Characterization of yeast cell surface displayed Aspergillus oryzae β-glucosidase 1 high hydrolytic activity for soybean isoflavone

The yeast Saccharomyces cerevisiae GRI-117-UK was transformed to either display or secrete β-glucosidase 1 (BGL1) from the koji mold, Aspergillus oryzae. The β-glucosidase activity of BGL1-displaying yeast strains reached 405.9 U/g dry cell mass after 72 h of cultivation in YPD medium. The optimal pH and temperature for BGL1 displayed on the cell surfaces of the yeast were 5.0 and 55 °C, while the optima for BGL1 secreted by the yeast were 4.0 and 55 °C. The displayed BGL1 was stable at higher pH compared with the secreted BGL1. In addition, the thermostability of BGL1 was improved by displaying the enzyme on the yeast cell surfaces. In addition, the displayed and secreted forms of BGL1 had similar substrate specificity. β-Glucosidase hydrolyzes daidzin and genistin, which are the glycoside forms of soybean isoflavones, to the aglycones. Isoflavone aglycones were efficiently produced by BGL1-displaying yeast from an isoflavone mixture; at optimal temperature and pH the rate of aglycone production was at least 15.8 g/(l h). After 144 h of reaction, almost isoflavones were converted to its aglycone by BGL1-displaying yeast. The results of the present study demonstrate that BGL1-displaying yeast strains are effective whole cell biocatalysts of isoflavone aglycone production.