The kinetics of calcium binding to calmodulin: Quin 2 and ANS stopped-flow fluorescence studies.

The rate of calcium dissociation from bovine testis calmodulin was measured by fluorescence stopped-flow using the calcium indicator Quin 2 or the fluorescence probe 8-anilinonaphthalene sulphonate. Two processes are resolved with Quin 2 corresponding to dissociation from the high affinity sites (kdiss 2 to 9 s-1 for T = 11 to 28 degrees C) and from the low affinity sites (kdiss 293 to 550 s-1 for T = 11 to 19 degrees C). These rates and the activation parameters as determined for the slow process delta H not equal to = 59 +/- 10 kJ X mol-1 and delta S not equal to = 30 +/- 30 JK-1 X mol-1 are in good agreement with values determined from the 43Ca NMR exchange rates. These experiments provide confirmation that the calcium induced conformational change cannot be resolved kinetically from the calcium binding or dissociation, and by inference this conformational change is not a rate-limiting process in the function of calmodulin.