Molecular cloning and biochemical characterization of a new coumarin glycosyltransferase CtUGT1 from Cistanche tubulosa

UDP-glycosyltransferases (UGTs) are an important and functionally diverse family of enzymes involved in secondary metabolite biosynthesis. Coumarin is one of the most common skeletons of natural products with candidate pharmacological activities. However, to date, many reported GTs from plants mainly recognized flavonoids as sugar acceptors. Only limited GTs could catalyze the glycosylation of coumarins. In this study, a new UGT was cloned from Cistanche tubulosa, a valuable traditional tonic Chinese herb, which is abundant with diverse glycosides such as phenylethanoid glycosides, lignan glycosides, and iridoid glycosides. Sequence alignment and phylogenetic analysis showed that CtUGT1 is phylogenetically distant from most of the reported flavonoid UGTs and adjacent to phenylpropanoid UGTs. Extensive in vitro enzyme assays found that although CtUGT1 was not involved in the biosynthesis of bioactive glycosides in C. tubulosa, it could catalyze the glucosylation of coumarins umbelliferone 1, esculetine 2, and hymecromone 3 in considerable yield. The glycosylated products were identified by comparison with the reference standards or NMR spectroscopy, and the results indicated that CtUGT1 can regiospecifically catalyze the glucosylation of hydroxyl coumarins at the C7-OH position. The key residues that determined CtUGT1's activity were further discussed based on homology modeling and molecular docking analyses. Combined with site-directed mutagenesis results, it was found that H19 played an irreplaceable role as the crucial catalysis basis. CtUGT1 could be used in the enzymatic preparation of bioactive coumarin glycosides.