Assemblies of Alzheimer’s peptides Aβ25–35 and Aβ31–35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction

Abstract Alzheimer’s β amyloid protein (Aβ) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimer’s disease (AD). The assemblies constituted from residues 25–35 (Aβ25–35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by Aβ25–35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of β-sheet structure. Moreover, the peptides Aβ31–35 (Ile–Ile–Gly–Leu–Met) and tachykinin analogue Aβ(Phe 31 )31–35 (Phe–Ile–Gly–Leu–Met) gave powder diffraction patterns to 2.8 A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a =9.36 A, b =15.83 A, and c =20.10 A for the native Aβ31–35 peptide, and a =9.46 A, b =16.22 A, and c =11.06 A for the peptide having the Ile31Phe substitution. The initial model was a β strand where the hydrogen bonding, chain, and intersheet directions were placed along the a , b , and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that Aβ is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like Aβ31–35 domain is a turn exposed at the Aβ oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.