Foguel D, Suarez M C, Barbosa C, Rodrigues J J, Sorenson M M, Smillie L B, Silva J L
Departamento de Bioquímica Médica, Universidade Federal do Rio de Janeiro, Brazil.
Proc Natl Acad Sci U S A. 1996 Oct 1;93(20):10642-6. 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 Ca2+ 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 Ca2+-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 Ca2+.
