Selective and high-yield production of ethyl α-d-glucopyranoside by the α-glucosyl transfer enzyme of Xanthomonas campestris WU-9701 and glucose isomerase.

Ethyl α-d-glucopyranoside (α-EG) is detected in sake (Japanese rice wine), that has moisturizing and skin conditioning effects. The production of α-EG by fermentation or enzymatic synthesis to date generates unwanted by-products such as maltooligosaccharides and/or organic acids. In this study, we employed a reaction involving selective α-glucosylation of ethanol by the α-glucosyl transfer enzyme (XgtA) of Xanthomonas campestris WU-9701. Under standard conditions, when 0.80 M ethanol and 1.2 M maltose were used as substrates with XgtA (2.5 units) and incubated in 30 mM HEPES-NaOH buffer (pH 8.0) at 45°C, only one form of ethyl glucopyranoside was selectively obtained as a product. The isolated product was identified as ethyl α-d-glucopyranoside by 1H NMR, 1H-1H COSY, and NOESY analyses. In the reaction mixture, other glucosylated products such as maltotriose and ethylmaltoside were not detected. Under optimum conditions, 180 mM (37.5 g/L) α-EG was produced in one batch production for 80 h. Further, the reaction rate of α-EG production decreased with an increase in glucose, especially more than 500 mM. In contrast, the addition of glucose isomerase decreased the concentration of glucose and was useful for maintaining a glucose concentration of less than 500 mM in the reaction mixture. Thus, owing to the enzymatic reaction with XgtA and glucose isomerase, as much as 260 mM (54.1 g/L) α-EG was produced in one batch production for 100 h. Altogether, this study reports the highest concentration of α-EG produced by enzymatic reaction.