Identification and Characterization of Glycosyltransferases Involved in Emodin-Type Anthraquinone Glycosylation in Rheum palmatum

The dried rhizomes and roots of Rheum palmatum are widely used in China, Russia, and Arabia for the treatment of various diseases, with anthraquinone glycosides being the principal bioactive constituents. However, the glycosyltransferases involved in the biosynthesis of these glycosides remain largely unexplored. In this study, we first report the discovery and characterization of four novel glycosyltransferases (GTs)─RpUGT8, RpUGT12, RpUGT19, and RpUGT26─responsible for anthraquinone glycosylation in R. palmatum. These enzymes catalyze the formation of key bioactive compounds such as emodin-6-O-β-d-glucoside and rhein-8-O-β-d-glucoside. These glycosides are important for their pharmacological activities. By performing multiple sequence alignment, molecular docking, and site-directed mutagenesis, we identified critical residues required for their catalytic activity. Additionally, these enzymes exhibit broad substrate promiscuity, facilitating glycoside formation from various compounds, such as flavonoids, chalcones, dihydroflavonoids, and dihydrochalcones. This study provides valuable insights into the glycosylation mechanisms of emodin-type anthraquinone glycosides and offers an efficient approach for synthesizing O-glycosides with medicinal potential.