The Ca2+‐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 Ca2+‐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.
|Number of pages||6|
|Journal||European Journal of Biochemistry|
|Publication status||Published - Dec 1994|
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