Fast dynamics of halophilic malate dehydrogenase and BSA measured by neutron scattering under various solvent conditions influencing protein stability

Protein thermal dynamics was evaluated by neutron scattering for halophilic malate dehydrogenase from Haloarcula marismortui (HmMa1DH) and BSA under d ifferent solvent conditions. As a measure of thermal stability in each case , loss of secondary structure temperatures were determined by CD. HmMaIDH r equires molar salt and has different stability behavior in H2O, D2O, and in NaCl and KCI solvents. BSA remains soluble in molar NaCl. The neutron expe riments provided values of mean-squared atomic fluctuations at the 0.1 ns t ime scale. Effective force constants, characterizing the mean resilience of the protein structure, were calculated from the variation of the mean-squa red fluctuation with temperature. For HmMa1DH, resilience increased progres sively with increasing stability, from molar NaCl in H2O, via molar KCl in D2O, to molar NaCl in D2O. Surprisingly, however, the opposite was observed for BSA; its resilience is higher in H2O where it is less stable than in D 2O. These results confirmed the complexity of dynamics-stability relationsh ips in different proteins. Softer dynamics for BSA in D2O showed that the h igher thermostability is associated with entropic fluctuations. In the halo philic protein, higher stability is associated with increased resilience sh owing the dominance of enthalpic terms arising from bonded interactions. Fr om previous data, it is suggested that these are associated with hydrated i on binding stabilizing the protein in the high-salt solvent.