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  Solid-state NMR study on the structure of amyloid-beta
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[Background and purpose]
Alzheimer's disease is caused by anomarous aggregation of amyloid-beta peptide inside a brain. When the amyloid-beta peptides aggregates with one another to form parallel beta-sheet structure, toxicity arises to nerve cells. There exists two major types of amyloid-beta peptides called amyloid-beta40 and amyloid-beta42 (Fig. 1), because they are composed of 40 and 42 amino acid chains, respectively. Since the latter is known to be more cohesive and thereby show stronger toxicity, secondary structure of the aggregating amyloid-beta42 peptide has been of considerable interest.

Fig. 1. Primary structures (i.e., amino acid sequences) of the amyloid-beta40 and amyloid-beta42 peptides.

Solid-state NMR is a suitable approach for this purpose compared to liquid-state NMR and X-ray diffraction, because the target material can neither be dissolved in liquids nor form a single crystal.

In collaboration with the Irie group at Graduate School of Agriculture in Kyoto University, we have been carrying out solid-state NMR studies on the structures of wild-type amyloid-beta42 as well as its variant E22K-amyloid-beta42 (Italian type), which shows even stronger toxicity.

Fig. 2. Secondary structure of the aggregated, toxic amyloid-beta42 peptide proposed by the Irie group.

From toxicities of various proline substituted mutants of the amyloid-beta peptides, the structure of the wild-type amyloid-beta peptide in the toxicic aggregation was predicted by the Irie group as shown in Fig. 2[1-2], in which turn structures arise around the 22-23th and 38-39 residues. Also, previous works using the DARR (Fig. 3)[3] and R2[4] techniques suggest the existence of intermolecular parallel beta-sheet structure. In order to further confirm the proposed structure, we carried out solid-state NMR analysis of the amyloid-beta peptide, particularly focusing on the turn structure at the 22-23th amino acid residues.

Fig. 3. A pulse sequence of the DARR technique.

[Investigation of the turn structure at the 22-23th residues]
Carbon-13 2D-exchange DARR NMR experiments have been carried out in a amyloid-beta sample in which all the carbons at the 21-22th residue and C-gamma carbon at the 23th residue were isotopically labeled with carbon-13. As shown in Fig. 4, cross peaks have been observed between Asp-gamma (COOH) and Lys-gamma, Asp-gamma and Lys-delta, which indicates that these carbons are spatially in the vicinity and thus supports the existence of the turn struncture as has been predicted.

Fig. 4. Carbon-13 2D-exchange DARR NMR spectrum obtained in amyloid-beta42.

Interested readers are suggested to refer to a paper on this topic[5].

[1] A. Morimoto, et al., J. Biol. Chem., 279, 52781 (2004).
[2] K. Murakami, et al., J. Am. Chem. Soc., 127, 15168 (2005).
[3] K. Takegoshi, et al., J. Chem. Phys, 118, 2325 (2003).
[4] D. P. Raleigh, et al., Chem.Phys.Lett., 136 ,71 (1988).
[5] Y. Masuda, et al., Bioorg.Med.Chem., 13, 6803 (2005).

[People in contribution
  Azusa Nakanishi


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