Structural and Computational Insights into the Noncanonical Aromatization in Fungal Polyketide Biosynthesis

Aromatic polyketides, with their diverse structures and biological activities, have attracted significant scientific interest. Aromatization reactions are typically performed by aromatases and cyclases associated with bacterial type II polyketide synthases (PKSs) or product template and Claisen cyclase domains found in fungal type I nonreducing PKSs. Intriguingly, our recent study discovered a noncanonical aromatization mechanism involving a cupin domain-containing enzyme, StrC, in fungal alkyl salicylaldehyde derivative biosynthesis. Nevertheless, the catalytic function of StrC largely remained enigmatic. In this study, an in-depth in vitro characterization of StrC was performed to gain a better understanding of the reaction mechanism for aromatization. In addition, the X-ray crystal structure of StrC in the complex with a substrate analogue was obtained to elucidate the unique active site. Subsequent mutational experiments identified important amino acid residues for aromatase activity. Furthermore, a quantum mechanics study provided a plausible reaction mechanism for aromatization and highlighted the vital role of StrC in the biosynthetic process. Thus, our study unveils an unknown paradigm for aromatization in polyketide biosynthesis. © 2024 American Chemical Society