Calcium binding to complexes of calmodulin and calmodulin binding proteins

Area of Science:

• Biochemistry
• Molecular Biology
• Calcium Signaling

Background:

• Calmodulin (CaM) is a crucial calcium-binding protein regulating numerous cellular processes.
• Understanding the energetics of CaM-protein interactions is key to deciphering calcium-dependent signaling pathways.

Purpose of the Study:

• To determine the free energy of coupling for Ca2+ and calmodulin-binding proteins (CBPs) binding to calmodulin.
• To compare these coupling energies with those of troponin I (TnI) and myosin light chain kinase (MLCK).

Main Methods:

• Quantified Ca2+ binding to CaM complexed with CBPs using Quin 2 to monitor free Ca2+ concentrations.
• Calculated dissociation constants (Kd) for Ca2+ binding in the presence of equimolar TnI, MLCK, and calmodulin-sensitive phosphodiesterase (CaM-PDE).

Main Results:

• Geometric mean Kd values for Ca2+ binding to CaM-TnI, CaM-MLCK, and CaM-CaM-PDE were 2.1, 1.1, and 0.55 µM, respectively.
• Free-energy couplings for the binding of Ca2+ and these proteins to CaM were -4.48, -6.00, and -7.64 kcal.
• Ca2+-independent Kd for CaM-PDE binding was 80 mM, suggesting no complex formation under low Ca2+ conditions.

Conclusions:

• Large free-energy coupling values indicate increased Ca2+ affinity of CaM when complexed with CBPs, demonstrating apparent positive cooperativity.
• In vitro differences in free-energy coupling suggest varying Ca2+ sensitivities for different calmodulin-regulated enzymes in vivo.