Interaction of Ca++ binding sites of troponin: significance for Ca++ movements

Ca++ binding data for seven preparations of troponin and 37 preparations of native tropomyosin were analyzed by the method of nonlinear least squares. The analysis was based on the assumption that two classes of independent binding sites exist. Data from one-third the preparations could not be fitted with all binding parameters at true least-squares minimum, and computed values of parameters for two-thirds the preparations were biologically uniterpretable. We conclude that troponin does not contain two classes of independent binding sites. Comparison of Scratchard plots of Ca++ binding by troponin and native tropomyosin modifies binding through a cooperative effect on troponin. Certain features of the Scratchard plots are imcompatible with the possibility that troponin possess more than two classes of independent sites. We interpret these results to mean that troponin's binding sites interact and that the interaction is increased by tropomyosin. The interaction would cause Ca++ affinity to vary with time during a muscle contraction. The effect of variable Ca++ affinity on intracellular Ca++ movements and the time course of the active state is discussed.