Ca(2+)-induced Conformational Shift of the COOH-domain of Eel Skeletal Muscle Troponin C in the Presence of Physiological Concentrations of Mg2+

The spectroscopic properties of Trp152 of eel skeletal muscle troponin C have been studied under conditions in which the COOH-domain is depleted of metal ions, titrated with Mg2+ and subsequently with Ca2+. This spectroscopic study clearly shows that the Mg2+ or Ca(2+)-bound states substitution lead to distinct conformations of the COOH-domain. The analysis of eel troponin C absorption and Trp152 fluorescence emission spectra indicates a more polar environment in the Mg(2+)-bound state of the protein. Steady state tryptophan polarization and lifetime distribution data indicate that the motion of the indole moiety is more restricted in the Mg2+ state of the protein than in the Ca(2+)-bound state. However, fluorescence quenching data using I- and Cs+ show that Trp152 is more accessible to the solvent in the Mg(2+)-bound state of eel troponin C. This spectroscopic analysis of the distinct Ca2+ and Mg(2+)-bound states of eel troponin C is consistent with the description of the three-dimensional structure of the corresponding states of pike (pI = 4.10) parvalbumin which is structurally homologous to the COOH-domain of troponin C. Since it appears that during muscular contraction, magnesium ions, which occupy the binding sites of the COOH-domain of troponin C in the resting cell are replaced by calcium ions, the structural shift occurring upon Mg2+/Ca2+ substitution, must have a physiological significance. The role of this domain is probably not limited to a structural role.