Influence of Protein Scaffold on Side-Chain Transfer Free Energies

The process by which membrane proteins fold involves the burial of side chains into lipid bilayers. Both structure and function of membrane proteins depend on the magnitudes of side-chain transfer free energies (ΔΔG_sc^o). In the absence of other interactions, ΔΔG_sc^o is an independent property describing the energetics of an isolated side chain in the bilayer. However, in reality, side chains are attached to the peptide backbone and surrounded by other side chains in the protein scaffold in biology, which may alter the apparent ΔΔG_sc^o. Previously we reported a whole protein water-to-bilayer hydrophobicity scale using the transmembrane β-barrel Escherichia coli OmpLA as a scaffold protein. To investigate how a different protein scaffold can modulate these energies, we measured ΔΔG_sc^o for all 20 amino acids using the transmembrane β-barrel E. coli PagP as a scaffold protein. This study represents, to our knowledge, the first instance of ΔΔG_sc^o measured in the same experimental conditions in two structurally and sequentially distinct protein scaffolds. Although the two hydrophobicity scales are strongly linearly correlated, we find that there are apparent scaffold induced changes in ΔΔG_sc^o for more than half of the side chains, most of which are polar residues. We propose that the protein scaffold affects the ΔΔG_sc^o of side chains that are buried in unfavorable environments by dictating the mechanisms by which the side chain can reach a more favorable environment and thus modulating the magnitude of ΔΔG_sc^o.