NMR studies of amyloid .beta.-peptides: proton assignments, secondary structure, and mechanism of an .alpha.-helix .fwdarw. .beta.-sheet conversion for a homologous, 28-residue, N-terminal fragment

Abstract

Beta-peptide is a major component of amyloid deposits in Alzheimer's disease. We report here a proton nuclear magnetic resonance (NMR) spectroscopic investigation of a synthetic peptide that is homologous to residues 1-28 of beta-peptide [abbreviated as beta-(1-28)]. The beta-(1-28) peptide produces insoluble beta-pleated sheet structures in vitro, similar to the beta-pleated sheet structures of beta-peptide in amyloid deposits in vivo. For peptide solutions in the millimolar range, in aqueous solution at pH 1-4 the beta-(1-28) peptide adopts a monomeric random coil structure, and at pH 4-7 the peptide rapidly precipitates from solution as an oligomeric beta-sheet structure, analogous to amyloid deposition in vivo. The NMR work shown here demonstrates that the beta-(1-28) peptide can adopt a monomeric alpha-helical conformation in aqueous trifluoroethanol solution at pH 1-4. Assignment of the complete proton NMR spectrum and the determination of the secondary structure were arrived at from interpretation of two-dimensional (2D) NMR data, primarily (1) nuclear Overhauser enhancement (NOE), (2) vicinal coupling constants between the amide (NH) and alpha H protons, and (3) temperature coefficients of the NH chemical shifts. The results show that at pH 1.0 and 10 degrees C the beta-(1-28) peptide adopts an alpha-helical structure that spans the entire primary sequence. With increasing temperature and pH, the alpha-helix unfolds to produce two alpha-helical segments from Ala2 to Asp7 and Tyr10 to Asn27. Further increases in temperature to 35 degrees C cause the Ala2-Asp7 section to become random coil, while the His13-Phe20 section stays alpha-helical. A mechanism involving unfavorable interactions between charged groups and the alpha-helix macrodipole is proposed for the alpha-helix----beta-sheet conversion observed at midrange pH.

Structure determination of extracellular fragments of amyloid proteins involved in Alzheimer's disease and Dutch‐type hereditary cerebral haemorrhage with amyloidosis

“…Neurochemical and histopathological aspects are being actively researched by various groups. Recent work (Barrow and Zagorski, 1991 ;Zagorski and Barrow, 1992) has demonstrated that PA4 and related peptide fragments are highly sensitive to concentration, solvent, pH and temperature effects. Information on the possible solution structure of PA4 should assist in the overall understanding of the molecular mechanisms involved in this fatal disease, as well as other related amyloidoses.…”

Solvent Accessibility to Aspartyl and Succinimidyl Residues at Positions 7 and 23 in the Amyloid β 1 - 28 Peptide

“…The amyloid ß (1-28) peptide obtained from the protein data bank (PDB) has several confor mational structures determined via NMR in solu tion (Barrow and Zagorski, 1991;Zagorski and Barrow, 1992;Talafous et al, 1994). These struc-tures have been averaged and the starting coordi nates for this peptide were obtained and used to perform the calculations of water accessibility to the aspartyl residues at positions 7 and 23.…”

“…The NMR structure retrieved from the PDB ar chives was obtained in an aqueous solution of 1,1,1-trifluoroethanol and it exists in a helical con formation (Barrow and Zagorski, 1991;Zagorski and Barrow, 1992). It has been shown that there is no helical conformation of this peptide in pure water (Lee et al, 1995).…”

“…It is plausible to mention that the aqueous solution of 1,1,1-trifluoroethanol might be a better representation of the intracellu lar media than pure water. Thus, it seems appro priate to limit this investigation to the helical con formation obtained from the NMR in 1,1,1-trifluoroethanol aqueous solution (Barrow and Zagorski, 1991;Zagorski and Barrow, 1992) and from molecular dynamics simulations (Szendrei et al, 1996). This validation procedure is necessary due to the fact that when the aspartyl residues are changed to the succinimidyl one (ASP -> SUC) pared to the backbone in the unmodified peptide, the peptide structure needs to be relaxed and this However, when the aspartyl is modified to a sucrelaxation is not available experimentally.…”

Identification of structural variations in the carboxyl terminus of Alzheimer's disease-associated βA4[1–42] amyloid using a monoclonal antibody

“…This structure is modified with increasing temperature and pH so that at pH 1±4 or above 7 the peptides adopt a random coil structure but aggregate in a b-sheet conformation between pH 4 and 7. In contrast, the region 0 1 2 encompassing bA4[29±42] has been shown to adopt a b-strand conformation regardless of temperature and pH, and as such has been reported to direct the protein folding of the entire bA4 sequence to produce the b-pleated sheet structure found in amyloid deposits [7,8,15,16] In light of these observations, this study has demonstrated a unique C-terminal conformation within bA4[1±42], as defined using MoAb 3B9 which binds the GVV epitope. These results support the hypothesis that key residues within the C-terminal region of bA4[1±42] generate a unique peptide conformation which may be responsible for initiating the bA4 polymerization and amyloid fibril formation observed in the neuritic and diffuse plaques of AD.…”