Polyglutamine Fibrils: New Insights into Antiparallel β‑Sheet Conformational Preference and Side Chain Structure

Understanding the structure of polyglutamine
(polyQ) amyloid-like
fibril aggregates is crucial to gaining insights into the etiology
of at least ten neurodegenerative disorders, including Huntington’s
disease. Here, we determine the structure of D2Q10K2 (Q10) fibrils using ultraviolet resonance Raman (UVRR)
spectroscopy and molecular dynamics (MD). Using UVRR, we determine
the fibril peptide backbone Ψ and glutamine (Gln) side chain
χ3 dihedral angles. We find that most of the fibril
peptide bonds adopt antiparallel β-sheet conformations; however,
a small population of peptide bonds exist in parallel β-sheet
structures. Using MD, we simulate three different potential fibril
structural models that consist of either β-strands or β-hairpins.
Comparing the experimentally measured Ψ and χ3 angle distributions to those obtained from the MD simulated models,
we conclude that the basic structural motif of Q10 fibrils is an extended
β-strand structure. Importantly, we determine from our MD simulations
that Q10 fibril antiparallel β-sheets are thermodynamically
more stable than parallel β-sheets. This accounts for why polyQ
fibrils preferentially adopt antiparallel β-sheet conformations
instead of in-register parallel β-sheets like most amyloidogenic
peptides. In addition, we directly determine, for the first time,
the structures of Gln side chains. Our structural data give new insights
into the role that the Gln side chains play in the stabilization of
polyQ fibrils. Finally, our work demonstrates the synergistic power
and utility of combining UVRR measurements and MD modeling to determine
the structure of amyloid-like fibrils.

洞悉聚谷氨酰胺（polyQ）类淀粉样纤维聚集体的结构对于揭示至少十种神经退行性疾病的病因至关重要。在本研究中，我们运用紫外共振拉曼光谱（UVRR）和分子动力学（MD）技术确定了D2Q10K2（Q10）纤维的结构。通过UVRR技术，我们测定了纤维肽骨架Ψ和谷氨酰胺（Gln）侧链χ3二面角。研究发现，大部分纤维肽键采取反平行β-折叠构象；然而，存在一小部分肽键处于平行β-折叠结构中。利用MD技术，我们模拟了三种不同的潜在纤维结构模型，这些模型由β-折叠或β-发夹结构组成。通过将实验测得的Ψ和χ3角度分布与MD模拟模型获得的分布进行比较，我们得出结论：Q10纤维的基本结构基元是扩展的β-折叠结构。重要的是，我们的MD模拟结果表明，Q10纤维的反平行β-折叠在热力学上比平行β-折叠更稳定。这解释了为何聚谷氨酰胺纤维优先采取反平行β-折叠构象，而非如大多数淀粉样肽那样采取对齐的平行β-折叠构象。此外，我们首次直接确定了Gln侧链的结构。我们的结构数据为谷氨酰胺侧链在聚谷氨酰胺纤维稳定化中所起的作用提供了新的见解。最后，我们的研究展示了将UVRR测量与MD建模相结合以确定淀粉样纤维结构的协同作用力和实用性。