Abstract
Class A amphipathic helixes present in exchangeable plasma apolipoproteins are characterized by the location of positively charged amino acid residues at the non-polar-polar interface and negatively charged amino acid residues at the center of the polar face. The objectives of the present study were: (i) to investigate the role of hydrocarbon side chain length of the interfacial positively charged amino acid residues in the lipid affinity of class A amphipathic helixes, and (ii) to investigate the importance of the nature of interfacial charge in the lipid affinity of class A amphipathic helixes. Toward this end, lipid interactions of the following two analogs of the class A amphipathic helix, Ac-18A-NH2 (acetyl-Asp-Trp-Leu-Lys-Ala-Phe-Tyr- Asp-Lys-Val-Ala-Glu-Lys-Leu-Lys-Glu-Ala-Phe-NH2), and Ac-18A(Lys > Haa)-NH2 (acetyl-Asp-Trp-Leu-Haa-Ala-Phe-Tyr-Asp-Haa-Val-Ala-Glu-Haa-Leu-Haa-Glu- Ala-Phe-NH2) (Haa = homoaminoalanine), were studied. The side chain of Haa has two CH2 groups less than that of lysine. The lipid affinities of these two peptide analogs were compared with that of Ac-18R-NH2, an analog of Ac-18A-NH2 with positions of the charged amino acid residues reversed. The techniques used in these studies were circular dichroism, fluorescence spectroscopy, right-angle light scattering measurements, and differential scanning calorimetry. The results of these studies indicated the following rank order of lipid affinity: Ac-18A-NH2 > Ac-18A(Lys > Haa)-NH2 > Ac-18R-NH2. These results are in agreement with the "snorkel" model proposed earlier to explain the higher lipid affinity of class A amphipathic helixes (Segrest, J. P., Loof, H. D., Dohlman, J. G., Brouillette, C. G., and Anantharamaiah, G. M. (1990) Proteins Struct. Funct. Genetics 8, 103-117). In addition, it was observed from the differential scanning calorimetry studies that Ac-18A-NH2 and Ac-18A(Lys > Haa)-NH2 interact more strongly than Ac-18R-NH2 with negatively charged dimyristoyl phosphatidylglycerol. The weaker interaction of Ac-18R-NH2 with dimyristoyl phosphatidylglycerol is suggested to be due to electrostatic repulsion between the negatively charged lipid and the interfacial negative charges of the peptide.
Highlights
From the Departments of Medicine and Biochemistry and the Atherosclerosis Research Unit, University of Alabama-Birmingham Medical Center, Birmingham, Alabama35294
Class A amphipathic helixes present in exchangeable is the amphipathica-helix [1].While many other proteins and plasma apolipoproteins are characterized by the loca- biologically activepeptides have been found to possess this tion of positively charged amino acid residues at the secondary structural motif, the exchangeable apolipoproteins non-polar-polarinterface and negatively charged amino are unique in that thpeoyssess multiple copies of amphipathic acid residues at the centerof the polar face
The amphipathic tively charged amino acid residues in the lipid affinity a-helixes with this kind of distribution of charged amino acid of classA amphipathic helixes, and (ii) toinvestigate the residues in thepolar face have been classified as class A(3-5)
Summary
(Received for publication, October 12, 1993,and in revised form, December 6, 1993). Vinod K. The tech- features of the amphipathic helix described in the original niques used in these studies were circular dichroism, fluorescence spectroscopy, right-angle light scattering measurements, and differential scanning calorimetry The results of these studies indicated the followingrank order of lipid affinity: Ac-18A-NHz > Ac-l8A(Lys>Haa)-. Apolipoprotein A-I (apoA-I),' the major protein constituentof plasma high density lipoproteins, and class Aamphipathic helical peptide analogs have been shown to stabilize the bilayer structure of phospholipids, and these properties were correlated to their ability to protect against lytic peptide-induced erythrocyte lysis [21]. These molecules exhibit anti-viral from Aldrich.All other chemicals were of highest purity commercially available.
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