Apolipoprotein A-II (apoA-II), the second most abundant protein of the human high-density lipoproteins, spontaneously associates with dimyristoylphosphatidylcholine (DMPC) to give multiple products whose composition, structure, and properties are a sensitive function of the temperature and of the initial lipid to protein ratio at which they are formed. We have studied the thermodynamics of this association by calorimetric and spectroscopic methods. Complexes having a DMPC/apoA-II molar ratio of 75:1 are formed at 20 and 24°C; a 240:1 complex is also formed at 24°C, and a 45:1 complex is formed at 30°C. Additionally, in the presence of a large excess of lipid at 24°C, the 75:1 complex can be converted to a 240:1 complex. These temperature regions are respectively below, at, and above the transition temperature, Tc, of DMPC (23.9°C). According to our analysis of the differential scanning calorimetric data, the 75:1 and 240:1 complexes contain a "boundary layer" of 45 lipid molecules/molecule of ApoA-II whereas the 45:1 complex has a boundary of about 37 DMPC molecules/ molecule of apoA-II. Batch calorimetry of the association of DMPC with apoA-II at 23.45, 24.5, and 30.0°C gives values of +90, -260, and -62 kcal/mol of apoA-II. The enthalpy of association of lysomyristoylphosphatidylcholine (LMPC) with apoA-II at 30.0°C was identical with that of DMPC. Correlation of circular dichroic and calorimetric data shows that the enthalpy of α-helix formation of apoA-II which accompanies its association with DMPC is exothermic with a value of -2.9 kcal/1% α-helical development (-2.0 kcal/α-helical residue). This is the major source of enthalpy in the association of apoA-II with DMPC at 30°C and with LMPC at all temperatures. Between the Tc of DMPC and that of its complexes with apoA-II the enthalpy of association is highly exothermic (-260 kcal/mol of apoA-II); the enthalpy in this temperature range is assigned to the sum of protein-induced, isothermal acyl chain crystallization and α-helix formation. Below Tc, the association is endothermic (+90 kcal/mol of apoA-II) but occurs spontaneously. Therefore, the entropic contribution (+TΔS) to the free energy of association is greater than -90 kcal/mol of apoA-II. This value compares favorably with the calculated free energy of transfer (-98 kcal/mol of apoA-II) of hydrophobic amino acid side chains of the nonpolar faces of amphipathic α-helical regions of apoA-II from an aqueous to a hydrophobic environment. These results demonstrate that the energetics of apolipoprotein association with phospholipids are a function of accompanying structural changes in both the lipid and the protein.
ASJC Scopus subject areas