INTERACTIONS OF SYNTHETIC PEPTIDE ANALOGS OF THE CLASS A AMPHIPATHIC HELIX WITH LIPIDS - EVIDENCE FOR THE SNORKEL HYPOTHESIS

Class A amphipathic helixes present in exchangeable plasma apolipoprot eins are characterized by the location of positively charged amino aci d residues at the non-polar-polar interface and negatively charged ami no acid residues at the center of the polar face. The objectives of th e 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 inv estigate the importance of the nature of interfacial charge in the lip id affinity of class A amphipathic helixes. Toward this end, lipid int eractions of the following two analogs of the class A amphipathic heli x, Ac-18A-NH2 -Asp-Lys-Val-Ala-Glu-Lys-Leu-Lys-Glu-Ala-Phe-NH2), and A c-18A(Lys>Haa)-NH2 cetyl-Asp-Trp-Leu-Haa-Ala-Phe-Tyr-Asp-Haa-Val-Ala- Glu-Haa-Leu-Haa-Glu-Ala-Phe-NH2) (Haa = homoaminoalanine), were studie d. The side chain of Has has two CH2 groups less than that of lysine. The lipid affinities of these two peptide analogs were compared with t hat of Ac-18R-NH2, an analog of Ac-18A-NH2 with positions of the charg ed 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 re sults of these studies indicated the following rank order of lipid aff inity: Ac-18A-NH2 > Ac-18A(Lys>Haa)NH2 > Ac-18R-NH2. These results are in agreement with the ''snorkel' model proposed earlier to explain th e 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 additio n, 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 phosphatidylglycero l is suggested to be due to electrostatic repulsion between the negati vely charged lipid and the interfacial negative charges of the peptide .