Molecular dynamics simulations of calcium-free calmodulin in solution

A 4-ns molecular dynamics simulation of calcium-free calmodulin in solution has been performed, using Ewald summation to treat electrostatic interactions. Our simulation results were mostly consistent with solution experimental studies, including NMR, fluorescence and x-ray scattering. The secondary structures within the N- and C-terminal domains were conserved in the simulation, with trajectory structures similar to the NMR-derived model structure ICFD. However, the relative orientations of the domains, for which there are no NMR restraints, differed in details between the simulation and the ICFD model. The most interesting information provided by the simulations is that the dynamics of calcium-free calmodulin in solution is dominated by slow rigid body reorientations of the domains. The interdomain distance fluctuated between 29 and 39 Angstrom, and interdomain orientation angle, defined as the pseudo-dihedral formed by the four calcium binding sites, varied between -2degrees and 108degrees. Similarly, the domain linker region also exhibited significant fluctuations, with its length varying in the 34-45 Angstrom range and its bond angle in the 10-100degrees range. The simulations are in accord with fluorescence results suggesting that calcium-free calmodulin is more compact and more flexible than the calcium activated form. Surprisingly, quite similar solvent accessibilities of the hydrophobic patches were seen in the calcium-free trajectory described in this work and previously generated calcium-loaded calmodulin simulations. Thus, our simulations suggest a reexamination of the standard model of the structural change of calmodulin upon calcium binding, involving exposure of the hydrophobic patches to solvent.