Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, D-69118 Heidelberg, Germany.
Phys Chem Chem Phys. 2013 Jun 14;15(22):8765-71. doi: 10.1039/c3cp00067b. Epub 2013 Apr 30.
Stacking of β-sheets results in a protein super secondary structure with remarkable mechanical properties. β-Stacks are the determinants of a silk fiber's resilience and are also the building blocks of amyloid fibrils. While both silk and amyloid-type crystals are known to feature a high resistance against rupture, their structural and mechanical similarities and particularities are yet to be fully understood. Here, we systematically compare the rupture force and stiffness of amyloid and spider silk poly-alanine β-stacks of comparable sizes using Molecular Dynamics simulations. We identify the direction of force application as the primary determinant of the rupture strength; β-sheets in silk are orientated along the fiber axis, i.e. the pulling direction, and consequently require high forces in the several nanoNewton range for shearing β-strands apart, while β-sheets in amyloid are oriented vertically to the fiber, allowing a zipper-like rupture at sub-nanoNewton forces. A secondary factor rendering amyloid β-stacks softer and weaker than their spider silk counterparts is the sub-optimal side-chain packing between β-sheets due to the sequence variations of amyloid-forming proteins as opposed to the perfectly packed poly-alanine β-sheets of silk. Taken together, amyloid fibers can reach the stiffness of silk fibers in spite of their softer and weaker β-sheet arrangement as they are missing a softening amorphous matrix.
β-折叠的堆积导致蛋白质具有显著的机械性能的超二级结构。β-堆叠是丝纤维弹性的决定因素，也是淀粉样纤维的构建块。虽然已知丝和淀粉样晶体都具有抗破裂的高阻力，但它们的结构和机械相似性和特殊性尚未完全理解。在这里，我们使用分子动力学模拟系统地比较了具有可比尺寸的淀粉样和蜘蛛丝聚丙氨酸β-堆叠的断裂力和刚性。我们确定力的施加方向是断裂强度的主要决定因素；丝中的β-折叠沿着纤维轴取向，即拉伸方向，因此需要在几纳米牛顿范围内的高力才能将β-链分开，而淀粉样中的β-折叠垂直于纤维，允许在亚纳米牛顿力下发生拉链式断裂。使淀粉样β-折叠比其蜘蛛丝对应物更软和更弱的第二个因素是由于淀粉样形成蛋白的序列变化导致β-折叠之间的侧链堆积不理想，而不是丝中完美堆积的聚丙氨酸β-折叠。总的来说，尽管淀粉样纤维的β-折叠排列较软且较弱，但由于缺少软化的无定形基质，它们可以达到丝纤维的刚性。
