Effect of end group blockage on the properties of a class A amphipathic helical peptide

In a recent classification of biologically active amphipathic α‐helixes, the lipid‐associating domains in exchangeable plasma apolipoproteins have been classified as class A amphipathic helixes (Segrest, J. P., De Loof, H., Dohlman, J. G., Brouillette, C. G., Anantharamaiah, G. M. Proteins 8:103–117, 1990). A model peptide analog with the sequence, Asp Trp Leu Lys Ala Phe Tyr Asp Lys Val Ala Glu Lys Leu Lys Glu Ala Phe (18A), possesses the characteristics of a class A amphipathic helix. The addition of an acetyl group at the α‐amino terminus and an amide at the α‐carboxyl terminus, to obtain Ac‐18A‐NH 2 , produces large increases in helicity for the peptide both in solution and when associated with lipid (for 18A vs Ac‐18A‐NH 2 , from 6 to 38% helix in buffer and from 49 to 92% helix when bound to dimyristoyl phosphatidylcholine in discoidal complexes). Blocking of the end‐groups of 18A stabilizes the α‐helix in the presence of lipid by approximately 1.3 kcal/mol. There is also an increase in the self‐association of the blocked peptide in aqueous solution. The free energy of binding to the PC–water interface is increased only by about 3% (from −8.0 kcal/mol for 18A to −8.3 kcal/mol for Ac‐18A‐NH 2 ). The Ac‐18A‐NH 2 has a much greater potency in raising the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine than does 18A. In this regard Ac‐18A‐NH 2 more closely resembles the behavior of the apolipoprotein A‐I, which is the major protein component of high‐density lipoprotein and a potent inhibitor of lipid hexagonal phase formation. The activation of the plasma enzyme lecithin: cholesterol acyltransferase by the Ac‐18A‐NH 2 peptide is greater than the 18A analog and comparable to that observed with the apo A‐I. In the case of Ac‐18A‐NH 2 , the higher activating potency may be due, at least in part, to the ability of the peptide to micellize egg PC vesicles. © 1993 Wiley‐Liss, Inc.