MIMICRY OF THE CALCIUM-INDUCED CONFORMATIONAL STATE OF TROPONIN-C BY LOW-TEMPERATURE UNDER PRESSURE

Calcium binding to the N-domain of troponin C initiates a series of co nformational changes that lead to muscle contraction, Calcium binding provides the free energy for a hydrophobic region in the core of N-dom ain to assume a more open configuration, Fluorescence measurements on a tryptophan mutant (F29W) show that a similar conformational change o ccurs in the absence of Ca2+ when the temperature is lowered under pre ssure, The conformation induced by subzero temperatures binds the hydr ophobic probe bis-aminonaphthalene sulfonate, and the tryptophan has t he same fluorescence lifetime (7 ns) as in the Ca2+-bound form. The de crease in volume (Delta V = -25.4 ml/mol) corresponds to an increase i n 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 Ca2+.