Oxepinamide F biosynthesis involves enzymatic D-aminoacyl epimerization, 3H-oxepin formation, and hydroxylation induced double bond migration

Total Synthesis of Cyclotripeptidic Natural Products Anacine, Aurantiomide C, Polonimides A and C, and Verrucine F

The total synthesis of cyclotripeptidic natural products possessing a central piperazino[2,1-b]quinazolin-3,6-dione core is described through an original strategy involving the pivotal cyclocondensation of an electrophilic homoserine lactone intermediate. The alkylidene group was spontaneously installed by autoxidation during the cyclocondensation process, while the propionamide side chain was introduced through the nickel-catalyzed aminocarbonylation of a bromoethyl intermediate. This last reaction is unprecedented on such highly functionalized intermediates. Finally, we explored structural modifications and interconversions of the natural products. Overall, this work led to anacine, aurantiomide C, polonimides A and C, and verrucine F.

Formation of Bridged Disulfide in Epidithiodioxopiperazines

Epidithiodioxopiperazine (ETP) alkaloids, featuring a 2,5-diketopiperazine core and transannular disulfide bridge, exhibit a broad spectrum of biological activities. However, the structural complexity has prevented efficient chemical synthesis and further clinical research. In the past few decades, many achievements have been made in the biosynthesis of ETPs. Here, we discuss the biosynthetic progress and summarize them as two comprehensible metabolic principles for better understanding the complex pathways of α, α'- and α, β'-disulfide bridged ETPs. Specifically, we systematically outline the catalytic machineries to install α, α'- and α, β'-disulfide by flavin-containing oxygenases. This concept would contribute to the medical and industrial applications of ETPs.

Four previously undescribed diketopiperazines from marine fungus Aspergillus puniceus FAHY0085 and their effects on liver X receptor α

Four previously undescribed diketopiperazine-type alkaloids including one oxepin-containing diketopiperazine-type alkaloid, oxepinamide L (1), three 4-quinazolinone alkaloids, puniceloids E-G (10-12), together with 12 known analogues, protuboxepin D (2), oxepinamides D-G, J-K and I (3-9), puniceloids B-D (13-15) and protubonine B (16), were isolated from the culture of the marine-derived fungus Aspergillus puniceus FAHY0085. The structures of the previously undescribed compounds were comprehensively elucidated by detailed interpretation of their NMR and HRESIMS data. Their absolute configurations were unambiguously determined by ROESY experiments, Marfey's method, calculated ECD experiments and single-crystal X-ray diffraction analysis. Compounds (3-4, 6-8, 14-15) were evaluated for their cytotoxic activity against HepG2, MCF-7, SW1116 and HeLa cells and compound 6 and 14 showed moderate cytotoxic activity against HeLa cells with IC50 49.61 ± 2.91 and 28.38 ± 1.57 μM, respectively. Compounds (1-8, 11-15) were screened for their transcriptional activation of liver X receptor α and compound 11 with known compounds 13-15 showed significant transcriptional activation of liver X receptor α with EC50 values in the range 2-50 μM.

Oxepine-containing pyrazinopyrimidine alkaloids and quinolinone derivatives produced by Aspergillus versicolor AS-212, a deep-sea-derived endozoic fungus

Four new oxepine-containing pyrazinopyrimidine alkaloids, versicoxepines A - D (1-4), two quinolinone alkaloid analogs including 3-hydroxy-6-methoxy-4-phenylquinolin-2(1H)-one (5) and 3-methoxy-6-hydroxy-4-phenylquinolin-2(1H)-one (6) which were new naturally occurring compounds, together with two known compounds (7 and 8) were isolated from Aspergillus versicolor AS-212, an endozoic fungus isolated from the deep-sea coral Hemicorallium cf. imperiale, which was collected from the Magellan Seamounts in the Western Pacific Ocean. Their structures were determined by extensive analysis of the spectroscopic and X-ray crystallographic data as well as by chiral HPLC analysis, ECD calculation, and DP4+ probability prediction. Structurally, versicoxepines B and C (2 and 3) represent the first example of a new oxepine-containing pyrazinopyrimidine alkaloid whose cyclic dipeptide moiety is composed of the same type of amino acid (Val or Ile). Compound 5 displayed antibacterial activity against aquatic pathogens, Vibrio harveyi and V. alginolyticus, with MICs of 8 μg/mL.

