Mapping free-energy landscapes has proved to be a powerful tool for studying reaction mechanisms. Many complex biomolecular assembly processes, however, have remained challenging to access using this approach, including the aggregation of peptides and proteins into amyloid fibrils implicated in a range of disorders. Here, we generalize the strategy used to probe free-energy landscapes in protein folding to determine the activation energies and entropies that characterize each of the molecular steps in the aggregation of the amyloid-β peptide (Aβ42), which is associated with Alzheimer’s disease. Our results reveal that interactions between monomeric Aβ42 and amyloid fibrils during fibril-dependent secondary nucleation fundamentally reverse the thermodynamic signature of this process relative to primary nucleation, even though both processes generate aggregates from soluble peptides. By mapping the energetic and entropic contributions along the reaction trajectories, we show that the catalytic efficiency of Aβ42 fibril surfaces results from the enthalpic stabilization of adsorbing peptides in conformations amenable to nucleation, resulting in a dramatic lowering of the activation energy for nucleation.

绘制自由能态势图已被证明是研究反应机理的有力工具。然而，许多复杂的生物分子组装过程仍然难以使用这种方法进行接近，包括将肽和蛋白质聚集成淀粉样蛋白原纤维，这牵涉到一系列疾病。在这里，我们概括了用于探测蛋白质折叠中自由能态势的策略，以确定激活能和熵，这些能量和熵表征了与阿尔茨海默氏病相关的淀粉样β肽（Aβ42）聚集中的每个分子步骤。我们的结果表明，在依赖于原纤维的次级成核过程中，单体Aβ42与淀粉样原纤维之间的相互作用从根本上逆转了该过程相对于初级成核的热力学特征，即使这两个过程都从可溶性肽生成聚集体。通过沿着反应轨迹绘制能量和熵的贡献，我们表明Aβ42原纤维表面的催化效率是由吸附肽的焓稳定化而形成的，该构象适合于成核，从而导致成核的活化能大大降低。