Date of Award
Stevens, Edwin D.
Complexes of the antibiotics novobiocin and clorobiocin with DNA gyrase are illustrative of the importance of bound water to binding thermodynamics. Mutants resistantto novobiocin as well as those with a decreased affinity for novobiocin over clorobiocinboth involve a less favorable entropy of binding, which more than compensates for amore favorable enthalpy, and additional water molecules at the proteinligandinterface.Free energy, enthalpy, and entropy for these water molecules were calculated by thermodynamicintegration computer simulations. The calculations show that addition of thewater molecules is entropically unfavorable, with values that are comparable to the measuredentropy differences. The free energies and entropies correlate with the change inthe number of hydrogen bonds due to the addition of water molecules.To examine the wide variety of cavities available to water molecules inside proteins,a model of the protein cavities is developed with the local environment treated at atomicdetail and the nonlocal environment treated approximately. The cavities are then changedto vary in size and in the number of hydrogen bonds available to a water molecule insidethe cavity. The free energy, entropy, and enthalpy change for the transfer of a watermolecule to the cavity from the bulk liquid is calculated from thermodynamic integration.The results of the model are close to those of similar cavities calculated using the fullprotein and solvent environment. As the number of hydrogen bonds resulting from theaddition of the water molecule increases, the free energy decreases, as the enthalpic gainof making a hydrogen bond outweighs the entropic cost. Changing the volume of thecavity has a smaller effect on the thermodynamics. Once the hydrogen bond contributionis taken into account, the volume dependence on free energy, entropy, and enthalpy issmall and roughly the same for a hydrophobic cavity as a hydrophilic cavity.The influences of bound water on protein structure and influences are also evaluatedby performing molecular dynamics simulation for proteins with and without boundwater. Four proteins are simulated, the wildtypebovine pancreatic trypsin inhibitor(BPTI), the wildtypehen egg white lysozyme (HEWL), and two variants of the wildtypeStaphylococcal nuclease (SNase), PHS and PHS/V66E. The simulation reveals that allthese four proteins suffer structural changes upon the removing of bound water molecules,as indicating by their increased RMSD values with respect to the crystal structures. Threeout of the four proteins, BPTI, HEWL, and the PHS mutant of SNase have increased flexibility,while no apparent flexibility change is seen in the PHS/V66E variant of SNase.
Yu, Hongtao, "Water in Protein Cavities: Free Energy, Entropy, Enthalpy, and its Influences on Protein Structure and Flexibility" (2011). University of New Orleans Theses and Dissertations. 341.