Effects of the core lipid on the energetics of binding of ApoA-I to model lipoprotein particles of different sizes.
Interaction of apolipoproteins (apo) with lipid surfaces plays crucial roles in lipoprotein metabolism and cholesterol homeostasis. To elucidate the thermodynamics of binding of apoA-I to lipid, we used lipid emulsions composed of triolein (TO) and egg phosphatidylcholine (PC) as lipoprotein models. Determination of the level of binding of wild-type (WT) apoA-I and some deletion mutants to large (120 nm diameter; LEM) and small (35 nm diameter; SEM) emulsions indicated that N-terminal (residues 44-65) and C-terminal (residues 190-243 and 223-243) deletions have large effects on lipid interaction, whereas deletion of the central region (residues 123-166) has little effect. Substitution of amino acids at either L230 or L230, L233, and Y236 with proline residues also decreases the level of binding, indicating that an alpha-helix conformation in this C-terminal region is required for efficient lipid binding. Calorimetry showed that binding of WT apoA-I to SEM generates endothermic heat (DeltaH approximately 30 kcal/mol) in contrast to the exothermic heat (ca. -85 kcal/mol) generated upon binding to LEM and egg PC small unilamellar vesicles (SUV). This exothermic heat arises from an approximately 25% increase in alpha-helix content, and it drives the binding of apoA-I to LEM and SUV. There is a similar increase in alpha-helix content of apoA-I upon binding to either SEM or SUV, but the binding of apoA-I to SEM is an entropy-driven process. These results suggest that the presence of a core triglyceride modifies the highly curved SEM surface packing and thereby the thermodynamics of apoA-I binding in a manner that compensates for the exothermic heat generated by alpha-helix formation.
Tanaka
Masafumi
M
The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-4318, USA.
Saito
Hiroyuki
H
Dhanasekaran
Padmaja
P
Wehrli
Suzanne
S
Handa
Tetsurou
T
Lund-Katz
Sissel
S
Phillips
Michael C
MC
D016632:Apolipoprotein A-I / Q000737:chemistry / Q000378:metabolism*; D002151:Calorimetry; D002247:Carbon Isotopes; D002942:Circular Dichroism; D004655:Emulsions; D006801:Humans; D008051:Lipid Bilayers / Q000737:chemistry* / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D010316:Particle Size; D010713:Phosphatidylcholines / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013499:Surface Properties; D013816:Thermodynamics; D014304:Triolein / Q000378:metabolism
D016632:Apolipoprotein A-I; D002247:Carbon Isotopes; D004655:Emulsions; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D014304:Triolein