Calorimetric study of manganese binding to concanavalin A

The reaction of concanavalin A with Mn2+ has been studied calorimetrically. The binding enthalpy was measured at two different temperatures, 25 and 30 degrees C, in 10(-3) M acetate buffer; it was found to be constant between pH 4.0 and 5.0, delta H250 = 95 kJ/mol and delta H300 = 65 kJ/mol, respectively. The two S1 binding sites are identical and independent. Experiments at pH 5.6 are distorted by the heat of aggregation, which is several times higher than the heat of binding. Aggregation was demonstrated by spectrophotometric experiments and by light scattering. The presence of Mn2+ increases the stability of the protein molecule.