The N-terminal (1-44) and C-terminal (198-243) peptides of apolipoprotein A-I behave differently at the triolein/water interface

Structure and intermolecular interactions in spheroidal high-density lipoprotein subpopulations

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High-density lipoprotein subpopulations have unique surface profiles and dynamics.

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Relative hydrophobic surface area decreases with increasing lipoprotein size.

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Core lipid exposure at the lipoprotein surface decreases with increasing size.

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Cholesterol molecules localise near apolipoprotein A-I central helices.

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Lipid and protein interactions stabilise multifoil models of apolipoprotein A-I.

Human serum high-density lipoproteins (HDLs) are a population of small, dense protein-lipid aggregates that are crucial for intravascular lipid trafficking and are protective against cardiovascular disease. The spheroidal HDL subfraction can be separated by size and density into five major subpopulations with distinct molecular compositions and unique biological functionalities: HDL_3c, HDL_3b, HDL_3a, HDL_2a and HDL_2b. Representative molecular models of these five subpopulations were developed and characterised for the first time in the presence of multiple copies of its primary protein component apolipoprotein A-I (apoA-I) using coarse-grained molecular dynamics simulations. Each HDL model exhibited size, morphological and compositional profiles consistent with experimental observables. With increasing particle size the separation of core and surface molecules became progressively more defined, resulting in enhanced core lipid mixing, reduced core lipid exposure at the surface, and the formation of an interstitial region between core and surface molecules in HDL_2b. Cholesterol molecules tended to localise around the central helix-5 of apoA-I, whilst triglyceride molecules predominantly interacted with aromatic, hydrophobic residues located within the terminal helix-10 across all subpopulation models. The three intermediate HDL models exhibited similar surface profiles despite having distinct molecular compositions. ApoA-I in trefoil, quatrefoil and pentafoil arrangements across the surface of HDL particles exhibited significant warping and twisting, but largely retained intermolecular contacts between adjacent apoA-I chains. Representative HDL subpopulations differed in particle size, morphology, intermolecular interaction profiles and lipid and protein dynamics. These findings reveal how different HDL subpopulations might exhibit distinct functional associations depending on particle size, form and composition.

Aromatic residues in the C terminus of apolipoprotein C-III mediate lipid binding and LPL inhibition

Plasma apoC-III levels correlate with triglyceride (TG) levels and are a strong predictor of CVD outcomes. ApoC-III elevates TG in part by inhibiting LPL. ApoC-III likely inhibits LPL by competing for lipid binding. To probe this, we used oil-drop tensiometry to characterize binding of six apoC-III variants to lipid/water interfaces. This technique monitors the dependence of lipid binding on surface pressure, which increases during TG hydrolysis by LPL. ApoC-III adsorption increased surface pressure by upward of 18 mN/m at phospholipid/TG/water interfaces. ApoC-III was retained to high pressures at these interfaces, desorbing at 21-25 mN/m. Point mutants, which substituted alanine for aromatic residues, impaired the lipid binding of apoC-III. Adsorption and retention pressures decreased by 1-6 mN/m in point mutants, with the magnitude determined by the location of alanine substitutions. Trp42 was most critical to mediating lipid binding. These results strongly correlate with our previous results, linking apoC-III point mutants to increased LPL binding and activity at lipid surfaces. We propose that aromatic residues in the C-terminal half of apoC-III mediate binding to TG-rich lipoproteins. Increased apoC-III expression in the hypertriglyceridemic state allows apoC-III to accumulate on lipoproteins and inhibit LPL by preventing binding and/or access to substrate.

Lipid-free Apolipoprotein A-I Structure: Insights into HDL Formation and Atherosclerosis Development

Apolipoprotein A-I is the major protein in high-density lipoprotein (HDL) and plays an important role during the process of reverse cholesterol transport (RCT). Knowledge of the high-resolution structure of full-length apoA-I is vital for a molecular understanding of the function of HDL at the various steps of the RCT pathway. Due to the flexible nature of apoA-I and aggregation properties, the structure of full-length lipid-free apoA-I has evaded description for over three decades. Sequence analysis of apoA-I suggested that the amphipathic α-helix is the structural motif of exchangeable apolipoprotein, and NMR, X-ray and MD simulation studies have confirmed this. Different laboratories have used different methods to probe the secondary structure distribution and organization of both the lipid-free and lipid-bound apoA-I structure. Mutation analysis, synthetic peptide models, surface chemistry and crystal structures have converged on the lipid-free apoA-I domain structure and function: the N-terminal domain [1-184] forms a helix bundle while the C-terminal domain [185-243] mostly lacks defined structure and is responsible for initiating lipid-binding, aggregation and is also involved in cholesterol efflux. The first 43 residues of apoA-I are essential to stabilize the lipid-free structure. In addition, the crystal structure of C-terminally truncated apoA-I suggests a monomer-dimer conversation mechanism mediated through helix 5 reorganization and dimerization during the formation of HDL. Based on previous research, we have proposed a structural model for full-length monomeric apoA-I in solution and updated the HDL formation mechanism through three states. Mapping the known natural mutations on the full-length monomeric apoA-I model provides insight into atherosclerosis development through disruption of the N-terminal helix bundle or deletion of the C-terminal lipid-binding domain.

A Pressure-dependent Model for the Regulation of Lipoprotein Lipase by Apolipoprotein C-II

Apolipoprotein C-II (apoC-II) is the co-factor for lipoprotein lipase (LPL) at the surface of triacylglycerol-rich lipoproteins. LPL hydrolyzes triacylglycerol, which increases local surface pressure as surface area decreases and amphipathic products transiently accumulate at the lipoprotein surface. To understand how apoC-II adapts to these pressure changes, we characterized the behavior of apoC-II at multiple lipid/water interfaces. ApoC-II adsorption to a triacylglycerol/water interface resulted in large increases in surface pressure. ApoC-II was exchangeable at this interface and desorbed on interfacial compressions. These compressions increase surface pressure and mimic the action of LPL. Analysis of gradual compressions showed that apoC-II undergoes a two-step desorption, which indicates that lipid-bound apoC-II can exhibit at least two conformations. We characterized apoC-II at phospholipid/triacylglycerol/water interfaces, which more closely mimic lipoprotein surfaces. ApoC-II had a large exclusion pressure, similar to that of apoC-I and apoC-III. However, apoC-II desorbed at retention pressures higher than those seen with the other apoCs. This suggests that it is unlikely that apoC-I and apoC-III inhibit LPL via displacement of apoC-II from the lipoprotein surface. Upon rapid compressions and re-expansions, re-adsorption of apoC-II increased pressure by lower amounts than its initial adsorption. This indicates that apoC-II removed phospholipid from the interface upon desorption. These results suggest that apoC-II regulates the activity of LPL in a pressure-dependent manner. ApoC-II is provided as a component of triacylglycerol-rich lipoproteins and is the co-factor for LPL as pressure increases. Above its retention pressure, apoC-II desorbs and removes phospholipid. This triggers release of LPL from lipoproteins.

Surfactant-like properties of an amphiphilic α-helical peptide leading to lipid nanodisc formation

Nanodiscs are self-assembled discoidal nanoparticles composed of amphiphilic α-helical scaffold proteins or peptides that wrap themselves around the circumference of a lipid bilayer in a beltlike manner. In this study, an amphiphilic helical peptide that mimics helix 10 of human apoA-I was newly synthesized by solid phase peptide synthesis using Fmoc chemistry, and its physicochemical properties, including surface tension, self-association, and solubilization abilities, were evaluated and related directly to nanodisc formation. The synthesized peptide having hydrophobic and hydrophilic faces behaves like a general surfactant, affording a critical association concentration (CAC) of 2.7 × 10(-5) M and a γCAC of 51.2 mN m(-1) in aqueous solution. Interestingly, only a peptide solution above its CAC was able to microsolubilize L-α-dimyristoylphosphatidylcholine (DMPC) vesicles, and lipid nanodiscs with an average diameter of 9.5 ± 2.7 nm were observed by dynamic light scattering and negative stain transmission electron microscopy. Moreover, the ζ potentials of the lipid nanodiscs were measured for the first time as a function of pH, and the values changed from positive (20 mV) to negative (-30 mV). In particular, nanodisc solutions at acidic pH 4 (20 mV) or basic pH 9 (-20 mV) were found to be stable for more than 6 months as a result of the electrostatic repulsion between the particles.

