Positive cooperative binding of calcium to bovine brain calmodulin

Publisher Website
Google Scholar
Abstract
Equilibrium dialysis measurements of the binding of Ca2+ to calmodulin have confirmed the existence of 4 high affinity Ca2+-binding sites (Kd between 3 .times. 10-6 and 2 .times. 10-5 M). In the presence of 3 mM Mg2+ the Kd for Ca2+ are increased 2- to 4-fold (Kd between 5 .times. 10-6 and 4 .times. 10-5 M). Positive cooperativity of Ca2+ binding was observed at low Ca2+ concentrations with Hill coefficients of 1.33 and 1.22 in the absence and presence of 3 mM Mg2+, respectively. The positive cooperativity is compatible with the steepness of the Ca2+ dependence of the conformational transition associated with the binding of 2 mol of Ca2+/mol of calmodulin. This conformational change, which affects the environment of the aromatic residues of calmodulin as measured by UV absorption and near-UV circular dichroism spectroscopy, is not the result of a monomer-dimer equilibrium mediated by Ca2+. Binding of Ca2+ to calmodulin is believed to occur by a sequential mechanism generating at least 4 different conformers of the protein in its free and liganded states. Even though the major conformational change is almost complete upon binding of 2 mol of Ca2+/mol of calmodulin, the activation of cyclic nucleotide phosphodiesterase measured in the presence of limiting concentrations of calmodulin suggests that a calmodulin .**GRAPHIC**. complex is required for interaction of calmodulin with the enzyme. On the basis of the strong affinity of the enzyme for the calmodulin.cntdot.Ca2+ complex (Kd = 1-3 .times. 10-9 M), the Ca2+ dependence of phosphodiesterase activation is highly cooperative and leads to a sharp threshold of Ca2+ concentration for control of enzyme activity.

This publication has 26 references indexed in Scilit: