Thermodynamic analysis of calcium and magnesium binding to calmodulin.
To elucidate some aspects still debated concerning the interaction of Ca2+ and Mg2+ with CaM, the thermodynamic binding parameters of Ca2+-CaM and Mg2+-CaM complexes were characterized by flow dialysis and isothermal microcalorimetry under different experimental conditions. In particular, the enthalpy and entropy changes associated with Ca2+ and Mg2+ binding to their sites were determined, allowing a better understanding of the mechanism underlying cation-CaM interactions. Ca2+-CaM interaction follows an enthalpy-entropy compensation relationship, suggesting that CaM explores a subspace of isoenergetical conformations which is modified by Ca2+ binding. This Ca2+-induced change in CaM dynamics is proposed to play a key role in CaM function, i.e. in its interaction with and/or activation of target proteins. Furthermore, data show that Mg2+ does not act as a direct competitor for Ca2+ binding on the four main Ca2+ binding sites, but rather as an allosteric effector. This implies that the four main Mg2+ binding sites are distinct from the EF-hand Ca2+ binding sites. Finally, Ca2+ is shown to interact with auxiliary binding sites on CaM. These weak affinity sites were thermodynamically characterized. The results presented here challenge the current accepted view of CaM ion binding.
Gilli
R
R
Faculté de Pharmacie, UPRESA CNRS 6032, Marseille, France.
Lafitte
D
D
Lopez
C
C
Kilhoffer
M
M
Makarov
A
A
Briand
C
C
Haiech
J
J
D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D002147:Calmodulin / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism*; D002151:Calorimetry / Q000379:methods; D003956:Dialysis; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D013816:Thermodynamics
D002147:Calmodulin; D008274:Magnesium; D002118:Calcium