Comparison of apoA-I helical structure and stability in discoidal and spherical HDL particles by HX and mass spectrometry

Palaniappan Sevugan Chetty,David Nguyen,Margaret Nickel,Sissel Lund-Katz,Leland Mayne,S.Walter Englander,Michael C Phillips

doi:10.1194/jlr.m034785

Palaniappan Sevugan Chetty, David Nguyen + Show 5 more

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https://doi.org/10.1194/jlr.m034785

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Abstract

Elucidation of apoA-I secondary structure in spherical plasma HDL particles is essential for understanding HDL structure and function at the molecular level. To provide this information, we have applied hydrogen exchange (HX) and mass spectrometry methods to compare apoA-I secondary structure in discoidal (two apoA-I molecules/particle) and spherical (five apoA-I molecules/particle) HDL particles. The HX kinetics indicate that the locations of helical segments within the apoA-I molecules are the same in both discoidal and spherical HDL particles (approximately 10 nm hydrodynamic diameter). Helix stabilities in both types of particles are 3–5 kcal/mol, consistent with the apoA-I molecules being in a highly dynamic state with helical segments unfolding and refolding in seconds. For the spherical HDL, apoA-I fragments corresponding to residues 115–158 exhibit bimodal HX kinetics consistent with this segment adopting an inter-converting (on the timescale of tens of minutes) helix-loop configuration. The segment adopting this configuration in the 10 nm disc is shorter because the surface area available to each apoA-I molecule is apparently larger. Loop formation in the central region of the apoA-I molecule contributes to the ability of the protein to adapt to changes in available space on the HDL particle surface. Overall, apoA-I secondary structure is largely unaffected by a change in HDL particle shape from disc to sphere.

Highlights

Elucidation of apoA-I secondary structure in spherical plasma high density lipoproteins (HDLs) particles is essential for understanding HDL structure and function at the molecular level
Chemical cross-linking studies have shown that the intermolecular contacts characteristic of the double-belt arrangement are maintained in spherical HDL particles of different sizes [8, 9]
The results show that the apoA-I intramolecular helix locations and stabilities are largely retained upon HDL transformation from a disc to a sphere of the same size

Summary

Introduction

Elucidation of apoA-I secondary structure in spherical plasma HDL particles is essential for understanding HDL structure and function at the molecular level. To better understand the structure of apoA-I and to resolve the above issues, we have compared in detail the secondary structure of apoA-I on spherical and discoidal HDL particles This task involved extending our previous HX-MS studies of apoA-I helix structure and stability in the lipidfree state [16, 17] and in discs of different sizes [7] to plasma HDL containing only apoA-I (LpA-I). These spherical particles which contain apoA-I as essentially their only protein constituent [9] comprise ‫ف‬25% of plasma HDL. The exception is that the number of amino acids that form a loop segment located near the center of the molecule is greater in the spherical LpA-I particle, apparently the result of an increase in apoA-I surface packing density

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Conclusion