Department of Biology, Boston College, Chestnut Hill, Massachusetts 02467, USA.
Proteins. 2010 Aug 1;78(10):2306-21. doi: 10.1002/prot.22743.
The sequence KLVFFAE (A beta 16-22) in Alzheimer's beta-amyloid is thought to be a core beta-structure that could act as a template for folding other parts of the polypeptide or molecules into fibrillar assemblies rich in beta-sheet. To elucidate the mechanism of the initial folding process, we undertook combined X-ray fiber/powder diffraction and infrared (IR) spectroscopy to analyze lyophilized A beta 16-22 and solubilized/dried peptide containing nitrile probes at F19 and/or F20. Solubilized/dried wild-type (WT) A beta 16-22 and the peptide containing cyanophenylalanine at F19 (19CN) or at F20 (20CN) gave fiber patterns consistent with slab-like beta-crystallites that were cylindrically averaged around the axis parallel to the polypeptide chain direction. The WT and 19CN assemblies showed 30-A period arrays arising from the stacking of the slabs along the peptide chain direction, whereas the 20CN assemblies lacked any such stacking. The electron density projection along the peptide chain direction indicated similar side-chain dispositions for WT and 20CN, but not for 19CN. These X-ray results and modeling imply that in the assembly of WT A beta 16-22 the F19 side chain is localized within the intersheet space and is involved in hydrophobic contact with amino acids across the intersheet space, whereas the F20 side chain localized near the slab surface is less important for the intersheet interaction, but involved in slab stacking. IR observations for the same peptides in dilute solution showed a greater degree of hydrogen bonding for the nitrile groups in 20CN than in 19CN, supporting this interpretation.
阿尔茨海默病β淀粉样蛋白中的 KLVFFAE（Aβ16-22）序列被认为是β-结构的核心，它可以作为模板，将多肽或分子的其他部分折叠成富含β-折叠的纤维状组装体。为了阐明初始折叠过程的机制，我们进行了 X 射线纤维/粉末衍射和红外（IR）光谱分析，以分析冻干的 Aβ16-22 和含有腈探针的溶解/干燥肽在 F19 和/或 F20 处。溶解/干燥的野生型（WT）Aβ16-22 和在 F19（19CN）或 F20（20CN）处含有氰基苯丙氨酸的肽给出了与板状β-微晶一致的纤维图案，这些微晶在平行于多肽链方向的轴周围被圆柱状平均化。WT 和 19CN 组装体显示出 30-A 周期阵列，这些阵列是由沿着多肽链方向堆叠的板产生的，而 20CN 组装体则没有任何这种堆叠。沿着多肽链方向的电子密度投影表明 WT 和 20CN 的侧链排布相似，但 19CN 则不然。这些 X 射线结果和建模表明，在 WT Aβ16-22 的组装中，F19 侧链定位于片层之间的空间内，并与跨片层空间的氨基酸发生疏水性接触，而定位于板表面附近的 F20 侧链对于片层相互作用不太重要，但参与板层堆叠。对于相同肽在稀溶液中的相同肽的 IR 观察表明，20CN 中腈基团的氢键程度大于 19CN，这支持了这种解释。
