In the present study we examine the enthalpy of binding of 2-methoxy-3-isobutylpyrazine (IBMP) to the mouse major urinary protein (MUP), using a combination of isothermal titration calorimetry (ITC), NMR, X-ray crystallography, all-atom molecular dynamics simulations, and site-directed mutagenesis. Global thermodynamics data derived from ITC indicate that binding is driven by favorable enthalpic contributions, rather than a classical entropy-driven signature that might be expected given that the binding pocket of MUP-1 is very hydrophobic. The only ligand-protein hydrogen bond is formed between the side-chain hydroxyl of Tyr120 and the ring nitrogen of the ligand in the wild-type protein. ITC measurements on the binding of IBMP to the Y120F mutant demonstrate a reduced enthalpy of binding, but nonetheless binding is still enthalpy dominated. A combination of solvent isotopic substitution ITC measurements and all-atom molecular dynamics simulations with explicit inclusion of solvent water suggests that solvation is not a major contributor to the overall binding enthalpy. Moreover, hydrogen/deuterium exchange measurements suggest that there is no significant contribution to the enthalpy of binding derived from "tightening" of the protein structure. Data are consistent with binding thermodynamics dominated by favorable dispersion interactions, arising from the inequality of solvent-solute dispersion interactions before complexation versus solute-solute dispersion interactions after complexation, by virtue of poor solvation of the binding pocket.

The contributions of solute-solute dispersion interactions to binding thermodynamics have generally been thought to be small, due to the surmised equality between solute-solvent dispersion interactions prior to the interaction versus solute-solute dispersion interactions following the interaction. The thermodynamics of binding of primary alcohols to the major urinary protein (MUP-I) indicate that this general assumption is not justified. The enthalpy of binding becomes more favorable with increasing chain length, whereas the entropy of binding becomes less favorable, both parameters showing a linear dependence. Despite the hydrophobicity of the interacting species, these data show that binding is not dominated by the classical hydrophobic effect, but can be attributed to favorable ligand-protein dispersion interactions.

No abstract