Clarifying the influence of core amino acid hydrophobicity, secondary structure propensity, and molecular volume on amyloid-β 16–22 self-assembly

Abstract

The self-assembly of amyloid peptides is influenced by hydrophobicity, charge, secondary structure propensity, and sterics. Previous experiments have shown that increasing hydrophobicity at the aromatic positions of the amyloid-β 16-22 fragment (Aβ(16-22)) without introducing steric restraints greatly increases the rate of self-assembly and thermodynamically stabilizes the resulting fibrils [Senguen et al., Mol. BioSyst., 2011, DOI: 10.1039/c0mb00080a]. Conversely, when increasing side chain hydrophobicity coincides with an increase in side chain volume, the increase in the rate of self-assembly is offset by a thermodynamic destabilization of the resulting amyloid fibrils when direct cross-strand side chain interactions occur. These findings indicate that steric effects also influence the self-assembly of amyloidogenic peptides. Herein, the aromatic Phe residues at positions 19, 20, and 19,20 of Aβ(16-22) have been systematically replaced by Val, Leu, Ile, or hexafluoroleucine (Hfl) and amyloid formation has been characterized. The Val variants, despite the high β-sheet propensity of Val, were thermodynamically destabilized (ΔΔG = +0.1-0.4 kcal mol(-1)) relative to the wild-type with the double mutant failing to self-assemble at the concentrations studied. Conversely, the Leu and Ile variants formed fibrils at enhanced rates relative to wild-type and exhibited similar, or in some cases enhanced thermodynamic stabilities relative to the wild-type (ΔΔG = 0-0.6 kcal mol(-1)). The more hydrophobic Hfl variants were greatly stabilized (ΔΔG = -0.3-2.1 kcal mol(-1)) relative to the wild-type. These data indicate that hydrophobicity and steric effects both influence peptide self-assembly processes, including nucleation and fibrillization rates and the thermodynamic stability of the resulting fibrils.

Side‐chain hydrophobicity and the stability of Aβ16–22 aggregates

“…Hence, not aromatic interactions but rather the hydrophobicity and b-sheet propensity of these residues at position 19 and 20 might be the requirement for the aggregation of Ab. A similar result was also obtained by Senguen et al 11 on nine variant of Ab [16][17][18][19][20][21][22] . Increasingly, molecular dynamic simulations complement experiments as a tool to research the assembly of oligomer and protofibrils, effects of mutation, mechanism of toxicity and inhibition of amyloid aggregate.…”

“…We conjecture that the low solvent exposure of the hydrophobic core region (Val18-21Ala) in the 19A mutant reduces the interactions with ''like'' peptides and contributes to the slower fibril formation. 10,11 In contrast, in the wild-type and the other two mutant systems, the hydrophobic region is more exposed to solvent. This increases the probability of these residues to interact with other hydrophobic residues facilitating aggregation and possibly leading to faster formation of fibrils as observed in previous experiments.…”

Stability of Amyloid Oligomers

“…This stability analysis of single and double point mutants located in the hydrophobic core region of Aβ 16À22 confirms that size and hydrophobicity determine the aggregation process. Our results are in agreement with pervious experiments (Armstrong, Chen, McKoy, & Hecht, 2011;Senguen, Doran, Anderson, & Nilsson, 2011) and indicate that rather than aromatic interactions, the hydrophobicity and β-sheet propensity of residues at positions 19 and 20 are responsible for the aggregation of Aβ. Hence, the substitution of residues in the hydrophobic region Aβ 16À22 (especially, Phe 19 and 20) with natural and nonnatural amino acids of similar size and hydrophobicity is a promising venue for designing Aβ aggregation inhibitors.…”

Turn Nucleation Perturbs Amyloid β Self-Assembly and Cytotoxicity

“…18, 19, 65 Evidence suggests that during fibril nucleation, turn formation plays a significant structural role in the equilibrium leading to fibrils (Figure 1, pathway B). 27, 40, 54 Sandberg, et al have recently proposed a two-pathway model for Aβ self-assembly in which fibril nucleation occurs through a β-sheet rich pathway.…”

“…16–19 During the lag phase, natively unfolded Aβ undergoes a chain reaction of association events that ultimately leads to the formation of cross-β fibrils. 20 Aβ fibrils have been characterized using solid-state NMR and these structural models have been used to extrapolate structures that may be present in earlier intermediates.…”