(185a) Expression of a Dianthus Flavonoid Glucosyltransferase in Saccharomyces Cerevisiae for Whole-Cell Biocatalysis

Many medicinally valuable compounds including antibiotics, alkaloids, and flavonoids are glycosides. The presence of saccharide groups on small molecules is often required for biological activity or for improved pharmacokinetic properties. Thus, there is an interest in developing chemical and enzymatic methods for performing glycosylation reactions. Glycosyltransferase enzymes are promising biocatalysts for the synthesis of small molecule glycosides.

In this study, Saccharomyces cerevisiae expressing a flavonoid glucosyltransferase (GT) from Dianthus caryophyllus (carnation) was investigated as a whole-cell biocatalyst. Two yeast expression systems were compared using the flavonoid naringenin as a model substrate. Under in vitro conditions, naringenin-7-O-glucoside was formed and a higher specific glucosyl transfer activity was found using a galactose inducible expression system compared to a constitutive expression system. However, S. cerevisiae expressing the GT constitutively was significantly more productive than the galactose inducible system under in vivo conditions. Interestingly, the glycosides were recovered directly from the culture broth and did not accumulate intracellularly. A previously uncharacterized naringenin glycoside formed using the D. caryophyllus GT was identified as naringenin-4'-O-glucoside using both MS and 1H-NMR. It was found that S. cerevisiae cells hydrolyze naringenin-7-O-glucoside during whole-cell biocatalysis. In addition, the effect of initial concentration of naringenin and medium composition on glycoside yield was examined. Up to 100 mg/L of naringenin glycosides were detected in the culture broth corresponding to 50% conversion of substrate. This study demonstrates that S. cerevisiae is a promising whole-cell biocatalyst host for the production of glycosides.