Comparative Analysis of the Structure and Fibrillogenesis of Aβ11-28 Peptide and the Inverse Sequence

Abstract

The inverse sequences of naturally occurring proteins display different folding and structural properties and fail to retain the functions of native proteins despite identical fundamental features, such as the amino acid composition, hydrophobicity, etc. To gain insight into the physical mechanisms underlying secondary structure formation and aggregation of direct and retro sequences of amyloidogenic peptides, we probed the fibrillogenesis and accompanying structural changes of amyloid β peptide fragment Aβ11-28 (EVHHQKLVFFAEDVGSNK) and its inverse sequence, Aβ28-11. These peptides are of interest because the aggregation of both Aβ11-28 or Aβ28-11 into parallel, in-register β-sheet (characteristic structure of Aβ fibrils) will be hampered by electrostatic repulsion between amino acids of like charges. Therefore, each peptide is expected to form antiparallel intermolecular β-sheet structure and their combination would form parallel β-sheet stabilized by ionic interactions between juxtaposed side chains with opposite charges. Circular dichroism data identified α-helical structure for both peptides and their equimolar combination in hexafluoroisopropanol and in dry state. Addition of aqueous buffer caused transition into unordered state, followed by formation of β-sheet/β-turn structure, faster for Aβ11-28. Thioflavin-T fluorescence showed fibril formation by Aβ11-28 and Aβ28-11, starting around 200 and 250 h, respectively. For the combination, ThT fluorescence stayed at low level for 360 h. FTIR data confirmed formation of β-sheet/β-turn structure in Aβ11-28 and Aβ28-11 and identified α-helix propensity of the retro sequence. The combination slowly transitioned from unordered to mostly β-turn structure. Amide I spectral features characteristic of antiparallel β-sheet were not detected for any of the peptide samples. These data suggest that ionic interactions between side chains play a minor role in amyloid peptide structure and aggregation, and the structural propensity of the inverse sequence is distinctly different from that of the parent peptide.