(+)-Sesamin-oxidising CYP92B14 shapes specialised lignan metabolism in sesame

Abstract

Sesamum spp. (sesame) are known to accumulate a variety of lignans in a lineage-specific manner. In cultivated sesame (Sesamum indicum), (+)-sesamin, (+)-sesamolin and (+)-sesaminol triglucoside are the three major lignans found richly in the seeds. A recent study demonstrated that SiCYP92B14 is a pivotal enzyme that allocates the substrate (+)-sesamin to two products, (+)-sesamolin and (+)-sesaminol, through multiple reaction schemes including oxidative rearrangement of α-oxy-substituted aryl groups (ORA). In contrast, it remains unclear whether (+)-sesamin in wild sesame undergoes oxidation reactions as in S. indicum and how, if at all, the ratio of the co-products is tailored at the molecular level. Here, we functionally characterised SrCYP92B14 as a SiCYP92B14 orthologue from a wild sesame, Sesamum radiatum, in which we revealed accumulation of the (+)-sesaminol derivatives (+)-sesangolin and its novel structural isomer (+)-7´-episesantalin. Intriguingly, SrCYP92B14 predominantly produced (+)-sesaminol either through ORA or direct oxidation on the aromatic ring, while a relatively low but detectable level of (+)-sesamolin was produced. Amino acid substitution analysis suggested that residues in the putative distal helix and the neighbouring heme propionate of CYP92B14 affect the ratios of its co-products. These data collectively show that the bimodal oxidation mechanism of (+)-sesamin might be widespread across Sesamum spp., and that CYP92B14 is likely to be a key enzyme in shaping the ratio of (+)-sesaminol- and (+)-sesamolin-derived lignans from the biochemical and evolutionary perspectives.