A theoretical analysis of binding to the Ca2+-specific sites on troponin incorporated into thin filaments

Ca2+ binding to cardiac troponin C in the myofilament lattice and its relation to the myofibrillar ATPase activity

The Ca(2+)-binding properties of troponin C in the intact myofilament lattice and their relation to the activation of ATPase were investigated with isolated porcine cardiac myofibrils. Ca2+ binding, which is composed of two classes of binding sites with different affinities (classes 1 and 2), was clearly detected by a novel method for subtracting the large background activity of myofibrillar Ca2+ binding. The classes 1 and 2 were equivalent stoichiometrically to the two high-affinity sites (sites III and IV) and a single low-affinity site (site II) of troponin C. In the presence of ATP, positive cooperativity was observed in the Ca2+ binding of class-2 sites and the Hill equation parameters were in excellent agreement with those for the Ca(2+)-activated myofibrillar ATPase activity, which indicated that the activation of ATPase is a linear function of the Ca2+ occupancy of site II. In the absence of ATP, a marked increase in the affinity of only class-2 sites was observed while the cooperativity was lost. These results provide direct evidence that some feedback mechanism exists between myosin crossbridge attachment and the Ca2+ binding to site II of troponin C, which may thus confer positive cooperativity on the Ca2+ activation of myofibrillar ATPase activity.

The effect of Mg2+ on the Ca2+ binding to troponin C in rabbit fast skeletal myofibrils

The effect of Mg2+ on the Ca2+ binding to rabbit fast skeletal troponin C and the CA2+ dependence of myofibrillar ATPase activity was studied in the physiological state where troponin C was incorporated into myofibrils. The Ca2+ binding to troponin C in myofibrils was measured directly by 45Ca using the CDTA-treated myofibrils as previously reported (Morimoto, S. and Ohtsuki, I. (1989) J. Biochem. 105, 435-439). It was found that the Ca2+ binding to the low and high affinity sites of troponin C in myofibrils was affected by Mg2+ competitively and the Ca2(+)- and Mg2(+)-binding constants were 6.20 x 10(6) and 1.94 x 10(2) M-1, respectively, for the low affinity sites, and 1.58 x 10(8) and 1.33 x 10(3) M-1, respectively, for the high affinity sites. The Ca2+ dependence of myofibrillar ATPase was also affected by Mg2+, with the apparent Ca2(+)- and Mg2(+)-binding constants of 1.46 x 10(6) and 276 x 10(2) M-1, respectively, suggesting that the myofibrillar ATPase was modulated through a competitive action of Mg2+ on Ca2+ binding to the low affinity sites, though the Ca2+ binding to the low affinity sites was not simply related to the myofibrillar ATPase.