Thermodynamics of the Binding of the C-Terminal Repeat Domain of Streptococcus sobrinus Glucosyltransferase-I to Dextran

Glucosyltransferases (GTFs) secreted by mutans streptococci and some other lactic acid bacteria catalyze glucan synthesis from sucrose, and possess a C-terminal glucan-binding domain (GBD) containing homologous, directly repeating units. We prepared a series of C-terminal truncated forms of the GBD of Streptococcus sobrinus GTF-I and studied their binding to dextran by isothermal titration calorimetry. The binding of all truncates was strongly exothermic. Their titration curves were analyzed assuming that the GBD recognizes and binds to a stretch of dextran chain, not to a whole dextran molecule. Both the number of glucose units constituting the dextran stretch (n) and the accompanying enthalpy change (ΔH°) are proportional to the molecular mass of the GBD truncate, with which the Gibbs energy change calculated by the relation ΔG° = -RT ln K (R, the gas constant; T, the absolute temperature; K, the binding constant of a truncate for a dextran stretch of n glucose units) also increases linearly. For the full-length GBD (508 amino acid residues), n = 33.9, K = 4.88 × 10⁷ M^-1, and ΔH° = -289 kJ mol^-1 at 25°C. These results suggest that identical, independent glucose-binding subsites, each comprising 14 amino acid residues on average, are arranged consecutively from the GBD N-terminus. Thus, the GBD binds tightly to a stretch of dextran chain through the adding up of individually weak subsite/glucose interactions. Furthermore, the entropy change accompanying the GBD/dextran interaction as given by the relation ΔS° = (ΔG° - ΔH°)/T has a very large negative value, probably because of a loss of the conformational freedom of dextran and GBD after binding.