The temperature influences the ratio of glucosidase and galactosidase activities of beta-glycosidases

The selectivity for the glycon part of a donor substrate of beta-glycosidases from almond, a mesophilic (Kluyveromyces fragilis) and three highly thermophilic organisms (Caldocellum saccharolyticum, Sulfolobus solfataricus and Pyrococcus furiosus) was investigated at various temperatures (25-90 degreesC ). On the basis of kinetic constants, the selectivity was calculated as the specificity constant (V-max/K-m) ratio or V-max ratio of glucoside to galactoside donor. In the almond beta-glucosidase and the mesostable enzyme one enzyme activity dominated whereas the thermostable enzymes expressed both high beta-glucosidase and high beta-galactosidase activities. Surprisingly, for beta-glycosidases from almond, K. fragilis, and C. saccharolyticum the donor selectivity decreased as the temperature increased. In contrast, two of the highly thermostable enzymes (from S. solfataricus and P. furiosus) had constant donor selectivity as the temperature increased. The results thus showed beta-glycosidases of differing origins to differ markedly in their substrate specificity and in the extent to which their selectivity for the glycon part of the donor substrate is influenced by the temperature.