Biocatalytic β-glucosylation/β-galactosylation of Rebaudioside C by glycosynthases

Extracts of Stevia rebaudiana Bertoni plant leaves contain a mixture of diterpene glycosides which possess high sweetness and are used as non-caloric sweeteners in foods and beverages. One of the most abundant of these steviol glycosides is Rebaudioside C (Reb C), which bears one esterified β-D-glucose and one glycosidically linked α-L-rhamnosyl-(1→2)-[β-D-glucosyl-(1→4)]-D-glucose trisaccharide. In this work, we isolated Reb C from commercial Stevia extracts using an orthogonal normal-phase × reversed-phase purification strategy. We then demonstrated that Reb C could be used as a donor substrate for enzymatic trans glycosylation reactions using chemically synthesized 1-deoxy-1-fluoro-D-glucose and 1-deoxy-1-fluoro-D-galactose donors and two engineered glycosynthases derived from Agrobacterium and Streptomyces β-glycosidases. This chemoenzymatic glucosylation/galactosylation strategy may be of use to extend the current repertoire of steviol glycosides analogues for analysis or the sensory evaluation of these novel steviol glycosides.