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Skeletal muscle myofibrils, in the presence of 2 mM MgCl(2) at pH 7.0, were found to have two classes of calcium-binding sites with apparent affinity constants of 2.1 x 10(6)M(-1) (class 1) and approximately 3 x 10(4)M(-1) (class 2), respectively. At free calcium concentrations essential for the activation of myofibrillar contraction ( approximately 10(-6)M) there would be significant calcium binding only to the class 1 sites. These sites could bind about 1.3 micromoles of calcium per g protein. Extraction of myosin from the myofibrils did not alter their calcium-binding parameters. Myosin A, under identical experimental conditions, had little affinity for calcium. The class 1 sites are, therefore, presumed to be located in the I filaments. The class 1 sites could only be detected in F actin and myosin B preparations which were contaminated with the tropomyosin-troponin complex. Tropomyosin bound very little calcium. Troponin, which in conjunction with tropomyosin confers calcium sensitivity on actomyosin systems, could bind 22 micromoles of calcium per g protein with an apparent affinity constant of 2.4 x 10(6)M(-1). In view of the identical affinity constants of the myofibrils and troponin and the much greater number of calcium-binding sites on troponin it is suggested that calcium activates myofibrillar contraction by binding to the troponin molecule.

The Site of Calcium Binding in Relation to the Activation of Myofibrillar Contraction