Surface behavior of apolipoprotein A-I and its deletion mutants at model lipoprotein interfaces

Apolipoprotein A-I (apoA-I) has a great conformational flexibility to exist in lipid-free, lipid-poor, and lipid-bound states during lipid metabolism. To address the lipid binding and the dynamic desorption behavior of apoA-I at lipoprotein surfaces, apoA-I, Δ(185-243)apoA-I, and Δ(1-59)(185-243)apoA-I were studied at triolein/water and phosphatidylcholine/triolein/water interfaces with special attention to surface pressure. All three proteins are surface active to both interfaces lowering the interfacial tension and thus increasing the surface pressure to modify the interfaces. Δ(185-243)apoA-I adsorbs much more slowly and lowers the interfacial tension less than full-length apoA-I, confirming that the C-terminal domain (residues 185-243) initiates the lipid binding. Δ(1-59)(185-243)apoA-I binds more rapidly and lowers the interfacial tension more than Δ(185-243)apoA-I, suggesting that destabilizing the N-terminal α-helical bundle (residues 1-185) restores lipid binding. The three proteins desorb from both interfaces at different surface pressures revealing that different domains of apoA-I possess different lipid affinity. Δ(1-59)(185-243)apoA-I desorbs at lower pressures compared with apoA-I and Δ(185-243)apoA-I indicating that it is missing a strong lipid association motif. We propose that during lipoprotein remodeling, surface pressure mediates the adsorption and partial or full desorption of apoA-I allowing it to exchange among different lipoproteins and adopt various conformations to facilitate its multiple functions.

Changes in helical content or net charge of apolipoprotein C-I alter its affinity for lipid/water interfaces

Amphipathic α-helices mediate binding of exchangeable apolipoproteins to lipoproteins. To probe the role of α-helical structure in protein-lipid interactions, we used oil-drop tensiometry to characterize the interfacial behavior of apolipoprotein C-I (apoC-I) variants at triolein/water (TO/W) and 1-palmitoyl-2-oleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. ApoC-I, the smallest apolipoprotein, has two amphipathic α-helices. Mutants had single Pro or Ala substitutions that resulted in large differences in helical content in solution and on phospholipids. The ability of apoC-I to bind TO/W and POPC/TO/W interfaces correlated strongly with α-helical propensity. On binding these interfaces, peptides with higher helical propensity increased surface pressure to a greater extent. Likewise, peptide exclusion pressure at POPC/TO/W interfaces increased with greater helical propensity. ApoC-I retention on TO/W and POPC/TO/W interfaces correlated strongly with phospholipid-bound helical content. On compression of these interfaces, peptides with higher helical content were ejected at higher pressures. Substitution of Arg for Pro in the N-terminal α-helix altered net charge and reduced apoC-I affinity for POPC/TO/W interfaces. Our results suggest that peptide-lipid interactions drive α-helix binding to and retention on lipoproteins. Point mutations in small apolipoproteins could significantly change α-helical propensity or charge, thereby disrupting protein-lipid interactions and preventing the proteins from regulating lipoprotein catabolism at high surface pressures.

Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic α-helices

Amphipathic α-helices (AαH) are the primary structural motif of exchangeable apolipoproteins. AαHs in exchangeable apolipoproteins adsorb, remodel, and desorb at the surface of plasma lipoproteins in response to changes in their size or composition. A triolein/water (TO/W) interface was used as a model surface to study adsorption and desorption of AαHs at a lipoprotein-like interface. We previously reported that AαH peptides spontaneously adsorb to a TO/W interface, but they only partially desorb from the surface when the excess peptide was removed from the system. This finding suggests that "exchangeable" apolipoproteins are in fact partially exchangeable and only desorb from a surface in response to compression or change in composition. Here, we develop a thermodynamic and kinetic model to describe this phenomenon based on the change in the interfacial pressure (Π) of the C-terminal 46 amino acids of apolipoprotein A-I (C46) at a TO/W interface. This model suggests that apolipoproteins have at least two interfacial conformations that are in a surface concentration and Π-dependent equilibrium. This two-state surface equilibrium model, which is based on experimental data and is consistent with dynamic changes in Π(t), provides insights into the selective metabolism and clearance of plasma lipoproteins and the process of lipoprotein remodeling.

