Differential scanning calorimetric study of the thermal unfolding of Taka-amylase A from Aspergillus oryzae

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Abstract
The thermally induced unfolding of Taka-amylase A, isolated from Aspergillus oryzae, was studied by differential scanning calorimetry. The experimental curves of excess apparent specific heat vs. termperature showed a single asymmetric peak. Curve resolution indicated that this asymmetry is due to the two-state unfolding of three domains in the molecule, with dissociation of the single tightly bound Ca2+ ion occurring during the unfolding of the last domain. Further indication of the dissociation of the specifically bound Ca2+ during denaturation is afforded by the fact that the temperature of maximal excess specific heat, tm, increases with increasing protein concentration in the absence of added excess Ca2+ and with increasing Ca2+ concentration in the presence of added Ca2+. Experiments in a variety of buffers with different enthalpies of ionization indicated that 11.8 .+-. 1.5 protons are lost from the protein during unfolding at pH 7.0. In apparent contradiction of this result, the value of tm was found to be essentially independent of pH in the range pH 7-8. No explanation of this anomaly is available. The enthalpy of unfolding at pH 7 and 62.degree.C in the absence of added Ca2+, corrected for the change in buffer protonation, is 2250 .+-. 40 kJ mol-1 (42.5 J g-1), and the permanent change in apparent heat capacity is 36.4 .+-. 4.1 kJ K-1 mol-1 (0.687 J g-1). Both of these quantities are unusually large for a globular protein.