Energetics of Target Peptide Binding by Calmodulin Reveals Different Modes of Binding

Abstract

Thermodynamic parameters of interactions of calcium-saturated calmodulin (Ca2+-CaM) with melittin, C-terminal fragment of melittin, or peptides derived from the CaM binding regions of constitutive (cerebellar) nitric-oxide synthase, cyclic nucleotide phosphodiesterase, calmodulin-dependent protein kinase I, and caldesmon (CaD-A, CaD-A*) have been measured using isothermal titration calorimetry. The peptides could be separated into two groups according to the change in heat capacity upon complex formation, ΔCp. The calmodulin-dependent protein kinase I, constitutive (cerebellar) nitric-oxide synthase, and melittin peptides have ΔCp values clustered around -3.2 kJ·mol-1·K-1, consistent with the formation of a globular CaM-peptide complex in the canonical fashion. In contrast, phosphodiesterase, the C-terminal fragment of melittin, CaD-A, and CaD-A* have ΔCp values clustered around -1.6 kJ·mol-1·K-1, indicative of interactions between the peptide and mostly one lobe of CaM, probably the Cterminal lobe. It is also shown that the interactions for different peptides with Ca2+-CaM can be either enthalpically or entropically driven. The difference in the energetics of peptide/Ca2+-CaM complex formation appears to be due to the coupling of peptide/Ca2+-CaM complex formation to the coil-helix transition of the peptide. The binding of a helical peptide to Ca2+-CaM is dominated by favorable entropic effects, which are probably mostly due to hydrophobic interactions between nonpolar groups of the peptide and Ca2+-CaM. Applications of these findings to the design of potential CaM inhibitors are discussed.