A Dynamic Model of Interactions of Ca2+, Calmodulin, and Catalytic Subunits of Ca2+/Calmodulin-Dependent Protein Kinase II
The thermodynamic free energy around a reaction loop must sum to zero. This principle (microscopic reversibility) constrains the relationship between the equilibrium constants in the loop. We define cooperativity coefficients s (for the N-terminus of CaM) and r (for the C-terminus of CaM) to quantify the relationship between the affinity of Ca2+ for free CaM and of Ca2+ for CaM when bound to CaMKII. The principle of microscopic reversibility indicates that these coefficients also quantify the relationship between the affinity of CaMKII for CaM with three bound Ca2+ ions, and the affinity of CaMKII for CaM4, as shown in the figure for the N-terminal coefficient s.