Transgalactosylation/Hydrolysis Ratios of Various b-Galactosidases Catalyzing Alkyl-b-Galactoside Synthesis in Single-Phased Alcohol Media

Three microbial galactosidases, Aspergillus oryzae, Escherichia coli and Klyveromyces marxianus b-galactosidase, were used as catalysts for transgalactosylation synthesis of alkyl-b-galactosides in single-phased alcohol media. Their selectivity towards different alcohol nucleophiles was quantified by determining the transgalactosylation/hydrolysis ratio in the water/alcohol mixtures containing water in concentrations below the level of saturation. p-Nitrophenyl-b-galactoside was used as a glycosyl donor at a concentration of 10 mM. Both the total reaction rate (transgalactosylation+hydrolysis) and the ratio between the transgalactosylation (alcoholysis) and hydrolysis increased with the increase of water activity. Although the A. oryzae b-galactosidase showed relatively low total activity (3.13 mmol/(min·mg protein)), it exhibited the highest selectivity towards the hexanol nucleophile among the examined enzymes (0.65). The selectivity values in all the examined cases were below one, which implies that the hydrolysis, and not the synthesis, was the dominating reaction. The total reaction rate (transgalactosylation+hydrolysis) was strongly affected by the water activity, and for the specific water activity in the different alcohols, it increased in the following order: n-octanol<n-hexanol<n-butanol.