BINDING OF APOLIPOPROTEIN-A-I MODEL PEPTIDES TO LIPID BILAYERS - MEASUREMENT OF BINDING ISOTHERMS AND PEPTIDE-LIPID HEADGROUP INTERACTIONS

Amphiphatic cu-helices are the lipid-binding motif in many apolipoprot eins. Two model peptides, namely he-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu-Ala-Phe (18A) and Lys-Trp-Leu-Asp-Ala-Phe-Tyr-Lys-Asp-Val -Ala-L ys-Glu-Leu-Glu-Lys-Ala-Phe (18R), have been synthesized previously to mimic the structural and functional properties of apolipoprotein A-1. Here a quantitative thermodynamic analysis of the binding process of 1 8A and 18R to neutral and negatively charged lipid membranes is provid ed. Peptide 18A has a higher lipid affinity than 18R, and both peptide s bind better to mixed 1-palmitoyl-2-oleoyl-3-sn-glycero /1-palmitoyl- 2-oleoyl-3-sn-glycero-phosphoglycerol (POPC/POPG) bilayers than to pur e POPC bilayers. At lipid-to-peptide ratios >100, the binding of 18A a nd 18R to phospholipid bilayers can be described by an apparent surfac e partition equilibrium with binding constants in the range of 40-900 M(-1). At high peptide concentrations, the membrane affinity of 18A an d 18R increases dramatically NMR studies provide evidence that peptide -peptide interactions make additional contributions to the binding ene rgy. A cooperative binding model is developed to describe the binding process over the whole concentration range. The cooperativity paramete r a is identical for 18A and 18R yielding a peptide-peptide interactio n energy of about -2.4 kcal/mol. The free energy of membrane insertion is about -6.5 kcal/mol for 18A and -5.5 kcal/mol for 18R. The binding reaction is driven by the hydrophobic surface energy which is partial ly balanced by the loss in translational and rotational degrees of fre edom. A molecular analysis of the free energy of binding predicts a 40 -60% insertion of the peptides into the hydrophobic membrane environme nt. Deuterium and phosphorus solid state NMR were used to monitor the influence of 18A and 18R on the long range and short range order of th e phospholipids. The sprectra are characteristic of fluid-like lipid b ilayers and provide no evidence for the formation of discoidal particl es. However, both peptides change the conformation of the phosphocholi ne dipoles, moving the N+ end of the latter toward the water phase. Th e rotation of the P---N+ dipoles is due to the interaction of the phos pholipids with the positive charges on 18A and 18R, with 18A being mor e effective than 18R. For 18R the NMR data predict a pK shift and a pa rtial charge neutralization of the carboxylate groups located at the e dge of the polar/nonpolar interface.