Two papers in PNAS add to the growing body of work aimed at understanding the molecular interactions involved in the formation of amyloid fibrils (see related news story). Writing in last week's early online edition, researchers from the European Molecular Biology Laboratory, Heidelberg, Germany, together with colleagues from the University of Cambridge, England, describe a molecular modeling approach to understanding fibril structure. Led by principal author Louis Serrano, this group used mathematical algorithms to study the forces driving the formation of polymeric β-sheet structures by hypothetical hexapeptides.
First author Manuela Lopez de la Paz et al. then put theory into practice by empirically measuring the ability of the peptides to self-associate into β-sheet structures in solution. The authors found an excellent fit between model and test tube, with peptides predicted to form stable β-sheets exhibiting circular dichroism spectra typical for this secondary structure. However, formation of β-sheets is not a guarantee that fibrils will form. For example, when the authors used the electron microscope (EM) to analyze charged peptides, they found that fibrils only formed when the net charge of the peptide was +/-1.
Taking EM images together with x-ray diffraction data, the authors constructed a model in which the protofilaments that stack together to make the fibril are composed of four antiparallel β-sheets.
In contrast, researchers from the NIH, Bethesda, Maryland, and Lulea University of Technology, Sweden, led by Robert Tycko, report in this week's online edition that the β-amyloid of Alzheimer's disease comprises hairpin-shaped β-sheet structures that are stacked upon each other in parallel.
Aneta Petkova et al. used solid-state NMR measurements of the human Aβ1-40 sequence to arrive at this conclusion. The straight parts of the hairpin, residues 12-24 and 30-40, are predicted to be β-sheet structures, while the bend itself (residues 25-29) contains amino acids that bring different Aβ molecules together through side-chain interactions.
The NMR data predict that each hairpin unit has a core between the β-sheets that contains only mostly neutral and hydrophobic amino acids. Two charged residues in this core, aspartic acid and lysine, are predicted to form a salt bridge between each length of the hairpin.—Tom Fagan
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- López de la Paz M, Goldie K, Zurdo J, Lacroix E, Dobson CM, Hoenger A, Serrano L. De novo designed peptide-based amyloid fibrils. Proc Natl Acad Sci U S A. 2002 Dec 10;99(25):16052-7. PubMed.
- Petkova AT, Ishii Y, Balbach JJ, Antzutkin ON, Leapman RD, Delaglio F, Tycko R. A structural model for Alzheimer's beta -amyloid fibrils based on experimental constraints from solid state NMR. Proc Natl Acad Sci U S A. 2002 Dec 24;99(26):16742-7. PubMed.