla. Here we report the isolation of a new binaphthalene, named rufoschweinitzin, from the basidiomycete Cortinarius rufoolivaceus. Rufoschweinitzin consists of two symmetrically 4,4′‐coupled torachrysone‐8‐O‐methyl ether moieties. Furthermore, we have identified a binaphthalene biosynthetic gene cluster in an unrelated fungus, the ascomycete Xylaria schweinitzii. Heterologous expression of the encoded cytochrome P450 enzyme verified its coupling activity: dimerization of torachrysone‐8‐O‐methyl ether led to the formation of rufoschweinitzin alongside a hitherto unknown regioisomer, now named alloschweinitzin. We have thus demonstrated enzymatic formation of the basidiomycete's metabolite rufoschweinitzin and made the regiochemistry of alloschweinitzin accessible with an ascomycete‐derived enzyme.