Amyloid β (1-40) Toxicity Depends on the Molecular Contact between Phenylalanine 19 and Leucine 34

The formation of the hydrophobic contact between phenylalanine 19 (F19) and leucine 34 (L34) of amyloid beta (1-40) (A beta(1-40)) is known to be an important step in the fibrillation of A beta(1-40) peptides. Mutations of this putatively early molecular contact were shown to strongly influence the toxicity of A beta(1-40) (Das et al. (2015) ACS Chem. Neurosci. 6, 1290-1295). Any mutation of residue F19 completely abolished the toxicity of A beta(1-40), suggesting that a proper F19 L34 contact is crucial also for the formation of transient oligomers. In this work, we investigate a series of isomeric substitutions of L34, namely, D-leucine, isoleucine, and valine, to study further details of this molecular contact. These replacements represent very minor alterations in the A beta(1-40) structure posing the question how these alterations challenge the fibrillation kinetics, structure, dynamics, and toxicity of the A beta(1-40) aggregates. Our work involves kinetic studies using thioflavin T, transmission electron microscopy, X-ray diffraction for the analysis of the fibril morphology, and nuclear magnetic resonance experiments for local structure and molecular dynamics investigations. Combined with cell toxicity assays of the mutated A beta(1-40) peptides, the physicochemical and biological importance of the early folding contact between F19 and L34 in A beta(1-40) is underlined. This implies that the F19 L34 contact influences a broad range of different processes including the initiation of fibrillation, oligomer stability, fibril elongation, local fibril structure, and dynamics and cellular toxicity. These processes do not only cover a broad range of diverse mechanisms, but also proved to be highly sensitive to minor modulations of this crucial contact. Furthermore, our work shows that the contact is not simply mediated by general hydrophobic interactions, but also depends on stereospecific mechanisms.