Synthesis of rebaudioside D, using glycosyltransferase UGTSL2 and in situ UDP-glucose regeneration

Steviol glycosides from Stevia rebaudiana leaves are used in stevia-based sweeteners for their intense sweetness and low calories. Rebaudioside D is present in leaves in minute quantities (∼0.4–0.5% w/w total dry weight), but it is ∼350 times sweeter than sucrose, and sweeter than the more abundant rebaudioside A and stevioside. In the present study, pathways for rebaudioside D synthesis and UDP-glucose recycling were developed by coupling recombinant UDP-glucosyltransferase UGTSL2 from Solanum lycopersicum and sucrose synthase StSUS1 from Solanum tuberosum. Reaction parameters, including substrate ratio, sucrose concentration, temperature, crude extract concentration, and reaction time, were evaluated, and 17.4 g/l of rebaudioside D (yield = 74.6%) was obtained from 20 g/l of rebaudioside A after 20 h, using UDP or UDP-glucose in recombinant cell crude extracts. Extending the reaction time generated rebaudioside M2 from further glycosylation of rebaudioside D. K_m values for UGTSL2 indicated a higher affinity for rebaudioside D than for rebaudioside A.