Production of bioactive ginsenosides Rh2 and Rg3 by metabolically engineered yeasts.

Ginsenosides Rh2 and Rg3 represent promising candidates for cancer prevention and therapy and have low toxicity. However, the concentrations of Rh2 and Rg3 are extremely low in the bioactive constituents (triterpene saponins) of ginseng. Despite the available heterologous biosynthesis of their aglycone (protopanaxadiol, PPD) in yeast, production of Rh2 and Rg3 by a synthetic biology approach was hindered by the absence of bioparts to glucosylate the C3 hydroxyl of PPD. In this study, two UDP-glycosyltransferases (UGTs) were cloned and identified from Panax ginseng. UGTPg45 selectively transfers a glucose moiety to the C3 hydroxyl of PPD and its ginsenosides. UGTPg29 selectively transfers a glucose moiety to the C3 glucose of Rh2 to form a 1-2-glycosidic bond. Based on the two UGTs and a yeast chassis to produce PPD, yeast cell factories were built to produce Rh2 and/or Rg3 from glucose. The turnover number (kcat) of UGTPg29 was more than 2500-fold that of UGTPg45, which might explain the higher Rg3 yield than that of Rh2 in the yeast cell factories. Building yeast cell factories to produce Rh2 or Rg3 from simple sugars by microbial fermentation provides an alternative approach to replace the traditional method of extracting ginsenosides from Panax plants.