Estimation of the maximum change in stability of globular proteins upon mutation of a hydrophobic residue to another of smaller size.

Although the hydrophobic effect is generally considered to be one of the most important forces in stabilizing the folded structure of a globular protein molecule, there is a lack of consensus on the precise magnitude of this effect. The magnitude of the hydrophobic effect is most directly measured by observing the change in stability of a protein molecule when an internal hydrophobic residue is mutated to another of smaller size. Results of such measurements have, however, been confusing because they vary greatly and are generally considerably larger than expected from the transfer free energies of corresponding small molecules. In this article, a thermodynamic argument is presented to show (1) that the variation is mainly due to that in the flexibility of the protein molecule at the site of mutation, (2) that the maximum destabilization occurs when the protein at the site of mutation is rigid, in which case the value of the destabilization is approximately given by the work of cavity formation in water, and (3) that the transfer free energy approximately gives the minimum of the range of variations. The best numerical agreements between the small molecule and the protein systems are obtained when the data from the small molecule system are expressed as the molarity-based standard free energies without other corrections.