(69c) Water-Restructuring Mutations Can Reverse the Thermodynamic Signature of Protein-Ligand Association

The organization of water surrounding interacting biomolecules can influenceâ??even dictateâ??the strength of their interaction; the mechanisms by which that influence is exerted, however, remain incompletely understood. This talk dicusses a study in which mutants of human carbonic anhydrase are used to examine how changes in the organization of water filling a binding pocket can alter the thermodynamics of protein-ligand association. Results from empirical (thermodynamic and crystallographic) and theoretical analyses suggest that most mutations strengthen networks of water-mediated hydrogen bonds, and reduce binding affinity by increasing both the enthalpic cost and, in an incompletely compensating fashion, the entropic benefit of rearranging those networks during binding. The organization of water filling a binding pocket can thus determine whether hydrophobic interactions involving that pocket are enthalpy-driven or entropy-driven. Findings highlight a possible asymmetry in protein-ligand association by suggesting that binding events associated with enthalpically favorable rearrangements of waterâ??rearrangements that can be enhanced by mutations that destabilize water networks over nonpolar regionsâ??are stronger than those associated with entropically favorable rearrangements of water.