TY - JOUR
T1 - Calculations suggest a pathway for the transverse diffusion of a hydrophobic peptide across a lipid bilayer
AU - Kessel, Amit
AU - Schulten, Klaus
AU - Ben-Tal, Nir
N1 - Funding Information:
This work was supported by Israel Science Foundation grant 683/97 and fellowships from the Wolfson and Alon Foundations to N.B.-T.
PY - 2000
Y1 - 2000
N2 - Alamethicin is a hydrophobic antibiotic peptide 20 amino acids in length. It is predominantly helical and partitions into lipid bilayers mostly in transmembrane orientations. The rate of the peptide transverse diffusion (flip-flop) in palmitoyl-oleyl-phosphatidylcholine vesicles has been measured recently and the results suggest that it involves an energy barrier, presumably due to the free energy of transfer of the peptide termini across the bilayer. We used continuum-solvent model calculations, the known x-ray crystal structure of alamethicin and a simplified representation of the lipid bilayer as a slab of low dielectric constant to calculate the flip-flop rate. We assumed that the lipids adjust rapidly to each configuration of alamethicin in the bilayer because their motions are significantly faster than the average peptide flip-flop time. Thus, we considered the process as a sequence of discrete peptide-membrane configurations, representing critical steps in the diffusion, and estimated the transmembrane flip-flop rate from the calculated free energy of the system in each configuration. Our calculations indicate that the simplest possible pathway, i.e., the rotation of the helix around the bilayer midplane, involving the simultaneous burial of the two termini in the membrane, is energetically unfavorable. The most plausible alternative is a two-step process, comprised of a rotation of alamethicin around its C-terminus residue from the initial transmembrane orientation to a surface orientation, followed by a rotation around the N-terminus residue from the surface to the final reversed transmembrane orientation. This process involves the burial of one terminus at a time and is much more likely than the rotation of the helix around the bilayer midplane. Our calculations give flip-flop rates of ~10-7/s for this pathway, in accord with the measured value of 1.7 x 10-6/s.
AB - Alamethicin is a hydrophobic antibiotic peptide 20 amino acids in length. It is predominantly helical and partitions into lipid bilayers mostly in transmembrane orientations. The rate of the peptide transverse diffusion (flip-flop) in palmitoyl-oleyl-phosphatidylcholine vesicles has been measured recently and the results suggest that it involves an energy barrier, presumably due to the free energy of transfer of the peptide termini across the bilayer. We used continuum-solvent model calculations, the known x-ray crystal structure of alamethicin and a simplified representation of the lipid bilayer as a slab of low dielectric constant to calculate the flip-flop rate. We assumed that the lipids adjust rapidly to each configuration of alamethicin in the bilayer because their motions are significantly faster than the average peptide flip-flop time. Thus, we considered the process as a sequence of discrete peptide-membrane configurations, representing critical steps in the diffusion, and estimated the transmembrane flip-flop rate from the calculated free energy of the system in each configuration. Our calculations indicate that the simplest possible pathway, i.e., the rotation of the helix around the bilayer midplane, involving the simultaneous burial of the two termini in the membrane, is energetically unfavorable. The most plausible alternative is a two-step process, comprised of a rotation of alamethicin around its C-terminus residue from the initial transmembrane orientation to a surface orientation, followed by a rotation around the N-terminus residue from the surface to the final reversed transmembrane orientation. This process involves the burial of one terminus at a time and is much more likely than the rotation of the helix around the bilayer midplane. Our calculations give flip-flop rates of ~10-7/s for this pathway, in accord with the measured value of 1.7 x 10-6/s.
UR - http://www.scopus.com/inward/record.url?scp=0033747198&partnerID=8YFLogxK
U2 - 10.1016/S0006-3495(00)76478-2
DO - 10.1016/S0006-3495(00)76478-2
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AN - SCOPUS:0033747198
SN - 0006-3495
VL - 79
SP - 2322
EP - 2330
JO - Biophysical Journal
JF - Biophysical Journal
IS - 5
ER -