Characterization of a glycosyltransferase from Paris polyphylla for application in biosynthesis of rare ophiopogonins and ginsenosides

Saponins are bioactive components of many medicinal plants, possessing complicated chemical structures and extensive pharmacological activities, but the production of high-value saponins remains challenging. In this study, a 6′-O-glucosyltransferase PpUGT7 (PpUGT91AH7) was functionally characterized from Paris polyphylla Smith var. yunnanensis (Franch.) Hand. -Mazz., which can transfer a glucosyl group to the C-6′ position of diosgenin-3-O-rhamnosyl-(1 → 2)-glucoside (1), pennogenin-3-O-rhamnosyl-(1 → 2)-glucoside (2), and diosgenin-3-O-glucoside (5). The K_M and K_cat values of PpUGT7 towards the substrate 2 were 8.4 μM and 2 × 10⁻³ s⁻¹, respectively. Through molecular docking and site-directed mutagenesis, eight residues were identified to interact with the sugar acceptor 2 and be crucial for enzyme activity. Moreover, four rare ophiopogonins and ginsenosides were obtained by combinatorial biosynthesis, including an undescribed compound ruscogenin-3-O-glucosyl-(1 → 6)-glucoside (10). Firstly, two monoglycosides 9 and 11 were generated using a known sterol 3-O-β-glucosyltransferase PpUGT80A40 with ruscogenin (7) and 20(S)-protopanaxadiol (8) as substrates, which were further glycosylated to the corresponding diglycosides 10 and 12 under the catalysis of PpUGT7. In addition, compounds 7–11 were found to show inhibitory effects on the secretion of TNF-α and IL-6 in macrophages RAW264.7. The findings provide valuable insights into the enzymatic glycosylation processes in the biosynthesis of bioactive saponins in P. polyphylla var. yunnanensis, and also serve as a reference for utilizing UDP-glycosyltransferases to construct high-value or rare saponins for development of new therapeutic agents.