Fluorescence titration and fluorescence stopped-flow studies on skeletal muscle troponin labeled with fluorescent reagent.

Incorporation of skeletal muscle troponin C (TN-C) subunit into skeletal muscle troponin (TN) induces a large increase in the apparent binding constant of Ca2+ to the low affinity Ca2+-binding sites of TN-C (from 1 X 10(5) M-1 to 5.6 X 10(6) M-1 in the presence of 2 mM MgCl2), and a large decrease in the rate constant of the Ca2+ removal reaction from the low affinity Ca2+-binding sites of TN-C (from 230 s-1 to 37 s-1 in the presence of 2 mM MgCl2). On the other hand, no significant modification in the molecular kinetic mechanism of the local conformational change due to the Ca2+ binding or removal reaction with the high affinity Ca2+-binding sites of TN-C is observed as TN-C is incorporated into TN.