Molecular characterization of a cytochrome P450 involved in the skeletal diversification of tricyclic coumarins in Angelica keiskei
TL;DR
New insights are provided into the molecular evolutionary process underlying ring-structure diversification and, more broadly, the adaptation of plants to their environments.
Abstract
Tricyclic coumarins (TCs) exhibit defensive properties against herbivorous insects and possess valuable pharmacological properties for humans. They are mainly distributed in the Apiaceae, Rutaceae, Moraceae, and Fabaceae families. The chemical structural diversity of TCs in these families primarily arises from differences in skeletal variety and attachment position of the third ring, as observed in unsaturated furanocoumarins (FCs) and saturated dihydrofuranocoumarins (DHFCs). In TC biosynthesis, the FC and DHFC pathways diverge through distinct oxidation patterns. The product specificity of the reported cytochrome P450 monooxygenase at the branching point suggests that these TC groups are produced in nearly identical ratios; however, some species of the genus Angelica (Apiaceae) accumulate DHFCs at higher levels than FCs. In this study, we investigated the enzymatic mechanism responsible for forming these two TC groups using A. keiskei, a Japanese native species used as a medicinal and edible herb. Organ-specific transcriptome analysis highlighted a P450 member CYP71AJ62 that exhibited a close correlation with TC accumulation across organs. Enzymatic characterization demonstrated that CYP71AJ62 predominantly forms a hydroxylated DHFC, vaginidiol, which may promote DHFC over FC skeletal formation in TC biosynthesis. Phylogenetic analysis suggested that CYP71AJ62 has been derived from a gene responsible for FC formation by functional shift without gene duplication. Subsequent three-dimensional structural modeling and site-directed mutagenesis revealed that two amino acid residues on substrate recognition sites cooperatively determine the product specificity of this P450 enzyme. This study provides new insights into the molecular evolutionary process underlying ring-structure diversification and, more broadly, the adaptation of plants to their environments.