Apolipoprotein C-I binds more strongly to phospholipid/triolein/water than triolein/water interfaces: a possible model for inhibiting cholesterol ester transfer protein activity and triacylglycerol-rich lipoprotein uptake

Apolipoprotein C-I (apoC-I) is an important constituent of high-density lipoprotein (HDL) and is involved in the accumulation of cholesterol ester in nascent HDL via inhibition of cholesterol ester transfer protein and potential activation of lecithin:cholesterol acyltransferase (LCAT). As the smallest exchangeable apolipoprotein (57 residues), apoC-I transfers between lipoproteins via a lipid-binding motif of two amphipathic α-helices (AαHs), spanning residues 7-29 and 38-52. To understand apoC-I's behavior at hydrophobic lipoprotein surfaces, oil drop tensiometry was used to compare the binding to triolein/water (TO/W) and palmitoyloleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. When apoC-I binds to either interface, the surface tension (γ) decreases by ~16-18 mN/m. ApoC-I can be exchanged at both interfaces, desorbing upon compression and readsorbing on expansion. The maximal surface pressures at which apoC-I begins to desorb (Π(max)) were 16.8 and 20.7 mN/m at TO/W and POPC/TO/W interfaces, respectively. This suggests that apoC-I interacts with POPC to increase its affinity for the interface. ApoC-I is more elastic on POPC/TO/W than TO/W interfaces, marked by higher values of the elasticity modulus (ε) on oscillations. At POPC/TO/W interfaces containing an increasing POPC:TO ratio, the pressure at which apoC-I begins to be ejected increases as the phospholipid surface concentration increases. The observed increase in apoC-I interface affinity due to higher degrees of apoC-I-POPC interactions may explain how apoC-I can displace larger apolipoproteins, such as apoE, from lipoproteins. These interactions allow apoC-I to remain bound to the interface at higher Π values, offering insight into apoC-I's rearrangement on triacylglycerol-rich lipoproteins as they undergo Π changes during lipoprotein maturation by plasma factors such as lipoprotein lipase.

C-terminus of apolipoprotein A-I removes phospholipids from a triolein/phospholipids/water interface, but the N-terminus does not: a possible mechanism for nascent HDL assembly

Apolipoprotein A-I (ApoA-I) is the principle protein component of HDL, also known as "good cholesterol," which is an inverse marker for cardiovascular disease. The N-terminal 44 amino acids of ApoA-I (N44) are predicted to be responsible for stabilization of soluble ApoA-I, whereas the C-terminal 46 amino acids (C46) are predicted to initiate lipid binding and oligomerization. In this work, we apply what we believe to be a novel application of drop tensiometry to study the adsorption and desorption of N44 and C46 at a triolein/POPC/water (TO/POPC/W) interface. The amount of peptide that adsorbed to the surface was dependent on the surface concentration of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and pressure (Π) before adsorption. At a TO/POPC/W interface, the exclusion pressure (Π(EX)) of C46 was 25.8 mN/m, and was 19.3 mN/m for N44. Once adsorbed, both peptides formed a homogeneous surface with POPC but were progressively ejected from the surface by compression. During a compression, C46 removed POPC from the surface whereas N44 did not. Repeated compressions caused C46 to deplete entirely the surface of phospholipid. If full-length ApoA-I could also remove phospholipid, this could provide a mechanism for the transfer of surface components of chylomicrons and very low density lipoprotein to high density lipoprotein with the assistance of phospholipid transfer protein.

Interfacial properties of apolipoprotein B292-593 (B6.4-13) and B611-782 (B13-17). Insights into the structure of the lipovitellin homology region in apolipoprotein B

