Amyloid β-Protein (17-42)

A peptide corresponding to the amino acid sequence of A beta 17-42 (LVFFAEDVGSNKGAIIGLMVGGVVIA) was isolated from Alzheimer Disease patient brains containing large deposits of diffuse-type amyloid. Brain homogenates were lysed in SDS and submitted to high speed centrifugations. A beta peptides were purified by size exclusion chromatography on Superose 12 and TSK 3000 SW columns. An A beta peptide with M(r) of 3,000 was recovered that on automatic gas-phase Edman degradation yielded the amino acid sequence of A beta starting at residue 17 (Leu). The 3-kDa peptide was subsequently hydrolyzed with trypsin and reacted with CNBr, and the resulting peptides were separated by reverse phase high pressure liquid chromatography and characterized by amino acid analyses, peptide microsequencing, and mass spectrometry. Hydrolysis of beta-amyloid precursor protein 695 at Lys612-Leu613 or at Lys16-Leu17 of its A beta 1-42 derivative prevents the generation of neurotoxic A beta filaments, thus leading to the formation of A beta 17-42 localized in the diffuse amyloid deposits. An outstanding feature in the pathology of Alzheimer Disease is that the predominant A beta peptides have their C termini at position 42, whether in the cores of the neuritic plaques, in the vascular walls, or in the diffuse deposits. Based on these observations, we postulate that the accumulation of insoluble A beta N-42 in Alzheimer Disease is due to the anomalous processing of the C-terminal region.

The aggregation of the Aβ peptide (Aβ1-42) to form fibrils is a key feature of Alzheimer's disease. The mechanism is thought to be a nucleation stage followed by an elongation process. The elongation stage involves the consecutive addition of monomers to one end of the growing fibril. The aggregation process proceeds in a stop-and-go fashion and may involve off-pathway aggregates, complicating experimental and computational studies. Here we present exploration of a well-defined region in the free and potential energy landscapes for the Aβ17-42 pentamer. We find that the ideal aggregation process agrees with the previously reported dock-lock mechanism. We also analyze a large number of additional stable structures located on the multifunnel energy landscape, which constitute kinetic traps. The key contributors to the formation of such traps are misaligned strong interactions, for example the stacking of F19 and F20, as well as entropic contributions. Our results suggest that folding templates for aggregation are a necessity and that aggregation studies could employ such species to obtain a more detailed description of the process.

Experimental studies have reported the possibility of affecting the growth/dissolution of amyloid fibres by the addition of organic salts of the room-temperature ionic-liquid family, raising the tantalizing prospect of controlling these processes under physiological conditions. The effect of [Tea][Ms] and [Tea][H2PO4] at various concentrations on the structure and stability of a simple model of Aβ42 fibrils has been investigated by computational means. Free energy computations show that both [Tea][Ms] and [Tea][H2PO4] decrease the stability of fibrils with respect to isolated peptides in solution, and the effect is significantly stronger for [Tea][Ms]. The secondary structure of fibrils is not much affected, but single peptides in solution show a marked decrease in their β-strand character and an increase in α-propensity, again especially for [Tea][Ms]. These observations, consistent with the experimental picture, can be traced to two primary effects, i.e., the difference in the ionicity of the [Tea][Ms] and [Tea][H2PO4] water solutions and the remarkable affinity of peptides for [Ms]- anions, due to the multiplicity of H-bonds.