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

In aqueous medium, the reaction catalyzed by Aspergillus oryzae beta-galactosidase with O-nitrophenyl-beta-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 (0.05-2.7 M xylose), organic co-solvents (dimethylformamide, acetone) and reaction time. In the absence of co-solvents ONPG was completely consumed in 2 h; with 0.5 M xylose the maximum yield of galactosylxylose (16%) was attained at 60 min, while with 2.7 M 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. (C) 2002 Elsevier Science B.V. All rights reserved.