Thermodynamic and kinetic stability of discoidal high-density lipoprotein formation from phosphatidylcholine/apolipoprotein A-I mixture.
ABSTRACT Nascent high-density lipoproteins (HDLs), which are also known as discoidal HDLs, are formed by the interaction of apolipoprotein A-I (apoA-I) with transmembrane ATP-binding cassette transporter A1 (ABCA1). However, the molecular mechanism governing disc formation is not fully understood. Here, we evaluated the thermodynamic and kinetic stability of disc formation from mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine and apoA-I by quantifying the discs and vesicles produced. Sodium cholate dialysis experiments revealed that the discs are thermodynamically more stable than the vesicle/apoA-I mixture (Delta*G = -52 kJ/disc mol at 37.0 degrees C) because the decrease in enthalpy (Delta*H = -620 kJ/disc mol) exceeds the decrease in entropy (TDelta*S = -570 kJ/disc mol). Circular dichroism spectral measurements ascribed 68% of the decrease in enthalpy during disc formation to the formation of helices in apoA-I. Fluorescence measurements suggested that phospholipids enclosed in the discs are more closely packed than those in the vesicles so that they are entropically destabilized. To determine if the disc could be spontaneously produced from vesicles, we measured the decrease in the turbidity of vesicles in response to the addition of apoA-I. However, the rate of disc formation was very slow, suggesting that the large kinetic barrier against disc formation makes the vesicle/apoA-I mixtures metastable. These results raise the possibility that ABCA1 may act to lower the activation energy, thereby facilitating disc formation.
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ABSTRACT: One of biology's most pervasive nanostructures, the phospholipid membrane, represents an ideal scaffold for a host of nanotechnology applications. Whether engineering biomimetic technologies or designing therapies to interface with the cell, this adaptable membrane can provide the necessary molecular-level control of membrane-anchored proteins, glycopeptides, and glycolipids. If appropriately prepared, these components can replicate in vitro or influence in vivo essential living processes such as signal transduction, mass transport, and chemical or energy conversion. To satisfy these requirements, a lipid-based, synthetic nanoscale architecture with molecular-level tunability is needed. In this regard, discrete lipid particles, including reconstituted high density lipoprotein (HDL), have emerged as a versatile and elegant solution. Structurally diverse, native biological HDLs exist as discoidal lipid bilayers of 5-8 nm diameter and lipid monolayer-coated spheres 10-15 nm in diameter, all belted by a robust scaffolding protein. These supramolecular assemblies can be reconstituted using simple self-assembly methods to incorporate a broad range of amphipathic molecular constituents, natural or artificial, and provide a generic platform for stabilization and transport of amphipathic and hydrophobic elements capable of docking with targets at biological or inorganic surfaces. In conjunction with top-down or bottom-up engineering approaches, synthetic HDL can be designed, arrayed, and manipulated for a host of applications including biochemical analyses and fundamental studies of molecular structure. Also highly biocompatible, these assemblies are suitable for medical diagnostics and therapeutics. The collection of efforts reviewed here focuses on laboratory methods by which synthetic HDLs are produced, the advantages conferred by their nanoscopic dimension, and current and emerging applications.ACS Nano 01/2011; 5(1):42-57. · 12.03 Impact Factor
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ABSTRACT: Discoidal high-density lipoprotein (HDL) particles are known to fractionalize into several discrete populations. Factors regulating their size are, however, less understood. To reveal the effect of lipid composition on their formation and characteristics, we prepared several reconstituted HDLs (rHDLs) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and sphingomyelin at phospholipid to apolipoprotein A-I ratios of 100 and 25. When reconstitution was conducted at 37°C, the efficiency of rHDL formation from POPC was decreased as compared with that conducted at 4°C. Moreover, large rHDLs with a Stokes diameter of 9.6nm became dominant over small rHDL with a diameter of 7.9nm, which was distinctly observed at 4°C. The aminophospholipids POPS and POPE promoted the formation of small rHDLs at 37°C, but fluorescence experiments revealed that they did so in a different fashion: Fluorescence lifetime data suggested that the head group of POPS reduces hydrophobic hydration, especially in small rHDLs, suggesting that this lipid stabilizes the saddle-shaped bilayer structure in small rHDLs. Fluorescence lifetime and anisotropy data showed that incorporation of POPE increases acyl chain order and water penetration into the head group region in large rHDLs, suggesting that POPE destabilizes the planar bilayer structure. These results imply that these aminophospholipids contribute to the formation of small rHDLs under biological conditions.Biochimica et Biophysica Acta 01/2013; · 4.66 Impact Factor