Effect of temperature and hydration on protein fluctuations: Molecular dynamics simulation of Cu, Zn superoxide dismutase at six different temperatures. Comparison with neutron scattering data

The equilibrium properties of solvated Cu,Zn superoxide dismutase have been sampled in the isothermal-isobaric ensemble at six different temperatures, namely at T=80, 130, 200, 225, 250, and 300 K. Molecular dynamics simulations covering the same range of temperatures have been also carried out for the protein in vacuo. Evaluation of the Debye-Waller factors as a function of temperature indicates that the "glasslike" transition, experimentally observed at approximately 200 K by neutron scattering experiments, is well reproduced when the protein is simulated in the presence of water solvent whereas the simulations without solvent are not able to reproduce the experimental results. Analysis of anharmonicity and anisotropy of the atomic motions indicates that these parameters are good indicators for the occurrence of the transition. Analysis of the atomic fluctuations of different protein shells, having different degrees of exposure to the solvent, shows that the transition is driven by the protein atoms belonging to the external shell, whose Debye-Waller factors are found to be larger in vacuo than in the solvated system. (C) 1998 American Institute of Physics.