Recently, the internationally renowned journal Journal of the American Chemical Society (JACS) published online a collaborative research work by Professor Xudong Qu’s team from the School of Life Sciences and Biotechnology / Zhangjiang Institute for Advanced Study, and Professor Shukun Luo’s team from the same school.The paper is titled “A Stereoselective Decarboxylative Aromatase/Cyclase Directs the Biosynthesis of an Axially Chiral Biphenyl Framework in Fasamycin”.Professors Xudong Qu and Shukun Luo are the co-corresponding authors, and doctoral students Kai Jiang and Cheng Zhu are co-first authors.Aromatic polyketides are a large class of natural products with various biological activities. Their biosynthesis is primarily governed by type II polyketide synthases (PKSs), where minimal PKSs assemble polyketide chains, followed by aromatization and cyclization by aromatase/cyclase enzymes (ARO/CYC) to form the fundamental aromatic backbone.These backbones serve as templates, which are further modified by PKS tailoring enzymes, thereby giving rise to the structural diversity of aromatic polyketides.However, despite the extensive post-PKS modifications, only eight distinct planar conjugated aromatic skeletons have been identified so far (Figure 1).Therefore, the discovery of ARO/CYCs capable of catalyzing novel cyclization modes is crucial for expanding the structural and functional diversity of aromatic polyketides.

Fasamycin (FAS) and its analogues are a class of aromatic polyketides that have attracted attention due to their potent activity against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE).Unlike typical planar aromatic skeletons, FAS features a unique axially chiral biphenyl framework (Figure 1), suggesting its biosynthesis involves a novel stereoselective ARO/CYC and a specialized biosynthetic mechanism.The group had previously elucidated the formation of the phenyldimethylanthrone (PDA) core in FAS, which is generated through a tetracyclization reaction catalyzed by a TcmI-type ARO/CYC (FasL) and assisted by a non-essential auxiliary ARO/CYC (FasD) (Proc. Natl. Acad. Sci. USA 2024, 121, e2321722121, Figure 2).However, the mechanism underlying the formation of the axially chiral biphenyl structure (ring E) remained unclear.

In this study, the joint team first reconstituted the biosynthetic pathway of the shared PDA core between ABX and FAS in vitro.By optimizing the pH and temperature of the enzymatic reaction system and using ^¹³C isotope labeling, they identified a previously undetected biosynthetic intermediate 1 as the true substrate of FasU.Both in vitro assays and in vivo complementation confirmed that FasU, a monooxygenase family protein, acts as a novel type of ARO/CYC that catalyzes, with strict S-stereospecificity, sequential decarboxylation, cyclization, and aromatization reactions of intermediate 1 to form the axially chiral biphenyl scaffold of FAS.

To further understand the unique decarboxylation/aromatization/ cyclization/stereoselective mechanism of FasU, the researchers resolved the crystal structure of FasU.By performing site-directed mutagenesis of its active site and combining this with molecular dynamics (MD) simulations, they identified key catalytic residues and demonstrated that FasU's stereospecificity originates from hydrophobic repulsion between the binding pocket and the hydroxyl groups at C21 and C23.This part of the study not only significantly expands the known types of ARO/CYCs in type II aromatic polyketide biosynthesis but also provides valuable insights for the future development of novel ARO/CYCs and axially chiral aromatic polyketide skeletons.

This research was supported by the National Natural Science Foundation of China (Grants 32425033, 22377074, 22107069), the Shanghai Outstanding Academic Leaders Program (22XD1421300), and the Shanghai Natural Science Foundation (23ZR1432800).

Paper link: https://pubs.acs.org/doi/10.1021/jacs.4c18376