Apoprotein stability and lipid-protein interactions in human plasma high density lipoproteins

Temperature-dependent conformational changes of the principal apoprotein of human plasma high density lipoprotein (HDL), apoA-I, have been studied in the isolated apoprotein, in complexes of apoprotein with phospholipid, and in intact HDL. Differential scanning calorimetry shows that in solution apoA-I undergoes a reversible, two-state thermal denaturation (midpoint temperature 54 degrees). The enthalpy (2.4 cal/g)(10.0 J/g) and specific heat change (0.08 cal/degrees C per g)(0.33 J/degrees C per g) associated with the denaturation were used to calculate the free energy difference (deltaG) between native and unfolded apoA-I at 37 degrees. DeltaG (2.4 kcal/mol)(10.0 kJ/mol) is less than that of other globular proteins (typically 8-14 kcal/mol)(33-59 kJ/mol), indicating that at 37 degrees native apoA-I has a loosely folded conformation. Turbidity studies show that apoA-I is able to solubilize phospholipid in its native but not in its denatured form. Mixtures of apo-HDL (the total apoprotein of HDL) or apoA-I with dimyristoyl lecithin show a thermal transition at about 85 degrees not present in the lecithin or the apoprotein alone, which indicates that the native conformation of the apoprotein is stabilized by phospholipid. Scanning calorimetry of intact HDL shows a high-temperature endotherm associated with disruption of the HDL particle, suggesting that in HDL the conformation of apoA-I is also stabilized by interaction with lipid. The loosely folded conformation of native, uncomplexed apoA-I may be especially adapted to the binding of lipid, since this process may involve both hydrophobic sites on the surface of the protein and concealed apolar amino acid residues that are exposed by a cooperative, low energy unfolding process.