Residue-Specific pK_aMeasurements of the β-Peptide and Mechanism of pH-Induced Amyloid Formation

Abstract

The aggregation of the β-peptide into amyloid is a key pathological event in Alzheimer's disease. This process (β-amyloidosis) involves the conversion of soluble random coil, α-helical or β-sheet conformations into insoluble, aggregated β-pleated sheet structures. The pH is a significant extrinsic factor that influences β-amyloidosis, which must be related to the presence of ionizable groups in the β-peptide. To further evaluate this effect, we determined the dissociation constants (pK a) of the side chains for the aspartic acid (Asp), glutamic acid (Glu), histidine (His), and tyrosine (Tyr) amino acid residues using NMR spectroscopy. The measurements were performed under different solution conditions, where the predominant conformation is either random coil or α-helix. We have used a peptide fragment that comprises residues 1−28 [β-(1−28)] of the natural β-(1−40) or β-(1−42) peptides, which is an appropriate model since the remaining 29−40 or 29−42 regions are devoid of polar and charged amino acid residues. The results demonstrate that the Glu and His residues have larger pK a values in sodium dodecyl sulfate solution, suggesting that electrostatic interactions are important in stabilizing the α-helix and preventing an α-helix → β-sheet rearrangement. A mechanism involving unfavorable interactions of the charged groups with the α-helix macrodipole is proposed for the pH-induced α-helix → β-sheet transformation in water−trifluoroethanol solution.

The Alzheimer β‐peptide shows temperature‐dependent transitions between left‐handed 31‐helix, β‐strand and random coil secondary structures

“…The shorter peptides show a lower degree or no cooperativity. The shortest fragment Ab (1)(2)(3)(4)(5)(6)(7)(8)(9) shows no cooperativity at all (r ¼ 1.00) in agreement with earlier observations that short segments do not exhibit cooperativity in the transition from PII to random coil [19].…”

“…There is a small contribution of PII at 0°C. Trying to break the structural states down into three different peptide segments, (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17), (18)(19)(20)(21)(22)(23)(24), (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40), we arrive at a simplified model with overall characteristics as described in Fig. 6.…”

Alzheimer's Aβ40 Studied by NMR at Low pH Reveals That Sodium 4,4-Dimethyl-4-silapentane-1-sulfonate (DSS) Binds and Promotes β-Ball Oligomerization

“…The ␦-values of protons in Asp and Glu residues indicate that they are in the neutral, carboxylic acid form, whereas those in His, Lys, and Arg show they are charged. These charged states are consistent with the thorough study of A␤ pK a values reported previously (42). All of the resonances of A␤40 assigned here have been deposited in the BioMagResBank data base (www.bmrb.wisc.edu) under accession number 6257.…”

Charge substitution shows that repulsive electrostatic interactions impede the oligomerization of Alzheimer amyloid peptides

“…These charges were then summed to calculate the total net charge on the peptide. The pK a values of Ma et al [20] measured for Ab in H 2 O gave very similar profiles of net charge vs. pH. The correlation of net charge with oligomerization was calculated using the paired Student t test implemented in Kaleidagraph 3.6 (Synergy Software).…”