Interaction of troponin and tropomyosin: spectroscopic and calorimetric studies

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Abstract
The thermodynamic parameters characterizing the interaction between rabbit fast skeletal muscle troponin and tropomyosin have been determined at 25.degree. C for three solution conditions: buffer containing (A) 1 mM CaCl2, simulating a "turned-on" state, (B) 3 mM MgCl2, simulating a "turned-off" state, and (C) 2 mM ethylenediaminetetraacetic acid, a reference state. The enthalpies were measured in two buffers with different heats of ionization to allow correction for dissociation or uptake of protons. The enthalpies corrected for proton effects are -22.1, -25.4, and -23.5 kcal/mol, respectively, in buffers A, B, and C. The interaction between troponin and tropomyosin in the presence of calcium is accompanied by release of 0.9 mol of proton per mole of complex. Proton effects in the presence of magnesium and in the absence of divalent metal ions were too small to quantitate. The association constants were measured by using tropomyosin labeled with the extrinsic fluorescent probe dansylaziridine, and binding was detected by enhancement of the probe fluorescence. The magnitudes of the association constants for unlabeled troponin are 7.5 .times. 105, 4.2 .times. 105, and 9.5 .times. 105 M-1, respectively, for the three solution conditions corresponding to unitary free energies of -10.4, -10.1, and -10.6 kcal/mol. The unitary entropies for the interaction are -39, -51, and -43 cal/(deg.cntdot.mol), respectively, for the three solution conditions. Under these conditions, the troponin-tropomyosin interaction is enthalpy driven, and a large unfavorable entropy must be overcome in the formation of the complex. The troponin-tropomyosin interaction is thought to be a crucial part of the protein interactions which regulate the actomyosin ATPase activity of skeletal muscle. These studies suggest that if changes in the troponin-tropomyosin interaction are part of the regulatory signal, they are small in terms of free energy and of its enthalpic and entropic components.