The N-terminal sequence of apolipoprotein B (apoB) is critical in triacylglycerol-rich lipoprotein assembly. The first 17% of apoB (B17) is thought to consist of three domains: B5.9, a beta-barrel, B6.4-13, a series of 17 alpha-helices, and B13-17, a putative beta-sheet. B5.9 does not bind to lipid, while B6.4-13 and B13-17 contain hydrophobic interfaces that can interact with lipids. To understand how B6.4-13 and B13-17 might interact with triacylglycerol during lipoprotein assembly, the interfacial properties of both peptides were studied at the triolein/water interface. Both B6.4-13 and B13-17 are surface active. Once bound, the peptides can be neither exchanged nor pushed off the interface. Some residues of the peptides can be ejected from the interface upon compression but readsorb on expansion. B13-17 binds to the interface more strongly. The maximum pressure the peptide can withstand without being partially ejected (Pi(max)) is 19.2 mN/m for B13-17 compared to 16.7 mN/m for B6.4-13. B13-17 is purely elastic at the interface, while B6.4-13 forms a viscous-elastic film. When they are spread at an air/water interface, the limiting area and the collapse pressures are 16.6 A(2)/amino acid and 31 mN/m for B6.4-13 and 17.8 A(2)/amino acid and 35 mN/m for B13-17, respectively. The alpha-helical B6.4-13 contains some hydrophobic helices that stay bound and prevent the peptide from leaving the surface. The beta-sheets of B13-17 bind irreversibly to the surface. We suggest that during lipoprotein assembly, the N-terminal apoB starts recruiting lipid as early as B6.4, but additional sequences are essential for formation of a lipid pocket that can stabilize lipoprotein emulsion particles for secretion.

Interaction between the N- and C-terminal domains modulates the stability and lipid binding of apolipoprotein A-I

The tertiary structures of human and mouse apolipoprotein A-I (apoA-I) are comprised of an N-terminal helix bundle and a separate C-terminal domain. To define the possible intramolecular interaction between the N- and the C-terminal domains, we examined the effects on protein stability and lipid-binding properties of exchanging either the C-terminal domain or helix between human and mouse apoA-I. Chemical denaturation experiments demonstrated that replacement of the C-terminal domain or helical segment in human apoA-I with the mouse counterparts largely destabilizes the N-terminal helix bundle. Removal of the C-terminal domain or alpha-helix in human apoA-I had a similar effect on the destabilization of the helix bundle against urea denaturation, indicating that the C-terminal helical segment mainly contributes to stabilizing the N-terminal helix bundle structure in the apoA-I molecule. Consistent with this, KI quenching experiments indicated that removal or replacement of the C-terminal domain or helix in human apoA-I causes Trp residues in the N-terminal domain to become exposed to solvent. Measurements of the heats of binding to egg phosphatidylcholine (PC) vesicles and the kinetics of solubilization of dimyristoyl PC vesicles demonstrated that the destabilized human N-terminal helix bundle can strongly interact with lipids without the hydrophobic C-terminal helix. In addition, site-specific labeling of the N- and C-terminal helices by acrylodan to probe the conformational stability and the spatial proximity of the two domains indicated that the C-terminal helix is located near the N-terminal helix bundle, leading to a relatively less solvent-exposed, more organized conformation of the C-terminal domain. Taken together, these results suggest that interaction between the N- and C-terminal tertiary structure domains in apoA-I modulates the stability and lipid-binding properties of the N-terminal helix bundle.

The adsorption of biological peptides and proteins at the oil/water interface. A potentially important but largely unexplored field

This review focuses on some new techniques to study the behavior of peptides and proteins bound to oil droplets. We will show how model peptides e.g., amphipathic alpha helices (AalphaH) and amphipathic beta strand (AbetaS) and some apolipoproteins adsorb to triacylglycerol (TAG) droplets and how they behave once adsorbed to the interface. While most of the studies described involve peptides and proteins at an oil/water interface, studies can also be carried out when the surface has been partially covered with phospholipids. This work is important because it examines biophysical changes that take place at lipid droplet interfaces and how this may relate to the metabolism of lipoproteins and lipid droplets.

Surface study of apoB1694-1880, a sequence that can anchor apoB to lipoproteins and make it nonexchangeable

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. It remains bound to secreted lipoproteins during lipid metabolism in plasma. The beta1 region (residues 827-1880) of apoB has a high amphipathic beta strand (AbetaS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbetaS-rich region between apoB37 and apoB41 (residues 1694-1880) was cloned, expressed, and purified. The interfacial properties were studied at the triolein/water (TO/W) and air/water (A/W) interfaces. ApoB[37-41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB[37-41] was removed from the aqueous phase. Adsorbed apoB[37-41] did not desorb and could not be forced off by increasing the surface pressure up to 23 mN/m. ApoB[37-41] adsorbed on the TO/W interface was completely elastic when compressed and expanded by +/-13% of its area. On an A/W interface, the apoB[37-41] monolayer became solid when compressed to 4 mN/m pressure indicating extended beta-sheet formation. It could be reversibly compressed and expanded between low pressure and its collapse pressure (35 mN/m). Our studies confirm that the AbetaS structure of apoB[37-41] is a lipid-binding motif that can irreversibly anchor apoB to lipoproteins.

