Three-dimensional structures of CaM domains and EF-hands 2 and 3 fragment. The apo-CaM and holo-CaM structures shown here are correspond to human CaM with PDB ID codes 1cfd and 1cll, respectively. (A) The closed, apo and open, holo conformations of nCaM consist of helices A/B and C/D with binding loops I and II, respectively. (B) The closed, apo and open, holo conformations of cCaM consist of helices E/F and G/H with binding loops III and IV, respectively. (C) The apo and holo conformations of CaM2/3 consist of helices C/D and E/F with binding loops II and III, respectively. These three-dimensional illustrations were made by using Visual Molecular Dynamics (VMD) (16).