A basidomycetous hydroxynaphthalene-prenylating enzyme exhibits promiscuity toward prenyl donors

The fungal prenyltransferase ShPT from Stereum hirsutum was believed to prenylate 4-hydroxybenzyl alcohol and thereby be involved in the vibralactone biosynthesis. In this study, we demonstrate that hydroxynaphthalenes instead of benzyl alcohol or aldehyde were accepted by ShPT for regular C-prenylation in the presence of both dimethylallyl and geranyl diphosphate. Although the natural substrate of ShPT remains unknown, our results provide one additional prenyltransferase from basidiomycetes, which are less studied, in comparison to those from other sources. Furthermore, this study expands the chemical toolbox for regioselective production of prenylated naphthalene derivatives. KEY POINTS: •Basidiomycetous prenyltransferase •Biochemical characterization •A DMATS prenyltransferase prenylating hydroxynaphthalene derivatives.

Diketopiperazine Alkaloids and Bisabolene Sesquiterpenoids from Aspergillus versicolor AS-212, an Endozoic Fungus Associated with Deep-Sea Coral of Magellan Seamounts

Two new quinazolinone diketopiperazine alkaloids, including versicomide E (2) and cottoquinazoline H (4), together with ten known compounds (1, 3, and 5-12) were isolated and identified from Aspergillus versicolor AS-212, an endozoic fungus associated with the deep-sea coral Hemicorallium cf. imperiale, which was collected from the Magellan Seamounts. Their chemical structures were determined by an extensive interpretation of the spectroscopic and X-ray crystallographic data as well as specific rotation calculation, ECD calculation, and comparison of their ECD spectra. The absolute configurations of (-)-isoversicomide A (1) and cottoquinazoline A (3) were not assigned in the literature reports and were solved in the present work by single-crystal X-ray diffraction analysis. In the antibacterial assays, compound 3 exhibited antibacterial activity against aquatic pathogenic bacteria Aeromonas hydrophilia with an MIC value of 18.6 μM, while compounds 4 and 8 exhibited inhibitory effects against Vibrio harveyi and V. parahaemolyticus with MIC values ranging from 9.0 to 18.1 μM.

Application of Gene Knockout and Heterologous Expression Strategy in Fungal Secondary Metabolites Biosynthesis

The in-depth study of fungal secondary metabolites (SMs) over the past few years has led to the discovery of a vast number of novel fungal SMs, some of which possess good biological activity. However, because of the limitations of the traditional natural product mining methods, the discovery of new SMs has become increasingly difficult. In recent years, with the rapid development of gene sequencing technology and bioinformatics, new breakthroughs have been made in the study of fungal SMs, and more fungal biosynthetic gene clusters of SMs have been discovered, which shows that the fungi still have a considerable potential to produce SMs. How to study these gene clusters to obtain a large number of unknown SMs has been a research hotspot. With the continuous breakthrough of molecular biology technology, gene manipulation has reached a mature stage. Methods such as gene knockout and heterologous expression techniques have been widely used in the study of fungal SM biosynthesis and have achieved good effects. In this review, the representative studies on the biosynthesis of fungal SMs by gene knockout and heterologous expression under the fungal genome mining in the last three years were summarized. The techniques and methods used in these studies were also briefly discussed. In addition, the prospect of synthetic biology in the future under this research background was proposed.

A terpene cyclase from Aspergillus ustus is involved in the biosynthesis of geosmin precursor germacradienol

The earthy odor of geosmin with a C12 skeleton is known from bacteria, fungi and plants. The sesquiterpenoid germacradien-11-ol (germacradienol) is a crucial intermediate in the biosynthesis of geosmin. A bifunctional terpene cyclase for germacradienol formation and its degradation to geosmin had been described in bacteria. Terpene cyclases were also suggested for geosmin formation in basidiomycetes, but not reported for ascomycetes. We identified a putative terpene cyclase in Aspergillus ustus with low sequence homology to N-termini of the bacterial germacradienol/geosmin synthases. Heterologous expression in Aspergillus nidulans and biochemical characterization led to the identification of the geosmin precursor germacradienol as the sole detected enzyme product. Germacradienol synthase (GdlS) uses strictly farnesyl diphosphate as substrate for cyclization and requires Mg2+ for its reaction. Multiple sequence alignments with known enzymes indicate the presence of the highly conserved catalytic residues including the DDXXD motif for Mg2+ binding. Phylogenetic analysis suggests different clades of bacterial germacradienol/geosmin synthases and terpene cyclases from fungi.

