β折叠Beta sheet

β 链几乎完全伸展，每个残基沿链延伸约 0.33–0.35 nm，二面角约 φ = −120°、ψ = +120°。相邻链方向相反时为反平行 β 折叠，氢键近似直线、较稳定；方向相同时为平行 β 折叠，氢键略倾斜。侧链交替伸向片层两侧。天然蛋白质中的 β 片层通常带有右手扭曲，常组成 β 桶、β 三明治等结构域。

β-Strands are almost fully extended, about 0.33–0.35 nm per residue, with dihedral angles near φ = −120° and ψ = +120°. When neighbouring strands run in opposite directions the sheet is antiparallel, with nearly linear, stable hydrogen bonds; when they run the same way it is parallel, with slightly angled hydrogen bonds. Side chains point alternately to either face. Natural sheets usually have a right-handed twist and often form domains such as β-barrels and β-sandwiches.

1930 年代阿斯特伯里在拉伸的羊毛和丝的 X 射线图谱中发现了“β 型”结构。1951 年鲍林和科里在提出 α 螺旋的同一年提出了平行与反平行“折叠片”模型。蚕丝中的丝心蛋白以甘氨酸-丙氨酸重复序列形成紧密堆积的反平行 β 折叠，赋予蚕丝高强度和低延展性。

In the 1930s Astbury found a 'β form' in X-ray patterns of stretched wool and silk. In 1951, the same year as the α-helix, Pauling and Corey proposed parallel and antiparallel pleated-sheet models. Silk fibroin forms tightly packed antiparallel β-sheets from glycine–alanine repeats, which gives silk its high strength and low extensibility.

许多神经退行性疾病与蛋白质错误折叠形成富含 β 折叠的淀粉样纤维有关，如阿尔茨海默病中的 β-淀粉样蛋白、帕金森病中的 α-突触核蛋白和朊病毒病中的 PrPˢᶜ。这些纤维呈“交叉 β”结构，β 链垂直于纤维轴排列，极其稳定。普鲁西纳因发现朊病毒获 1997 年诺贝尔生理学或医学奖。

Many neurodegenerative diseases involve misfolded proteins forming β-sheet-rich amyloid fibrils: amyloid-β in Alzheimer's disease, α-synuclein in Parkinson's disease and PrPˢᶜ in prion diseases. The fibrils have a cross-β structure, with strands running perpendicular to the fibre axis, and are extremely stable. Prusiner received the 1997 Nobel Prize in Physiology or Medicine for discovering prions.