The primary structure of the A

The primary structure of the Aβ peptide can be divided into four regions based on hydrophobicity. The N-terminal residues 1–16 comprise the first hydrophilic region, which also contains the metal binding site. More specifically, side chain carboxylate of D1 and the side chain nitrogens of H6, H13 and H14 have been proposed to coordinate Cu2+ ions [36]. The second (central) hydrophilic region is shorter, spanning residues E22-G29 and is encompassed by the central hydrophobic region (L17-A21) often denoted as the central hydrophobic cluster (CHC) and the C-terminal (A30-V40/A42) hydrophobic region [38]. Studies of the peptide at room temperature utilizing CD have revealed that in solution absent of membrane mimicking agents Aβ adopts mainly random coil conformations [81], [82]. Detailed analysis of the NMR chemical shifts, J-coupling constants and relaxation parameters of the peptide has provided similar evidence [78], [83], [84], [85]. Furthermore, the translational diffusion coefficient of Aβ has been estimated using pulsed field gradient (PFG) NMR experiments, which is consistent with an extended monomeric structure [83], [84], [85]. Thus, several lines of evidence demonstrate that Aβ peptide in solution does not adopt a stable fold and that it can be considered as an intrinsically disordered peptide/protein (IDP).
Structural studies of Aβ intermediate Cyclosporin H receptor and implications for toxicity However, structural investigations of Aβ oligomers are limited by the low stability of the species and the high tendency to aggregate into mature fibrils. To overcome these issues, specific solution conditions must be employed to prevent further aggregation of the metastable species [63], [101], [109]. Other methods for obtaining a homogenous and stable oligomer sample include protein engineering [106], [110] and photo- or chemical-crosslinking [111]. Further complications arise from the observation of various intermediate species composed of strikingly heterogeneous structures. However, there is still no consensus on the classification and nomenclature of these aggregates. These intermediate states can be categorized as “on-pathway” or “off-pathway” species. On-pathway aggregates are species formed along the reaction coordinate from monomer to fibril, while off-pathway aggregates must first dissociate into monomers before being integrated into fibrils [112]. Depending on structural morphology, the soluble Aβ aggregates are often classified as oligomers and protofibrils. The latter displays non-spherical, curvilinear, irregular morphology. PFs have been shown to possess a molecular mass up to 250 MDa. Furthermore, these species exhibit β-sheet propensity, bind ThT and have been reported to seed the growth of fibrils [113]. Hence, they are considered to be on-pathway species [112]. Oligomers incorporate fewer Aβ molecules, their molecule masses range from 9 kDa to a few MDa and they have spherical appearance [114]. While oligomers often contain β-sheet secondary structure, they do not always bind ThT or seed fibril formation [65], [115]. Depending on solution conditions, different types of oligomers have been described, including amyloid-derived-diffusible ligands (ADDLs) [55], [116], [117], globulomers [63], [118], amylospheroids (ASPD) [119], [120], protofibrils (PF) [114], [121], [122] and β-barrel pore-forming Aβ1-42 oligomers (βPFOsAβ42) [54].
Aβ fibril structure Amyloid fibrils represent the final product of the peptide aggregation process. Fibril structural investigations can promote the understanding of later aggregation stages such as elongation and secondary nucleation steps. Furthermore, Aβ fibrils possess reactive surfaces for generation of toxic oligomers from monomers through secondary nucleation mechanisms [73], [74]. Thus, elucidating structural features of fibrils could illuminate the underlying mechanism of this catalytic process. Moreover, these structures could facilitate the development of diagnostic and therapeutic strategies for AD [32].