Figure 5.

Correlation of measured hydrophobicity of cysteine with the intrinsic hydrophobicity of the residue. Scatter plots representing partitioning free energies derived for the midplane and interface host site substituents are plotted as a function of the β-strand (from one to eight) in which the host site is located. A rainbow color scheme (N→C-terminus) is used for the scatter plots. The estimated energetic cost of substitution (Xxx→Cys) calculated from the whole protein hydrophobicity scale for midplane mutants (8) and the Wimley-White interface scale derived from peptide insertion into lipid membranes for the interface mutants (5) are depicted as gray histograms. We find that the estimated energetic costs correlate reasonably well with the ΔΔG⁰ for the midplane mutants (center panel). However, in the case of interface mutants (left and right panels), there is a significant difference between the estimated energetic cost of cysteine incorporation and the observed destabilization computed in the form of ΔΔG⁰. Additionally, the Mann-Whitney U test delivers a p-value of 0.0251, indicating a statistically significant difference between the ΔΔG values of the interface and the midplane mutants. This deviation could be explained by the difference in the model systems used to generate the two respective hydrophobicity scales. The whole protein hydrophobicity scale used mutations at a host site in a β-barrel membrane protein (used here for comparison with midplane mutants), whereas the Wimley-White interface scale measured the insertion of an array of helical peptides into phospholipid bilayers (used here for comparison with interface mutants). To see this figure in color, go online.