Hydrodynamic Properties of Bovine Cardiac Troponin C

The size and shape of bovine cardiac troponin C (TN-C) in solution have been examined by gel filtration, ultracentrifugation, and viscosity in the presence and absence of Ca²⁺. Cardiac TN-C (-Ca²⁺) has an intrinsic sedimentation coefficient, S_20,w⁰ of 1.87 S and a Stokes radius, R_S, of 26.3 A as determined by gel chromatography on Sephacryl S-300. In 2 mM Ca²⁺, 20-W is increased to 2.04 S and Rs is decreased to 24.3 A, indicating a conformational change to a more compact structure. Furthermore, the intrinsic viscosity of TN-C in the absence of Ca²⁺ (6.4 mL/g) is reduced to 5.4 mL/g when Ca²⁺ is added. Sedimentation equilibrium studies indicate that the effects of Ca²⁺ are not due to changes in the molecular weight of the protein. The hydrodynamic data suggest that TN-C is a moderately asymmetric protein with an axial ratio of 4-6.