A calorimetric study of Ca2+ binding by the parvalbumin of the toad (Bufo): distinguishable binding sites in the molecule

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Abstract
Microcalorimetric titrations of the major isotype of parvalbumin (tPA) from toad (Bufo) skeletal muscle, with Ca2+ in the presence and absence of Mg2+ and with Mg2+ in the absence of Ca2+, have been carried out at 25°C and pH 7.O. The results indicate that the two binding sites in each molecule are distinguishable from each other for both Ca2+ binding and Mg2+ binding. Such a characteristic is distinctly different from those of other parvalbumins. The enthalpy changes determined are distinctly different from those of bullfrog parvalbumins on Ca2+ or Mg2+ binding, but are similar to those on Mg2+‐Ca2+ exchange. The results indicate that the reaction of Mg2+‐Ca2+ exchange is driven almost entirely by the large favorable enthalpy change.