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

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

AvmM catalyses macrocyclization through dehydration/Michael-type addition in alchivemycin A biosynthesis.

Macrocyclization is an important process that affords morphed scaffold in biosynthesis of bioactive natural products. Nature has adapted diverse biosynthetic strategies to form macrocycles. In this work, we report the identification and characterization of a small enzyme AvmM that can catalyze the construction of a 16-membered macrocyclic ring in the biosynthesis of alchivemycin A (1). We show through in vivo gene deletion, in vitro biochemical assay and isotope labelling experiments that AvmM catalyzes tandem dehydration and Michael-type addition to generate the core scaffold of 1. Mechanistic studies by crystallography, DFT calculations and MD simulations of AvmM reveal that the reactions are achieved with assistance from the special tenuazonic acid like moiety of substrate. Our results thus uncover an uncharacterized macrocyclization strategy in natural product biosynthesis.

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

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