Interfacial properties of a complex multi-domain 490 amino acid peptide derived from apolipoprotein B (residues 292-782)

ApolipoproteinB (ApoB) is a lipid binding protein that is a nonexchangeable component of chylomicrons, VLDL, and LDL. In the liver and intestinal cells ApoB recruits lipid to form nascent triacylglycerol rich particles cotranslationally in the endoplasmic reticulum membrane which are then processed and secreted to form plasma lipoproteins. The N-terminal domain, which comprises the first 22% of apoB, recruits lipid in a controlled manner. The first 6% (residues 1-291) of the N-terminus does not bind lipid. The first lipid binding domain, including residues 292-782 (B6-17), forms a lipid binding pocket which is predicted to consist of 17 alpha-helices and 6 beta-strands. A structural model based on the X-ray structure of the homologues protein lipovitellin suggests that the N-terminal 6-8 helices and the beta-sheet interact with lipid while the C-terminal helices form a structural unit stabilizing the beta-sheet. Using isothermal drop tensiometry we showed that ApoB6.4-17 is surface active and binds to a triolein/water interface and exerts 16-19 mN/m of pressure (Pi) on that surface. The protein initially adsorbs slowly from aqueous solution to the surface but following compression and re-expansion it reaches equilibrium much faster. When Pi exceeds 16.9 mN/m part of the protein is ejected from the surface, but when compressed to high Pi the protein is never completely ejected indicating that part of the peptide is irreversibly anchored to the interface. The surface dilation modulus (epsilon) varies between 25-38 mN/m, and is predominantly elastic with a small viscous component. When compressed at an air/water interface ApoB6.4-17 has a limiting area of approximately 11 A2 per amino acid at lift off and only approximately 7 A2 per amino acid at the collapse Pi (28 mN/m). These values are about half the anticipated values if all the residues are at the surface. This suggests that ApoB6.4-17 retains some globular structure at an interface and does not completely denature at the surface, as many other globular proteins do. We suggest that while bound to the surface ApoB6.4-17 exhibits properties of both alpha and beta structure giving it unique and versatile characteristics at a hydrophobic interface.

Conformational flexibility of the N-terminal domain of apolipoprotein a-I bound to spherical lipid particles

Lipid binding of human apolipoprotein A-I (apoA-I) occurs initially through the C-terminal alpha-helices followed by conformational reorganization of the N-terminal helix bundle. This led us to hypothesize that apoA-I has multiple lipid-bound conformations, in which the N-terminal helix bundle adopts either open or closed conformations anchored by the C-terminal domain. To investigate such possible conformations of apoA-I at the surface of a spherical lipid particle, site-specific labeling of the N- and C-terminal helices in apoA-I by N-(1-pyrene)maleimide was employed after substitution of a Cys residue for Val-53 or Phe-229. Neither mutagenesis nor the pyrene labeling caused discernible changes in the lipid-free structure and lipid interaction of apoA-I. Taking advantage of a significant increase in fluorescence when a pyrene-labeled helix is in contact with the lipid surface, we monitored the behaviors of the N- and C-terminal helices upon binding of apoA-I to egg PC small unilamellar vesicles. Comparison of the binding isotherms for pyrene-labeled apoA-I as well as a C-terminal helical peptide suggests that an increase in surface concentration of apoA-I causes dissociation of the N-terminal helix from the surface leaving the C-terminal helix attached. Consistent with this, isothermal titration calorimetry measurements showed that the enthalpy of apoA-I binding to the lipid surface under near saturated conditions is much less exothermic than that for binding at a low surface concentration, indicating the N-terminal helix bundle is out of contact with lipid at high apoA-I surface concentrations. Interestingly, the presence of cholesterol significantly induces the open conformation of the helix bundle. These results provide insight into the multiple lipid-bound conformations that the N-terminal helix bundle of apoA-I can adopt on a lipid or lipoprotein particle, depending upon the availability of space on the surface and the surface composition.