A Dynamic Model of Interactions of Ca2+, Calmodulin, and Catalytic Subunits of Ca2+/Calmodulin-Dependent Protein Kinase II
Figure 4
Constraining of s and r cooperativity coefficients for on and off rates by fitting to experimental data.
Three independent sets of experimental data were used to constrain the values of the cooperativity coefficients s and r, that represent the ratios between the on and off binding constants for Ca2+ to the N- and C-termini of free CaM (respectively) and the corresponding binding constants for Ca2+ to the same termini in the K•CaM complex. The simplex method for gradient descent was used to fit the parameters to each set of data. A) Fits to data for dissociation of CaM from CaMKII in 50 µM Ca2+ (data from Figure 2B in [30]); B) Fits to data for dissociation of CaM from CaMKII in 200 nM Ca2+ (data from Figure 2B in [30]); and C) Fits to data for dissociation of Ca2+ from Ca2+/CaM/CaMKII (data renormalized from Figure 4A in [57]). Black, real data; Blue, best fit when all the cooperativity was assumed to reflect a change in on rates; Green, best fit when all the cooperativity was assumed to reflect a change in off rates; Red, best fit when cooperativity in on and off rates were allowed to vary simultaneously. (See Text S1 for details.)