Folding of amphipathic alpha-helices on membranes: Energetics of helix formation by melittin

Membranes have a potent ability to promote secondary structure formation in a wide range of membrane-active peptides, believed to be due to a reductio n through hydrogen bonding of the energetic cost of partitioning peptide bo nds. This process is of fundamental importance for understanding the mechan ism of action of toxins and antimicrobial peptides and the stability of mem brane proteins. A classic example of membrane-induced folding is the bee-ve nom peptide melittin that is largely unstructured when free in solution, bu t strongly adopts an amphipathic alpha-helical conformation when partitione d into membranes. We have determined the energetics of melittin helix forma tion through measurements of the partitioning free energies and the helicit ies of native melittin and of a diastereomeric analog with four D-amino aci ds (D-4,L-melittin). Because D-4,L-melittin has little secondary structure in either the free or bound forms, it serves as a model for the experimenta lly inaccessible unfolded bound form of native melittin. The partitioning o f native melittin into large unilamellar phosphocholine vesicles is 5.0(+/- 0.7) kcal mol(-1) more favorable than the partitioning of D-4,L-melittin (1 cal = 4.186 J). Differences in the circular dichroism spectra of the two f orms of melittin indicate that bound native melittin is more helical than b ound D-4,L-melittin by about 12 residues. These findings disclose that the free energy reduction per residue accompanying the folding of melittin in m embrane interfaces is about 0.4 kcal mol(-1), consistent with the hypothesi s that hydrogen bonding reduces the high cost of partitioning peptide bonds . A value of 0.6 kcal mol(-1) per residue has been observed for beta-sheet formation by a hexapeptide model system. These two values provide a useful rule of thumb for estimating the energetic consequences of membrane-induced secondary structure formation. (C) 1999 Academic Press.