Temperature dependence of molecular interactions involved in defining stability of glutamine binding protein and its complex with L-glutamine.

The temperature dependence of dynamic parameters derived from nuclear magnetic resonance (NMR) relaxation data is related to conformational entropy of the system under study. This provides information such as macromolecules stability and thermodynamics of ligand binding. We studied the temperature dependence of NMR order parameter of glutamine binding protein (GlnBP), a periplasmic binding protein (PBP) highly specific to L-glutamine associated with its ABC transporter, with the goal of elucidating the dynamical differences between the respective ligand bound and free forms. We found that the protein-ligand interaction, which is stabilized at higher temperature, has a striking effect on the stability of the hydrophobic core of the large domain of GlnBP. Moreover, in contrast to what was found for less specific PBPs, the decreasing backbone motion of the hinge region at increasing temperature supports the idea that the likelihood that GlnBP can adopt a ligand free closed conformation in solution diminishes at higher temperatures. Our results support the induced-fit model as mode of action for GlnBP. In addition, we found that the backbones of residues involved in a salt bridge do not necessarily become more rigid as the temperature rises as it was previously suggested [Vinther, J. M., et al. (2011) J. Am. Chem. Soc., 133, 271-278]. Our results show that for this to happen these residues have to also directly interact with a region of the protein that is becoming more rigid as the temperature increases.