Thermal unfolding of human high-density apolipoprotein A-1: implications for a lipid-free molten globular state.

Gursky, Olga; Atkinson, David

doi:10.1073/pnas.93.7.2991

Cited by 181 publications

(241 citation statements)

References 33 publications

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“…Thermal denaturation curves indicated that MetO-apoA-I began to denature at much lower temperatures than native apoA-I and that both samples were completely denatured above 90°C. Fits of these data to a two-state unfolding model yielded melting temperatures (T m ) of 58.7 AE 0.2°C for native apoA-I, consistent with previously published data (25), and 48.2 AE 0.4°C for MetO-apoA-I. The reduction in T m of MetO-apoA-I by around 10.5°C, with respect to that of native apoA-I, indicates that the structural stability of the oxidized protein is significantly impaired.…”

Section: Meto-apoa-i Is Partially Unfolded and Has Lower Thermal Stabsupporting

confidence: 70%

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

Wong

Binger²,

Howlett³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

104

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Apolipoprotein A-I (apoA-I) is the major protein component of HDL, where it plays an important role in cholesterol transport. The deposition of apoA-I derived amyloid is associated with various hereditary systemic amyloidoses and atherosclerosis; however, very little is known about the mechanism of apoA-I amyloid formation. Methionine residues in apoA-I are oxidized via several mechanisms in vivo to form methionine sulfoxide (MetO), and significant levels of methionine oxidized apoA-I (MetO-apoA-I) are present in normal human serum. We investigated the effect of methionine oxidation on the structure, stability, and aggregation of full-length, lipid-free apoA-I. Circular dichrosim spectroscopy showed that oxidation of all three methionine residues in apoA-I caused partial unfolding of the protein and decreased its thermal stability, reducing the melting temperature (T m ) from 58.7°C for native apoA-I to 48.2°C for MetO-apoA-I. Analytical ultracentrifugation revealed that methionine oxidation inhibited the native self association of apoA-I to form dimers and tetramers. Incubation of MetO-apoA-I for extended periods resulted in aggregation of the protein, and these aggregates bound Thioflavin T and Congo Red. Inspection of the aggregates by electron microscopy revealed fibrillar structures with a ribbon-like morphology, widths of approximately 11 nm, and lengths of up to several microns. X-ray fibre diffraction studies of the fibrils revealed a diffraction pattern with orthogonal peaks at spacings of 4.64 Å and 9.92 Å, indicating a cross-β amyloid structure. This systematic study of fibril formation by full-length apoA-I represents the first demonstration that methionine oxidation can induce amyloid fibril formation.aggregation | amyloidosis | atherosclerosis | protein misfolding

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Section: Meto-apoa-i Is Partially Unfolded and Has Lower Thermal Stabsupporting

confidence: 70%

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

Wong

Binger²,

Howlett³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

104

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“…2 , HX experiments indicate that the individual helices in apoA-I exist transiently, opening and closing on a timescale of seconds. The above characteristics are consistent with the helix bundle existing in a molten globular state, as has been inferred from studies of the thermal denaturation of apoA-I ( 46 ). Thus, the apoA-I N-terminal domain ( Fig.…”

