Enthalpy-Driven Apolipoprotein A-I and Lipid Bilayer Interaction Indicating Protein Penetration upon Lipid Binding

The interaction of lipid-free apolipoprotein A-I (apoA-I) with small unilamellar vesicles (SUVs) of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) with and without free cholesterol (FC) was studied by isothermal titration calorimetry and circular dichroism spectroscopy. Parameters reported are the affinity constant (K a), the number of protein molecules bound per vesicle (n), enthalpy change (ΔH°), entropy change (ΔS°), and the heat capacity change (ΔCp °). The binding process of apoA-I to SUVs of POPC plus 0−20% (mole) FC was exothermic between 15 and 37 °C studied, accompanied by a small negative entropy change, making enthalpy the main driving force of the interaction. The presence of cholesterol in the vesicles increased the binding affinity and the α-helix content of apoA-I but lowered the number of apoA-I bound per vesicle and the enthalpy and entropy changes per bound apoA-I. Binding affinity and stoichiometry were essentially invariant of temperature for binding to SUVs of POPC/FC at a molar ratio of 6/1 at (2.8−4) × 106 M-1 and 2.4 apoA-I molecules bound per vesicle or 1.4 × 102 phospholipids per bound apoA-I. A plot of ΔH° against temperature displayed a linear behavior, from which the ΔCp ° per mole of bound apoA-I was calculated to be −2.73 kcal/(mol·K). These results suggested that binding of apoA-I to POPC vesicles is characterized by nonclassical hydrophobic interactions, with α-helix formation as the main driving force for the binding to cholesterol-containing vesicles. In addition, comparison to literature data on peptides suggested a cooperativity of the helices in apoA-I in lipid interaction.