Interfacial Properties of an Amphipathic α-Helix Consensus Peptide of Exchangeable Apolipoproteins at Air/Water and Oil/Water Interfaces

Abstract

Amphipathic ␣-helices are the main structure and the major lipid binding motif of exchangeable apolipoproteins. To understand how these apolipoproteins behave at an hydrophobic lipoprotein interface, the interfacial properties of a consensus sequence peptide (CSP) derived from three exchangeable apolipoproteins (A-I, A-IV, and E) were studied using an oil drop tensiometer at air/water (A/W) and dodecane/water (DD/W) interfaces. CSP ((PLAEELRARLRAQLEELRERLG)2-NH 2 ) contains two 22-amino acid tandem repeat sequences that form amphipathic ␣-helices. CSP, when added into the aqueous phase, lowered the interfacial tension (␥) of A/W and DD/W in a concentration-dependant fashion. The ␥ A/W was lowered ϳ24 mN/m, and ␥ DD/W ϳ31 mN/m, indicating a greater affinity of CSP for DD/W. Using the Gibbs equation for surface, the surface area per CSP molecule was estimated at ϳ702 Å 2 (ϳ16 Å 2 /amino acid) on A/W and ϳ622 Å 2 on DD/W (ϳ14 Å 2 /amino acid) suggesting that adsorbed CSP lies flat with ␣-helices in the plane of both interfaces. At equilibrium ␥, CSP desorbed from the interface when compressed and re-adsorbed when expanded. The adsorption rate was concentrationdependant, but the desorption rate was not. Less CSP desorbed from DD/W than A/W indicating that CSP has higher affinity for DD/W. Dynamic analysis of elasticity shows that the faster the oscillation period (4, 8 s) and the lower the oscillation amplitude the more elastic the surfaces. CSP can be compressed 6 -12% while remaining on the surface, but large increases in pressure eject it from the surface. We suggest that surface pressuremediated desorption and readsorption of amphipathic ␣-helices provide lipoprotein stability during remodeling reactions in plasma. High density lipoproteins (HDLs)1 are small aggregates of exchangeable apolipoproteins (apoA-I, apoA-II, apoC's, and apoE, etc.) and lipids that reside in the plasma and are important mediators in reverse cholesterol transport. They also play roles as cofactors for a number of plasma lipoprotein modifying enzymes (e.g. lechthin:cholesterol acetyltransferase, lipoprotein lipase). For many years it has been known (1) that apolipoproteins can exchange off of HDL onto other lipoproteins like very low density lipoprotein and move back and forth between different lipoprotein classes. Thus they are located at the lipoprotein/water interface. The predominant secondary structural feature of exchangeable apolipoproteins is the amphipathic ␣-helix (2), which has been described and studied by a number of investigators (1-3). These structural units are about 20 amino acids in length, fold into an ␣-helix, and present one face, sub-tending less than 180°, as a band of hydrophobic amino acids. It is this hydrophobic face along the amphipathic ␣-helix that allows it to bind to the lipoprotein surfaces. A variety of different kinds of amphipathic ␣-helices have been described (4), but the specific function of the different classes is not clear. The class A amphipathic helix (3, 4) is a major lipid binding motif of the...

Apolipoproteins C-I and C-III Inhibit Lipoprotein Lipase Activity by Displacement of the Enzyme from Lipid Droplets

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Role of Secondary Structure in Protein−Phospholipid Surface Interactions: Reconstitution and Denaturation of Apolipoprotein C-I:DMPC Complexes

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