Fundamental and biotechnological applications of neutron scattering measurements for macromolecular dynamics.

To explore macromolecular dynamics on the picosecond timescale, we used neutron spectroscopy. First, molecular dynamics were analyzed for the hyperthermophile malate dehydrogenase from Methanococcus jannaschii and a mesophilic homologue, the lactate dehydrogenase from Oryctolagus cunniculus muscle. Hyperthermophiles have elaborate molecular mechanisms of adaptation to extremely high temperature. Using a novel elastic neutron scattering approach that provides independent measurements of the global flexibility and of the structural resilience (rigidity), we have demonstrated that macromolecular dynamics represents one of these molecular mechanisms of thermoadaptation. The flexibilities were found to be similar for both enzymes at their optimal activity temperature and the resilience is higher for the hyperthermophilic protein. Secondly, macromolecular motions were examined in a native and immobilized dihydrofolate reductase (DHFR) from Escherichia coli. The immobilized mesophilic enzyme has increased stability and decreased activity, so that its properties are changed to resemble those of the thermophilic enzyme. Are these changes reflected in dynamical behavior? For this study, we performed quasielastic neutron scattering measurements to probe the protein motions. The residence time is 7.95 ps for the native DHFR and 20.36 ps for the immobilized DHFR. The average height of the potential barrier to local motions is therefore increased in the immobilized DHFR, with a difference in activation energy equal to 0.54 kcal/mol, which is, using the theoretical rate equation, of the same order than expected from calculation.