Titratable Amino Acid Solvation in Lipid Membranes as a Function of Protonation State

Knowledge about the insertion and stabilization of membrane proteins is a key step toward understanding their function and enabling membrane protein design. Transmembrane helices are normally quite hydrophobic so as to efficiently insert into membranes, but there are many exceptions with polar or titratable residues. An obvious example is the S4 helices of voltage-gated ion channels with up to 4 arginines, leading to vivid discussion about whether such helices can insert spontaneously, and if so, what their conformation, protonation state, and cost of insertion really are. To address this question, we have determined geometric and energetic solvation properties for different protonation states of the titrateable amino acids, including hydration, side chain orientation, free energy profiles, and effects on the membrane thickness. As expected, charged states are significantly more expensive to insert (8-16 kcal/mol) than neutral variants (1-3 kcal/mol). Although both sets of values exhibit quite high relative correlation with experimental in vivo hydrophobicity scales, the magnitudes of the in vivo hydrophobicity scales are much lower and strikingly appears as a compressed version of the calculated values. This agrees well with computational studies on longer lipids but results in an obvious paradox: the differences between in vivo insertion and simulations cannot be explained by methodological differences in force fields, possible limited hydrophobic thickness of the endoplasmic reticulum (ER) membrane, or parameters; even anionic lipid head groups (PG) only have limited effect on charged side chains, and virtually none for hydrophobic ones. This leads us to propose a model for in vivo insertion that could reconcile these differences and explain the correlation: if there are considerable hydrophobic barriers inside the translocon, the experimental reference state for the solvation free energy when comparing insertion/translocation in vivo would be quite close to the bilayer environment rather than water.