Biosynthesis of Glycyrrhetinic Acid-3-O-monoglucose Using Glycosyltransferase UGT73C11 from Barbarea vulgaris

Glycyrrhetinic acid (GA) is the pentacyclic triterpenoid hydrophobic aglycone with many pharmacological effects and biological activities. Glycosylation is often used to improve the aglycone's properties such as solubility, stability, and pharmacological potency. UDP-glycosyltransferases (UGTs) are the main enzymes to catalyze this conversion via transferring glycosyl moiety to corresponding acceptor substrates in nature. However, a glycosyltransferase which can transfer glucose to GA has not been reported yet. The glycosyltransferase UGT73C11 from the plant Barbarea vulgaris was reported with the glycosylation function to compounds which are similar to GA in chemical structure. In this study, UGT73C11 was selected to express functionally in Escherichia coli and purified as the biocatalyst for the glycosylation of GA. As a result, the recombinant UGT73C11 catalyzed UDP-glucose and GA to produce a new compound GA-3-O-monoglucose. The product GA-3-O-monoglucose was characterized by HPLC and LC-ESI-MS spectrometry with an exact mass of 633, and the glucose was linked with an O atom at the GA C-3 position with a β-glycosidic bond by IR and NMR analyses. Under optimal reaction conditions, the recombinant UGT73C11 showed the highest activity at 40 °C with pH 7.0, and the highest conversion was found at the substrate molar ratio UDP-glucose/GA of 5:1. Last, 98% of GA was converted into the corresponding GA-3-O-monoglucose under optimized conditions at 6 h. GA-3-O-monoglucose improved significantly the solubility and bioactivity of the parent GA according to data from the water solubility and antibacterial activity testing. These results indicate that the recombinant UGT73C11 was potentially exploited as biocatalyst for the glycosylation of GA in industrial and pharmaceutical use.