Mimicry of the Calcium-Induced Conformational State of Troponin C by Low Temperature under Pressure

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 93; no. 20; pp. 10642 - 10646
• Main Authors: Foguel, Debora, Suarez, Marisa C., Barbosa, Cristiane, Rodrigues, Jorge J., Sorenson, Martha M., Smillie, Lawrence B., Silva, Jerson L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-10-1996; National Acad Sciences
• Subjects: Animals; Atmospheric pressure; Biochemistry; Calcium - physiology; Calcium-Binding Proteins - chemistry; Calcium-Binding Proteins - ultrastructure; Chickens; Cold Temperature; Enthalpy; Entropy; Fluorescence; Hydrostatic Pressure; Low temperature; Mutagenesis, Site-Directed; Pollutant emissions; Protein Conformation; Solubility; Surface areas; Thermodynamics; Troponin C - chemistry; Troponin C - ultrastructure; Water pressure
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.93.20.10642

Calcium binding to the N-domain of troponin C initiates a series of conformational changes that lead to muscle contraction. Calcium binding provides the free energy for a hydrophobic region in the core of N-domain to assume a more open configuration. Fluorescence measurements on a tryptophan mutant (F29W) show that a similar conformational change occurs in the absence of Ca$^{2+}$ when the temperature is lowered under pressure. The conformation induced by subzero temperatures binds the hydrophobic probe bis-aminonaphthalene sulfonate, and the tryptophan has the same fluorescence lifetime (7 ns) as in the Ca$^{2+}$-bound form. The decrease in volume ($\Delta $V = -25.4 ml/mol) corresponds to an increase in surface area. Thermodynamic measurements suggest an enthalpy-driven conformational change that leads to an intermediate with an exposed N-domain core and a high affinity for Ca$^{2+}$.