Discovery of Semi-Pinacolases from the Epoxide Hydrolase Family during Efficient Assembly of a Fungal Polyketide

Absolute Configuration of Oxabornyl Polyenes Prugosenes A1-A3 and Structural Revision of Prugosene A2

Oxabornyl polyenes represent a unique group of polyketides characterized by a central polyene core flanked by a conserved oxabornyl moiety and a structurally diverse oxygen heterocyclic ring. They are widely distributed in fungi and possess a variety of biological activities. Due to the significant spatial separation between the two stereogenic ring systems, it is difficult to establish their overall relative configurations. Here, we isolated three oxabornyl polyenes, prugosenes A1-A3 (1-3), from Talaromyces sp. JNU18266-01. Although these compounds were first reported from Penicillium rugulosum, their overall relative and absolute configurations remained unassigned. By employing ozonolysis in combination with ECD calculations, we were able to establish their absolute configurations, and additionally obtained seven new chemical derivatives (4-10). Notably, through NMR data analysis and quantum chemical calculations, we achieved the structural revision of prugosene A2. Furthermore, prugosenes A1-A3 exhibited potent antiviral activity against the respiratory syncytial virus, with compound 1 displaying an IC50 value of 6.3 μM. Our study thus provides a valuable reference for absolute configuration assignment of oxabornyl polyene compounds.

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

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

Complex Interplay and Catalytic Versatility of Tailoring Enzymes for Efficient and Selective Biosynthesis of Fungal Mycotoxins

Mycotoxins have substantial impacts on agricultural production and food preservation. Some have high similarities in bioactivity but subtle differences on structures from various fungal producers. Understanding of their complex cross-biosynthesis will provide new insights into enzyme functions and food safety. Here, based on structurally related mycotoxins, such as aurovertins, asteltoxin, and citreoviridin, we showed that methyltransferase (MT)-catalyzed methylation is required for efficient oxidation and polyketide stability. MTs have broad interactions with polyketide synthases and flavin-containing monooxygenases (FMOs), while MT AstB is required for FMO AstC functionality in vivo. FMOs have common catalysis on pyrone-polyene intermediates but different catalytic specificity and efficiency on oxidative intermediates for the selective production of more toxic and complex mycotoxins. Thus, the subtle protein interaction and elaborate versatile catalysis of biosynthetic enzymes contribute to the efficient and selective biosynthesis of these structure-related mycotoxins and provide the basis to re-evaluate and control mycotoxins for agricultural and food safety.

Norditerpenoids biosynthesized by variediene synthase-associated P450 machinery along with modifications by the host cell Aspergillus oryzae

The chemical diversity of terpenoids is typically established by terpene synthase-catalyzed cyclization and diversified by post-tailoring modifications. Fungal bifunctional terpene synthase (BFTS) associated P450 enzymes have shown significant catalytic potentials through the development of various new terpenoids with different biological activities. This study discovered the BFTS and its related gene cluster from the plant endophytic fungus Didymosphaeria variabile 17020. Heterologous expression of the BFTS in Saccharomyces cerevisiae resulted in the characterization of a major product diterpene variediene (1), along with two new minor products neovariediene and neoflexibilene. Further heterologous expression of the BFTS and one cytochrome P450 enzyme VndE (CYP6138B1) in Aspergillus oryzae NSAR1 led to the identification of seven norditerpenoids (19 carbons) with a structurally unique 5/5 bicyclic ring system. Interestingly, in vivo experiments suggested that the cyclized terpene variediene (1) was modified by VndE along with the endogenous enzymes from the host cell A. oryzae through serial chemical conversions, followed by multi-site hydroxylation via A. oryzae endogenous enzymes. Our work revealed that the two-enzymes biosynthetic system and host cell machinery could produce structurally unique terpenoids.

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 anisotanol A from Anisodus tanguticus.