Ca²⁺-induced conformational changes in cardiac troponin C as measured by N-(1-pyrene)maleimide fluorescence

Bovine cardiac troponin C was modified by N-(1-pyrene)maleimide at Cys-35 and Cys-84; the Ca²⁺-induced conformational changes were followed by measuring pyrene fluorescence. In isolated troponin C, the saturation of Ca²⁺, Mg²⁺-sites leads to a simultaneous increase in the pyrene monomer as well as to a decrease in the pyrene excimer fluorescence, whereas the saturation of Ca²⁺-specific sites results in a slight decrease in the fluorescence of pyrene monomer. Troponin T does not influence the dependence of pyrene-troponin C fluorescence on Ca²⁺ concentration. Within the equimolar complex of troponin C and troponin I, the saturation of Ca²⁺, Mg²⁺-sites has no effect on pyrene fluorescence, whereas the saturation of Ca²⁺-specific sites leads to a simultaneous decrease of both pyrene monomer and pyrene excimer fluorescence. It is supposed that troponin I diminishes the conformational changes in troponin C that are induced by the saturation of Ca²⁺, Mg²⁺-sites and enhances the conformational changes induced by the saturation of Ca²⁺-specific sites of troponin C.