Enzymatic synthesis of galactosyl–xylose by Aspergillus oryzae β-galactosidase

Abstract

In aqueous medium, the reaction catalyzed by Aspergillus oryzae β -galactosidase with O-nitrophenyl-β-d-galactopyranoside (ONPG) in the presence of an acceptor leads to the synthesis of transglycosylation compounds in addition to the hydrolysis products (ONP and galactose). Our goal was to develop a simple system for the synthesis of galactosyl–xylose, a disaccharide of possible application to diagnostics. To maximize synthesis yields, we have studied the effect of several conditions: increase of acceptor concentration (0.05–2.7M xylose), organic co-solvents (dimethylformamide, acetone) and reaction time.In the absence of co-solvents ONPG was completely consumed in 2h; with 0.5 M xylose the maximum yield of galactosyl–xylose (16%) was attained at 60min, while with 2.7M xylose the yield reached 21%. Both co-solvents tested decreased the kinetics of ONPG convertion into products and 50% (v/v) dimethylformamide was deleterious to the synthesis. However, in 50% (v/v) acetone the synthesis yield was 12% and interestingly, the proportion of transglycosylation with respect to the reacted substrate was higher than in buffer.The synthesis of galactosyl–ethyleneglycol was also studied; it was achieved with extremely high yield and no detectable hydrolysis products. This proves that other acceptor alcohols can be preferred over water in some conditions.