Joshua Wand, University of Pennsylvania.
11:00 AM, WL-Aud
Molecular recognition by proteins is fundamental to almost every biological process, particularly the protein associations underlying cellular signal transduction. Understanding the basis for protein-protein interactions requires the full characterization of the thermodynamics of their association. Historically it has been virtually impossible to experimentally estimate changes in residual protein entropy, a potentially important component of the change in the free energy of protein association. However, solution NMR spectroscopy has recently emerged as a powerful tool for characterizing the dynamics of proteins and has thereby potentially gained access to their residual entropy.1 Our approach to this problem will be outlined with emphasis on various technical aspects such as the robustness of the model-free treatment in this context. The methodology will be illustrated with results with calmodulin, which binds a range of target proteins with high affinity. We have examined the interaction of calmodulin with six calmodulin-binding domains. We find that the change in internal dynamics of the protein calmodulin varies significantly upon binding a variety of target domains. Surprisingly, the apparent change in the corresponding residual entropy is linearly related to the change in the overall binding entropy.2 We have also completed a study of the dynamics of the bound calmodulin-binding domains. This allows for the calibration of the dynamical �entropy meter� and reveals a remarkably precise quantitative measure of the change in conformational entropy.3 The results indicate that changes in residual protein conformational entropy can indeed contribute significantly to the free energy of protein-ligand association. It seems evident that protein entropy can be exploited in the maturation of high affinity interactions either by biological evolution or by human intervention such as in the design of protein-targeted pharmaceuticals. This is not a generally held view of the origins of protein-ligand association. Thus, though the basic approach requires further refinement, the initial view is tantalizing. Supported by NIH grant DK 39806.