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Eur J Biochem. 1983 Apr 15;132(1):85-8.

Stability of the Ca2+-specific and Ca2+-Mg2+ domains of troponin C. Effect of pH.


The unfolding of rabbit fast skeletal muscle troponin C has been examined as a function of pH using differential scanning calorimetry as a probe. Studies were conducted between pH 7.2 and 5.25 in the absence of both Ca2+ and Mg2+. At pH 7.2, a single unfolding transition, with a tm of 58 degrees C, is observed. Decreasing the pH to 5.5 increases the tm for this transition to 67 degrees C. At pH values less than or equal to 5.5, a second unfolding transition is observed, which has a tm of 51 degrees C. These studies suggest that troponin C consists of two types of structural domains: one has a compact structure throughout the pH range examined; the other has a poorly ordered structure at pH 7.2 which becomes more compact as the pH is decreased. We conclude that stabilization of both types of domains occurs through protonation of carboxylate groups clustered in each of the four Ca2+-binding loops. Our data are consistent with assignment of the higher-temperature transition to unfolding of the N-terminal portion of the molecule, which contains the Ca2+-specific domains and of the lower-temperature transition to unfolding of the C-terminal portion, which contains the Ca2+-Mg2+ domains.

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