Design and synthesis of novel quinazolinyl-bisspirooxindoles as potent anti-tubercular agents: an ultrasound-promoted methodology

The essential need for the potent anti-tubercular (anti-TB) agents with high selectivity and safety profile prompted us to synthesize a new series of quinazolinyl-bisspirooxindoles. The title compounds were synthesized by one-pot multicomponent [3 + 2] cycloaddition reaction under ultrasonication. Further, in vitro anti-TB activity was evaluated against Mycobacterium tuberculosis H37Rv. Among the screened compounds, two compounds (4q and 4x) showed potent activity with MIC value 1.56 µg/mL and four compounds exhibited significant activity (MIC = 3.125 µg/mL), and also cytotoxicity studies against RAW 264.7 cell lines reveal that most active compounds were less toxic to humans. In addition, in order to demonstrate the inhibitory properties, molecular docking studies were carried out and the results showed that the target compounds have good binding energy and better binding affinity within the active pocket, thus these compounds may consider to be as potent inhibitors toward selective targets.

(±)-Usphenethylones A-C, three pairs of heterodimeric polyketide enantiomers from Aspergillus ustus 3.3904

Three pairs of new heterodimeric polyketide enantiomers, (±)-usphenethylones A-C (1-3), were isolated from the culture extract of Aspergillus ustus 3.3904. Compounds 1-3 present two heterodimerization patterns by a phenylethyl unit connected to an α-pyrone moiety, of which usphenethylones A-B (1-2) feature a 2,6,18-trioxa-tetracyclo-[8.8.0.03,8.011,16]octadecane core and usphenethylone C (3) possesses a 2-phenyl-3,4-dihydro-pyrano[4,3-b]pyran-5-one scaffold. The structures of (±)-1-3 were elucidated based on spectroscopic data analyses, and their absolute configurations were determined by single-crystal X-ray diffraction analysis and ECD calculation. Plausible biosynthetic pathways for 1-3 were proposed. Compounds (+)-3 and (-)-3 exhibited moderate inhibitory effects against ConA-induced T cell and LPS-induced B cell proliferation.

Pyrazinopyrimidine alkaloids from a mangrove-derived fungus Aspergillus versicolor HDN11-84

Pyrazinopyrimidine-type alkaloids bearing a pyrazino[1,2-a] pyrimidine moiety, often have different functional groups substituted at C-8' or C-2'/C-8', generally further forming unique spiro-/conjugated ring systems. Four undescribed pyrazinopyrimidine-type alkaloids, including three natural products pyrasplorines A-C and an artifact deg-pyrasplorine B, as well as a biogenetically related versicoloid A were discovered from the extract of a mangrove-derived fungus Apergillus verisicolor HDN11-84. Pyrasplorine A contains unique spiral-type skeleton (composed of cyclopentenone ring with the pyrazino[1,2-a] pyrimidine core) which is unprecedented in pyrazinopyrimidine-type alkaloids. The deg-pyrasplorine B could be spontaneously converted from pyrasplorine B in mild conditions. Their structures including absolute configurations were elucidated based on NMR spectroscopic analysis, computational calculations and Marfey's method. The absolute configuration of versicoloid A was re-assigned in this study. All the isolated compounds are non-cytotoxic and deg-pyrasplorine B showed anti-influenza A virus H1N1 activity with the IC₅₀ of 50 μM.

Oxepin Formation in Fungi Implies Specific and Stereoselective Ring Expansion

Oxepinamides are fungal oxepine-pyrimidinone-ketopiperazine derivatives. In this study, we elucidated the biosynthetic pathway of oxepinamide D in Aspergillus ustus by gene deletion, heterologous expression, feeding experiments, and enzyme assays. We demonstrated that the cytochrome P450 enzymes catalyzed highly specific and stereoselective oxepin ring formation.

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.