Calcium Ion-Dependent Increase in Thermostability of Dextran Glucosidase from Streptococcus mutans

Dextran glucosidase from Streptococcus mutans (SmDG), which belongs to glycoside hydrolase family 13 (GH13), hydrolyzes the non-reducing terminal glucosidic linkage of isomaltooligosaccharides and dextran. Thermal deactivation of SmDG did not follow the single exponential decay but rather the two-step irreversible deactivation model, which involves an active intermediate having 39% specific activity. The presence of a low concentration of CaCl₂ increased the thermostability of SmDG, mainly due to a marked reduction in the rate constant of deactivation of the intermediate. The addition of MgCl₂ also enhanced thermostability, while KCl and NaCl were not effective. Therefore, divalent cations, particularly Ca²⁺, were considered to stabilize SmDG. On the other hand, CaCl₂ had no significant effect on catalytic reaction. The enhanced stability by Ca²⁺ was probably related to calcium binding in the β→α loop 1 of the (β?α)₈ barrel of SmDG. Because similar structures and sequences are widespread in GH13, these GH13 enzymes might have been stabilized by calcium ions.