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 titrationcalorimetry and circular dichroism spectroscopy. Parameters reported are the affinity constant (K;,), the number of protein molecules bound per vesicle (n), enthalpy change (ΔH°), entropy change (ΔS°), and the heat capacity change (ΔC p o ). 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) x 10 6 M - 1 and 2.4 apoA-I molecules bound per vesicle or 1.4 x 10 2 phospholipids per bound apoA-I. A plot of ΔH° against temperature displayed a linear behavior, from which the ΔC p o 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.