Charge states rather than propensity for beta-structure determine enhanced fibrillogenesis in wild-type Alzheimer's beta-amyloid peptide compared to E22Q Dutch mutant

Abstract

The activity of the Alzheimer's amyloid beta-peptide is a sensitive function of the peptide's sequence. Increased fibril elongation rate of the E22Q Dutch mutant of the Alzheimer's amyloid beta-peptide relative to that of the wild-type peptide has been observed. The increased activity has been attributed to a larger propensity for the formation of beta structure in the monomeric E22Q mutant peptide in solution relative to the WT peptide. That hypothesis is tested using four nanosecond timescale simulations of the WT and Dutch mutant forms of the Abeta(10-35)-peptide in aqueous solution. The simulation results indicate that the propensity for formation of beta-structure is no greater in the E22Q mutant peptide than in the WT peptide. A significant measure of "flickering" of helical structure in the central hydrophobic cluster region of both the WT and mutant peptides is observed. The simulation results argue against the hypothesis that the Dutch mutation leads to a higher probability of formation of beta-structure in the monomeric peptide in aqueous solution. We propose that the greater stability of the solvated WT peptide relative to the E22Q mutant peptide leads to decreased fibril elongation rate in the former. Stability difference is due to the differing charge state of the two peptides. The other proposal leads to the prediction that the fibril elongation rates for the WT and the mutant E22Q should be similar under acid conditions.

Fig. 1.

The result of the URMS analysis of the WT and E22Q mutant trajectory simulations are plotted. In the uppermost figures, the propensity for α-helical structural fluctuations is shown where red indicates strong α-like character, for the WT and E22Q mutant trajectories, respectively. In the lowermost figures, the propensity for β-sheet structure is shown where blue indicates strong β-like character, for the WT and E22Q mutant trajectories, respectively.

Fig. 2.

The secondary-structure formation probed through an analysis of the dihedral angle, φ_i, derived by mapping the all-atom model dynamics on a coarse-grained representation. The local backbone conformation was said to be helical if |φ_i − 60°| ≤ 30° (red) or a β-strand structure if |φ_i − 180°| ≤ 30° (green). All other conformations were said to be coil (blue).

Fig. 3.

This matrix shows the pairs of side-chain contacts and hydrogen-bonding interactions most frequently observed in the course of our simulations of the WT and E22Q mutant peptides. The side-chain contacts are shown above the diagonal; the backbone hydrogen-bonding pairs are shown below the diagonal. All side-chain contact and hydrogen-bonding pairs that are noted occur with a probability > 50%. Data is represented for the WT peptide (black spots) and E22Q mutant peptide (red spots). The full spots are contact pairs and the open spots are hydrogen-bonding pairs.

Fig. 4.

Conformational energy for the four trajectories of the WT peptide (black) and of the E22Q mutant peptide (red). The top panel shows the energies calculated from the coarse-grained model; the bottom panel shows those from the all-atom model.

Fig. 5.

The E22Q mutant form of the congener Aβ(10–35)-NH₂ peptide is depicted. From the N terminus the groups are Tyr 10–Glu 11–Val 12–His 13–His 14–Gln 15–Lys 16 (blue), Leu 17–Val 18–Phe 19–Phe 20–Ala 21 (red), Glu 22 (green), Asp 23 (blue), Val 24–Gly 25–Ser 26–Asn 27 (yellow), and Lys 28–Gly 29–Ala 30–Ile 31–Ile 32–Gly 33–Leu 34–Met 35 (blue). The figure compares the all-atom model employed in the peptide simulations with a coarse-grained representation of the peptide used in the side-chain contact analysis.

Fig. 6.

A schematic diagram describing the definition of the URMS order parameter used to measure the fluctuations of local peptide main-chain geometries consistent with β-structure.