Structure and dynamics of calmodulin in solution

To characterize the dynamic behavior of calmodulin in solution, we have carried out molecular dynamics (MD) simulations of the Ca2+-loaded structure. The crystal structure of calmodulin was placed in a solvent sphere of radius 44 Angstrom, and 6 Cl- and 22 Na+ ions were included to neutralize the system and to model a 150 mM salt concentration. The total number of atoms was 32,867. During the 3-ns simulation, the structure exhibits large conformational changes on the nanosecond time scale. The central alpha-helix, which has been shown to unwind locally upon binding of calmodulin to target proteins, bends and unwinds near residue Arg(74). We interpret this result as a preparative step in the more extensive structural transition observed in the "flexible linker" region 74-82 of the central helix upon complex formation. The major structural change is a reorientation of the two Ca2+-binding domains with respect to each other and a rearrangement of alpha-helices in the N-terminus domain that makes the hydrophobic target peptide binding site more accessible. This structural rearrangement brings the domains to a more favorable position for target binding, poised to achieve the orientation observed in the complex of calmodulin with myosin light-chain kinase. Analysis of solvent structure reveals an inhomogeneity in the mobility of water in the vicinity of the protein, which is attributable to the hydrophobic effect exerted by calmodulin's binding sites for target peptides.