Protein chemistry at membrane interfaces: Non-additivity of electrostatic and hydrophobic interactions

Non-specific binding of proteins and peptides to charged membrane interface s depends upon the combined contributions of hydrophobic (DeltaG(H phi)) an d electrostatic (DeltaG(ES)) free energies. If these are simply additive, t hen the observed free energy of binding (DeltaG(obs)) will be given by Delt aG(obs) = DeltaG(H Phi) + DeltaG(ES), where DeltaG(H Phi) = -sigma (NP)A(NP ) and DeltaG(ES) = zF phi. Ln these expressions, A(NP) is the non-polar acc essible area, sigma (NP) the non-polar solvation parameter, z the formal pe ptide valence, F the Faraday constant, and phi the membrane surface potenti al. But several Lines of evidence suggest that hydrophobic and electrostati c binding free energies of proteins at membrane interfaces, such as those a ssociated with cell signaling, are not simply additive. In order to explore this issue systematically, we have determined the interfacial partitioning free energies of variants of indolicidin, a cationic proline-rich antimicr obial peptide. The synthesized variants of the 13 residue peptide covered a wide range of hydrophobic free energies, which allowed us to examine the e ffect of hydrophobicity on electrostatic binding to membranes formed from m ixtures of neutral and anionic Lipids. Although DeltaG(obs) was always a li near function of DeltaG(H Phi) the slope depended upon anionic Lipid conten t: the slope was 1.0 for pure, zwitterionic phosphocholine bilayers and 0.3 for pure phosphoglycerol membranes, DeltaG(obs) also varied linearly with surface potential, but the slope was smaller than the expected value, zF. A s observed by others, this suggests an effective peptide valence z,, that i s smaller than the formal valence z. Because of our systematic approach, we were able to establish a useful rule-of-thumb: z,, is reduced relative to z by about 20 % for each 3 kcal mol(-1) (1 kcal = 4.184 kJ) favorable incre ase in DeltaG(H Phi). For neutral phosphocholine interfaces, we found that DeltaG(obs) could be predicted with remarkable accuracy using the Wimley-Wh ite experiment-based interfacial hydrophobicity scale. (C) 2001 Academic Pr ess.