The Ca2+ association to calcium binding proteins (CaBPs) represents an essential step in Ca2+ signal transduction. This study presents a characterization of Ca interactions with two CaBPs, calmodulin and DREAM, using time-resolved photothermal and fluorescence techniques. Calcium binding to the calmodulin C-terminal domain is associated with a volume change of 40 mL mol-1and an enthalpy change of 35 ± 16 kcal mol-1. These parameters are consistent with the Ca2+ triggered exposure of hydrophobic patches on the calmodulin surface. Also, the rate limiting step for Ca2+ binding to calmodulin is the closed-to-open transition of the C-terminal domain that occurs with a lifetime of 400 μs. Unlike calmodulin, DREAM exists in a dynamic equilibrium of two conformations and Ca2+ binding shifts the equilibrium towards a more compact conformation. These data clearly demonstrate that conformational dynamics play a crucial role in the transmission of Ca2+ signals.
