Lipid–protein interactions in the membrane: Studies with model peptides
We have used fluorescence quenching of tryptophan-containing trans-membrane peptides by bromine-containing phospholipids to study the specificity of peptide–lipid interactions. We have synthesized peptides Ac-K2GLmWLnK2A-amide where m=7 and n=9 (L16) and m=10 and n=12 (L22). Binding constants of L22 for dioleoylphosphatidylserine [di(C18:1)PS] or dioleoylphosphatidic acid [di(C18:1)PA] relative to dieoleoylphosphatidylcholine [di(C18:1)PC] were close to 1. However, for L16, whilst the bulk of the di(C18:1)PA molecules bound with a binding constant relative to di(C18:1)PC close to 1, a small number of di(C18:1)PA molecules bound much more strongly. Assuming just one high affinity binding site on L16 for anionic lipid, the affinity of the site for di(C18:1)PS was calculated to be ca. 8 times that for di(C18:1)PC. The relative binding constant was little affected by ionic strength and close contact between the anionic headgroup of di(C18:1)PS and a lysine residue on the peptide was suggested. The relative binding constant for di(C18:1)PS at this high affinity site was less than for di(C18:1)PA. Cholesterol interacts with L22 with an affinity about 0.7 of that of di(C18:1)PC. The structure of the peptide itself is important. The peptide Ac-KKGYL6WL8YKKA-amide (Y2L14) incorporated into bilayers of dinervonylphosphatidylcholine [di(C24:1)PC] whereas L16 did not incorporate into this lipid. It is suggested that thinning of a lipid bilayer around a peptide to give optimal hydrophobic matching is less energetically unfavourable when a Tyr residue is located in the lipid/water interfacial region.