A kinetic model for the binding of Ca²⁺ to the regulatory site of troponin from cardiac muscle

The kinetics of the binding of Ca²⁺ to the single regulatory site of cardiac muscle troponin was investigated by using troponin reconstituted from the three subunits, using a monocysteine mutant of troponin C (cTnC) labeled with the fluorescent probe 2-[(4'-(iodoacetamido)anilino]naphthalene-6- sulfonic acid (IAANS) at Cys-35. The kinetic tracings of binding experiments for troponin determined at free [Ca²⁺] > 1 μM were resolved into two phases. The rate of the fast phase increased with increasing [Ca²⁺], reaching a maximum of about 35 s^-1 at 4 °C, and the rate of the slow phase was approximately 5 s^-1 and did not depend on [Ca²⁺]. Dissociation of bound Ca²⁺ occurred in two phases, with rates of about 23 and 4 s^-1. The binding and dissociation results obtained with the binary complex formed between cardiac troponin I and the IAANS-labeled cTnC mutant were very similar to those obtained from reconstituted troponin. The kinetic data are consistent with a three-step sequential model similar to the previously reported mechanism for the binding of Ca²⁺ to a cTnC mutant labeled with the same probe at Cys-84 (Dong et al. (1996) J. Biol. Chem. 271, 688-694). In this model, the initial binding in the bimolecular step to form the Ca²⁺- troponin complex is assumed to be a rapid equilibrium, followed by two sequential first-order transitions. The apparent bimolecular rate constant is 5.1 x 10⁷ M^-1 s^-1, a factor of 3 smaller than that for cTnC. The rates of the first-order transitions are an order of magnitude smaller for troponin than for cTnC. These kinetic differences form a basis for the enhanced Ca²⁺ affinity of troponin relative to the Ca²⁺ affinity of isolated cTnC. Phosphorylation of the monocysteine mutant of troponin I by protein kinase A resulted in a 3-fold decrease in the bimolecular rate constant but a 2-fold increase in the two observed Ca²⁺ dissociation rates. These changes in the kinetic parameters are responsible for a 5-fold reduction in Ca²⁺ affinity of phosphorylated troponin for the specific site.