Relative Free Enthalpies for Point Mutations in Two Proteins with Highly Similar Sequences but Different Folds

Enveloping distribution sampling was used to calculate free-enthalpy changes associated with single amino acid mutations for a pair of proteins, G(A)95 and G(B)95, that show 95% sequence identity yet fold into topologically different structures. Of the L -> A, I -> F, and L -> Y mutations at positions 20, 30, and 45, respectively, of the 56-residue sequence, the first and the last contribute the most to the free-enthalpy difference between the native and non-native sequence-structure combinations, in agreement with the experimental findings for this protein pair. The individual free-enthalpy changes are almost sequence-independent in the four-strand/one-helix structure, the stable form of G(B)95, while in the three-helix bundle structure, the stable form of G(A)95, an interplay between residues 20 and 45 is observed.