X-ray diffraction from intraneuronal paired helical filaments and extraneuronal amyloid fibers in Alzheimer disease indicates cross-beta conformation.

Abstract

Information about the structure of the paired helical filaments (PHF) that accumulate within human neurons and the amyloid fibers that accumulate in the extracellular spaces between neurons in Alzheimer disease has so far depended on electron microscopy of thin-sectioned or negatively stained material. To determine the protein conformation of these abnormal fibers, we have obtained x-ray diffraction patterns from unfixed human brain fractions highly enriched in PHF and from purified amyloid cores isolated from senile plaques. The predominant x-ray scatter evident from both types of samples, either wet or dry, is a sharp reflection at 4.76-A spacing and a diffuse one at about 10.6-A spacing. These features are characteristic of a beta-pleated sheet type of protein conformation. In doubly oriented dried pellets of PHF fractions, the two reflections are accentuated at right angles to each other and the arc at 4.76-A spacing is in the fiber direction indicating a cross-beta conformation. From the integral widths of the reflections we estimate the cross-beta crystallite to be about 80 A long in the fiber direction and about 40 A thick. These dimensions correspond to approximately four pleated sheets, each of which consists of approximately 16 hydrogen-bonded polypeptide chains running normal to the fiber direction. The cross-beta conformation of PHF and amyloid fibers that we have found from x-ray diffraction is in contrast to the predominant alpha-helical coiled-coil conformation of the neurofilaments with which they share epitopes and from which they have been postulated to derive.

Alternate Aggregation Pathways of the Alzheimer β-Amyloid Peptide

“…Staining with Congo red and examination under polarized light produces green birefringence (10) indicating that amyloid deposits are composed of peptide polymers with ␤-sheet structure. This conformational characteristic of A␤ fibrils is confirmed by x-ray diffraction analysis, which shows a cross-␤-fiber diffraction pattern with a distance between polypeptide chains of 4.76 Å and a distance between sheets of 10.6 Å (11).…”

Aβ(31–35) and Aβ(25–35) fragments of amyloid beta‐protein induce cellular death through apoptotic signals: Role of the redox state of methionine‐35

“…X-ray diffraction data have shown that the conformation of Ab is characterized by an antiparallel cross-b pleated sheet [3], although more recent solid state NMR evidence suggests that the peptide has a parallel b-sheet structure [4]. Nevertheless, aggregation occurs because of hydrogen bonding between b-strands, and the resulting fibrils have axes perpendicular to the b-strand and parallel to the cross-linking hydrogen bonds [3].…”

Probing Solvent Accessibility of Transthyretin Amyloid by Solution NMR Spectroscopy

“…Because the macroscopic properties of amyloid, in terms of aggregation and solubility, prevent the use of single crystal x-ray crystallography, as well as solution NMR spectroscopy, the atomic resolution of the molecular structure of amyloid fibrils is poorly understood. So far, structural information has mainly arisen from fiber diffraction studies (2), cryoelectron microscopy (3), mass spectrometry (4), and solid state NMR spectroscopy (5). From these data a general picture has emerged implying the occurrence of parallel or antiparallel ␤-sheets, in which the ␤-strands lie perpendicular to the direction of the long fibril axis.…”