A Substrate-Dependent Bifunctional Dioxygenase from Fraxinus chinensis for O-Demethylation and C8-Hydroxylation of Coumarins

Fraxinus chinensis Roxb. (Qinpi), a traditional Chinese medicinal plant, accumulates abundant coumarins that contribute to its anti-inflammatory and other bioactivities. However, the enzymatic basis for coumarin structural diversification in this species remains largely unexplored. Here, through transcriptome-wide identification of the 2-oxoglutarate-dependent dioxygenase (2OGD) family in F. chinensis, followed by phylogenetic analysis, heterologous expression, and in vitro enzyme assays, we identified FcDOH2, a member of the DOXC31 subfamily, which exhibits substrate-dependent bifunctionality, catalyzing the C6-O-demethylation of scopoletin (3) to esculetin (2) and the C8-hydroxylation of umbelliferone (1) to daphnetin (6). Using AlphaFold3-based structural modeling, molecular docking, and alanine scanning mutagenesis, we revealed that residues R155 and R221 are essential for both activities through stabilizing hydrogen bonds, whereas residue F312 acts as a functional switch, being critical for demethylation but negatively regulating hydroxylation. These findings uncover a rare bifunctional 2OGD with substrate-dependent catalytic plasticity, providing mechanistic insights into coumarin diversification in medicinal plants and a structural basis for future enzyme engineering.