Section: Tertiary Structuresupporting

confidence: 67%

New insights into the determination of HDL structure by apolipoproteins

Phillips

2013

Journal of Lipid Research

149

106

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This article is available online at http://www.jlr.orgThe purpose of this review is to summarize current understanding of how apolipoproteins (apoA-I in particular) determine the structures of HDL particles. The focus is on recent advances in the fi eld, so the coverage of the literature on HDL structure is not comprehensive. Knowledge of HDL structure at the molecular level is critical for understanding how this lipoprotein achieves the multiple functions elaborated in other reviews in this thematic series.In human plasma, HDL is a heterogeneous collection of particles ranging 7-12 nm in diameter and 1.063-1.21 g/ml in density ( 1-4 ). The predominant species of HDL are spherical microemulsion particles, in which a core of neutral cholesteryl ester (CE) and triacylglycerol (TG) is encapsulated by a monolayer of phospholipid (PL), unesterifi ed (free) cholesterol (FC), and protein ( 5 ). The protein and PL constituents comprise approximately 50 and 25%, respectively, of the mass of such particles, with the CE, FC, and TG components making up the remainder. Larger, less dense HDL particles have a higher lipidto-protein mass ratio. Approximately 70% of total plasma HDL protein is apoA-I (which is present in normolipidemic human plasma at ‫ف‬ 130 mg/dl), and it is located in essentially every HDL particle. The second most abundant protein is apoA-II, which comprises 15-20% of total plasma HDL protein, but this component is not present in all HDL particles. In human plasma, about 25% of apoA-I is present in HDL particles containing only apoA-I (LpA-I); the remaining HDL particles contain both apoA-I and apoA-II (LpA-I+A-II), typically in a molar ratio of 1-2/1 ( 6 ). ApoA-I and apoA-II are the "scaffold" proteins that primarily determine HDL particle structure. Other members of the exchangeable apolipoprotein gene family that are Abstract Apolipoprotein (apo)A-I is the principal protein component of HDL, and because of its conformational adaptability, it can stabilize all HDL subclasses. The amphipathic ␣ -helix is the structural motif that enables apoA-I to achieve this functionality. In the lipid-free state, the helical segments unfold and refold in seconds and are located in the N-terminal two thirds of the molecule where they are loosely packed as a dynamic, four-helix bundle. The C-terminal third of the protein forms an intrinsically disordered domain that mediates initial binding to phospholipid surfaces, which occurs with coupled ␣ -helix formation. The lipid affi nity of apoA-I confers detergent-like properties; it can solubilize vesicular phospholipids to create discoidal HDL particles with diameters of approximately 10 nm. Such particles contain a segment of phospholipid bilayer and are stabilized by two apoA-I molecules that are arranged in an anti-parallel, double-belt conformation around the edge of the disc, shielding the hydrophobic phospholipid acyl chains from exposure to water. The apoA-I molecules are in a highly dynamic state, and they stabilize discoidal particles of different sizes by certain s...

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“…Our present study focuses on the lipid association of apoA-I with spherical lipoprotein particles and the conformational changes involved in the adsorption and pressure changes are likely to occur during all these remodeling and reactions. ApoA-I is a very fl exible protein in that it adopts a structure similar to the molten globular state ( 68 ). The multiple A ␣ Hs in apoA-I exhibit stabilization free energies in the range of 3-5 kcal/mol indicating high fl exibility and dynamic folding and refolding in seconds ( 37,66 ).…”

Section: Discussionmentioning

confidence: 99%

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

Wang

Mei

Atkinson

et al. 2014

Journal of Lipid Research

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This article is available online at http://www.jlr.org properties of apoA-I arise primarily through its important roles in the pathway of reverse cholesterol transport; where apoA-I stabilizes and maintains the structure of HDL particles, promotes cellular cholesterol effl ux by binding to specifi c ATP binding cassette transporters, interacts with the enzyme lecithin:cholesterol acyltransferase (LCAT) to drive the maturation of HDL particles, binds and modifi es the lipoprotein surface to facilitate enzyme reactions, and interacts with the scavenger receptor class B type 1 for selective cholesterol ester (CE) uptake by the liver for excretion ( 2-4 ).ApoA-I exists in lipid-free, lipid-poor, and lipid-bound states in plasma, and exchanges among HDL and TAGrich lipoprotein particles like very low density lipoproteins (VLDLs) and chylomicrons (CMs) ( 5 ). ApoA-I interacts primarily with the hydrocarbon chains of the phospholipids (PL) on discoidal and spherical HDLs ( 6-8 ) and may also interact with the hydrophobic CE core of spherical HDL and the hydrophobic TAG core of TAG-rich lipoproteins. The conformational fl exibility of apoA-I allows it to adopt a variety of conformations involving lipid adsorption, partial or full desorption, and fl exible unfolding and refolding in diverse physical environments to facilitate its multiple functions. Studies of the molecular mechanisms of the lipid association and the conformational fl exibility of apoA-I are essential for understanding the structure-function relationships.ApoA-I has an exon 3 encoded region (residues 1-43), and an exon 4 encoded region (residues 44-243) that contains 10 11/22-mer tandem repeat amino acid segments that are predicted to form class A and class Y amphipathic ␣ -helices (A ␣ Hs) ( 9, 10 ). These A ␣ Hs (helix 1-10) are the lipid binding motif of apoA-I and the structural basis for its multiple functions. Segment deletion and point mutation studies have elucidated the possible conformation and functions for each helical segment (11)(12)(13)(14)(15)(16) Apolipoprotein A-I (apoA-I) is a major protein of high density lipoproteins (HDLs) and is also present on large triacylglycerol (TAG)-rich lipoproteins. Reduced plasma levels of HDL and apoA-I are the key risk factors for atherosclerosis and cardiovascular disease ( 1 ). The anti-atherogenic

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Thermal unfolding of human high-density apolipoprotein A-1: implications for a lipid-free molten globular state.

Cited by 181 publications

References 33 publications

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

New insights into the determination of HDL structure by apolipoproteins

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

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