Bioconversion of Stevioside to Rebaudioside E Using Glycosyltransferase UGTSL2

Rebaudioside E, one of the minor components of steviol glycosides, was first isolated and identified fromStevia rebaudianain 1977. It is a high-intensity sweetener that tastes about 150-200 times sweeter than sucrose and is also a precursor for biosynthesis of rebaudioside D and rebaudioside M, the next-generationSteviasweeteners. In this work, new unknown steviol glycosides were enzymatically synthesized from stevioside by coupling UDP-glucosyltransferase UGTSL2 fromSolanum lycopersicumand sucrose synthase StSUS1 fromSolanum tuberosum. Rebaudioside E was speculated to be the main product of glucosylation of the Glc(beta 1 -> C-19) residue of stevioside along with the formation of a (beta 1 -> 2) linkage based on the analysis of the regioselectivity and stereoselectivity of UGTSL2, and verified afterwards by LC-MS/MS with standard. In a 20-ml bioconversion reaction of 20 g/l stevioside by UGTSL2 and StSUS1, 15.92 g/l rebaudioside E was produced for 24 h.