In Vitro Binding of Synthetic Acylated Lipid-Associating Peptides to High-Density Lipoproteins: Effect of Hydrophobicity

To measure the effect of hydrophobicity on the binding of model apoproteins to lipoproteins, we synthesized a 15 amino acid lipid-associating peptide (LAP) with acyl chains of various lengths (0-18 carbons) bound to the N-terminal amino acid through a peptide bond. The acylated LAPs preferentially bound to high-density lipoprotein (HDL) and were activators of lecithin:cholesterol acyltransferase. Circular dichroic spectra indicated that the LAP association with phospholipid was accompanied by increased α-helical structure. The LAPs self-associated in solution as judged from tryptophan fluorescence analysis. These characteristics, which are comparable to those of apolipoprotein A-I, were strongly dependent upon the acyl chain length of the LAPs. The equilibrium constants (K_eq) for the association of LAPs to reassembled HDL were measured by equilibrium dialysis at several temperatures. At 37°C, K_eq increased by 3 orders of magnitude as the number of carbon units was increased from 0 to 16; there was a log-linear relationship between K_eq and the acyl chain length. The free energy of association (ΔG_a) decreased by a constant value for each methylene unit added to the acyl chain (0.35 kcal mol^-1), clearly demonstrating a strict hydrophobic effect. This change of ΔG_a was enthalpy rather than entropy driven. Our data show that, with all other parameters including putative α-helicity, sequence, and molecular weight being constant, the binding of a lipid-associating peptide to lipoprotein is governed by its hydrophobicity.