Oligopeptide-Mediated Stabilization of the α-Helix of a Prion Protein Peptide

Control of helix sense in protein-mimicking backbone by the noncovalent chiral effect

We have reviewed our previous work regarding induction or control of a peptide helix sense through chiral stimulus to the peptide chain terminus. An optically inactive 3(10)-helix designed mainly with unusual alpha-amino acid residues was commonly employed. Such an N-terminal-free peptide generates a preferred helix sense by chiral acid molecule. A helix sense pre-directed in chiral sequence is also influenced or controlled by the chiral sign of such external molecule. Here free amide groups in the 3(10)-helical N-terminus participate in the formation of a multipoint coordinated complex. The terminal asymmetry produces the noncovalent chiral domino effect (NCDE) to influence the whole helix sense. The NCDE-mediated control of helicity provides the underlying chiral nature of protein-mimicking helical backbones: notably, chiral recognition at the terminus and modulation of helical propensity through chiral stimulus. The above items from our previous reports have been outlined and reviewed together with their significance in biopolymer science and chiral chemistry.

Nonfibrous β-structured aggregation of an Aβ model peptide (Ad-2α) on GM1/DPPC mixed monolayer surfaces

Adsorption and aggregation of transformed peptides and proteins onto the cell membrane surface is commonly associated with forms of amyloidosis such as Alzheimer's disease and prion disease. To address dynamic features of these pathological phenomena molecularly, the in situ Ad-2alpha model peptide deposition on glycolipid-containing monolayers was studied by using a 9 MHz quartz-crystal microbalance (QCM). The Ad-2alpha peptide has two amphiphilic alpha-helix segments, each modified with a 1-adamantanecarbonyl group at the N-terminal as a hydrophobic defect. The peptide folds in a 2alpha-helix structure in the bulk solution. In the presence of mixed monolayers of glycolipids (GM1, asialo-GM1, GM3, or LacCer) and/or dipalmitoyl phosphatidylcholine (DPPC) laminated on the QCM plate, the peptide deposition and the conformational change to beta-structure on the monolayers were accelerated. The adsorption kinetics and the amount of Ad-2alpha were dependent on the sort and contents of the glycolipid in the DPPC matrix. Although the Ad-2alpha peptide adsorbs onto most of the glycolipid membranes as monolayer coverage, it adsorbed largely onto the GM1/DPPC (30/70 mol%) mixed monolayer with characteristic kinetic behaviors. The accumulation of beta-structured nonfibrous aggregations was confirmed by AFM and fluorescence microscopy with Thioflavin T (ThT).

Oligopeptide-mediated helix stabilization of model peptides in aqueous solution

Oligopeptide-mediated helix stabilization of peptides in hydrophobic solutions was previously found by NMR and CD spectroscopic studies. The oligopeptide included the hydrophobic amino acids found in its parent peptide and were interposed by relevant basic oracidic amino acids. The strength of the interactions depended on the amino acid sequences. However, no helix-stabilizing effect was seen for the peptides in phosphate buffer solution, because the peptides assumed a random-coil structure. In order to ascertain whether the helix-stabilizing effect of an oligopeptide on its parent peptide could operate in aqueous solution, model peptides EK17 (Ac-AEAAAAEAAAKAAAAKA-NH2) and IFM17 (Ac-AEAAAAEIFMKAAAAKA-NH2) that may assume an alpha-helix in aqueous solutions were synthesized. Interactions were examined between various oligopeptides (EAAAK, KAAAE, EIFMK, KIFME, KIFMK and EYYEE) and EK17 or IFM17 in phosphate buffer and in 80% trifluoroethanol (TFE)-20% H2O solutions by CD spectra. EAAAK had little effect on the secondary structures of EK17 in both buffer and TFE solutions, while KAAAE, which has the reverse amino acid sequence of EAAAK, had a marked helix-destabilizing effect on EK17 in TFE. EIFMK and KIFME were found to stabilize the alpha-helical structure of EK17 in phosphate buffer solutions, whereas KIFMK and EYYEE destabilized the alpha-helical structure of EK17. EIFMK and KIFME had no effect on IFM17, because unexpectedly, IFM17 had appreciable amounts of beta-sheet structure in buffer solution. It was concluded that in order for the helix-stabilizing (1) the model peptide, the alpha-helical conformation of which is to be stabilized, should essentially assume an alpha-helical structure by nature, and (2) the hydrophobicity of the side-chains of the oligopeptide should be high enough for the oligopeptide to perform stable specific side chain-side chain intermolecular hydrophobic interactions with the model peptide.

Helix-stabilizing effects of the pentapeptide KIFMK and its related peptides on the sodium channel inactivation gate peptides

We have previously found by NMR and CD spectroscopic studies that the helical content of the sodium channel inactivation gate-related peptide (Ac-GGQDIFMTEEQK-NH2; MP-1A) in 80% trifluoroethanol solutions was increased by adding a pentapeptide, KIFMK. In order to study in further detail whether the presence of the IFM motif and the two lysine residues is a prerequisite for stabilizing the helical conformation, we examined interactions between various oligopeptides (RIFMR, KIFMTK, KIQMK, KAFAK, KIIIK) and MP-1A and its related peptides; that is, MP-2A in which Phe was replaced by Gln, MP-1MMA in which Thr was replaced by Met, MP-1TA in which Thr was removed from MP-1A, and MP-1A' in which L-Phe was replaced by D-Phe. It was found that the IFM motif was absolutely necessary in both the oligopeptide and the inactivation gate peptide. This finding means that hydrophobic interactions are operative between KIFMK and MP-1A. In contrast, KIFMK destabilized the helical structure of MP-1MMA, MP-1TA, and MP-1A', showing that the conformation around the IFM motif in the inactivation gate peptides is an important factor. It was concluded that the IFM motif and the two Lys residues are a prerequisite for effectively stabilizing the alpha-helix of MP-1A.