Contribution of the hydrophobic effect to globular protein stability

Abstract

The decrease in conformational stability, Δ(ΔG), has been measured for 72 aliphatic side-chain mutants from four proteins in which a larger side-chain is replaced by a smaller side-chain so that steric effects are minimal. When these Δ(ΔG) values are corrected to the same accessibility, namely 100% buried, then the following- Δ(ΔG) values per -CH 2-group (in kcal/mol) are obtained: Ile → Val (1.26), Ala (1.26), Gly (1.26); Leu → Ala (1.16), Gly (1.21); Val → Ala (1.23), Gly (1.53). The average of these values is 1.27(±0.07) kcal/mol. The 72 individual values range from 0 to 2.4 kcal/mol with an average value of 1.27(±0.51) (standard deviation) kcal/mol. When the ΔG tr values from n-octanol to water are corrected for the difference in volume between the solutes and the solvents, the average value for the same substitutions is 1.25(±0.05) kcal/mol. This suggests that proteins gain 1.3(± 0.5) kcal/ mol in stability for each -CH 2-group buried in folding, and, furthermore, that the volume corrected ΔG tr values for n-octanol for the amino acid side-chains provide good estimates of the contribution of the hydrophobic effect to globular protein stability.