Synthetic model peptides for apolipoproteins. I. Design and properties of synthetic model peptides for the amphipathic helices of the plasma apolipoproteins.
Brasseur, Robert ; ; et al
in Biochimica et Biophysica Acta (1993), 1170(1), 1-7
Amphipathic helical peptides are the lipid-binding motives of the plasma apolipoproteins, and synthetic peptide analogs have been used to unravel the mechanism of lipid association within this class of ... [more ▼]
Amphipathic helical peptides are the lipid-binding motives of the plasma apolipoproteins, and synthetic peptide analogs have been used to unravel the mechanism of lipid association within this class of proteins. Hydrophobic interactions between the apolar amino acid residues belonging to the hydrophobic face of the amphipathic helices and the lipids are the major driving forces in the peptide-lipid association to form discoidal complexes. Ionic interactions and salt bridge formation between contiguous peptide chains in the complex can, however, contribute to the overall stability of the lipid-protein particle. This was studied by designing peptide analogs to the helical repeats of the apolipoproteins with variable degrees of salt bridge formation between adjacent peptide chains. The most stable conformation for pairs of synthetic peptides was calculated by energy minimisation together with the energy of interaction between peptides. The sequence of the peptides was derived from that of the 18A peptide synthesized by Segrest et al., and the theoretical calculations confirmed that ionic interactions between residues close to each other, along the edge of two adjacent anti-parallel peptides, can significantly contribute towards the stability of a peptide-phospholipid complex. [less ▲]Detailed reference viewed: 11 (0 ULg)
Synthetic model peptides for apolipoproteins. II. Characterization of the discoidal complexes generated between phospholipids and synthetic model peptides for apolipoproteins.
; ; et al
in Biochimica et Biophysica Acta (1993), 1170(1), 8-16
The structure, composition and physico-chemical properties of complexes generated between phospholipids and synthetic model peptides for the amphipathic helices of the plasma apolipoproteins were studied ... [more ▼]
The structure, composition and physico-chemical properties of complexes generated between phospholipids and synthetic model peptides for the amphipathic helices of the plasma apolipoproteins were studied. The sequences of the peptides were derived from that of the 18A peptide and designed to either enhance or decrease ionic interactions between pairs of peptides, as described in the accompanying paper. Complexes were prepared with dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), or with DPPC and cholesterol, and isolated on a Superose 6HR column. Association kinetics for the DMPC-peptides complexes were followed by measuring the turbidity as a function of the temperature. The diameters of the DPPC-peptide complexes, measured by gradient gel electrophoresis (GGE), were about 120 A. Fluorescence polarization measurements after labeling with diphenyl hexatriene (DPH) yielded transition temperatures of, respectively, 40.6, 41.5 and 41.8 degrees C for the DPPC/18AM1-, DPPC/18AM4- and DPPC/18A-peptide complexes. These values were confirmed by differential scanning calorimetry. Circular dichroism and infrared spectroscopy revealed that the peptides adopt an alpha-helical structure in solution and this percentage increased from 30-40% in the free peptides up to 50-60% in the complexes. Attenuated total reflection (ATR) infrared measurements of the complexes indicated that the peptides are oriented parallel to the acyl chains of the phospholipid bilayer. Denaturation of the peptides and of the peptide-lipid complexes was monitored by Trp fluorescence under addition of increasing amounts of GdmCl. The mid-points of the denaturation curves lie at, respectively, 0.05, 0.25 and 0.35 M GdmCl for the 18AM4, 18A and 18AM1 peptide and are shifted towards higher GdmCl concentrations after peptide-lipid binding. GdmCl denaturation decreased the alpha-helical content of the peptides and of the complexes, as monitored by circular dichroism measurement. The helix to random coil structure transition occurred at, respectively, 2.1, 2.2, and 2.0 M GdmCl for 18A, 18AM1 and 18AM4, compared to 5.1, 5.0, and 5.3 M in the corresponding complexes. These data suggest altogether that the structural properties, the mode of lipid-protein association and the stability of the phospholipid-peptide complexes are similar to those of native plasma apolipoproteins. The 18A and 18AM4 peptides which contain charged residues along the edge of the helix, leading to salt bridge formation between peptides were shown to mimic the amphipathic helices of the plasma apolipoproteins. [less ▲]Detailed reference viewed: 21 (0 ULg)