通过酶水解揭示酪蛋白胶束的内部结构：小角X射线散射（SAXS）研究
Unmasking the internal structure of casein micelles through enzymatic hydrolysis: A SAXS study
AI总结：
本研究通过时间分辨SAXS探测凝乳酶诱导的酪蛋白胶束胶凝过程，揭示了酪蛋白胶束的内部结构，证实κ-酪蛋白对胶束组织的贡献，为解读复杂软物质散射数据提供新途径。
AI中文摘要：
酪蛋白胶束是研究最为深入的天然缔合胶体之一，是构成乳制品基质基本结构单元的酪蛋白与胶体磷酸钙的超分子组装体。尽管已有大量研究，酪蛋白胶束的内部结构仍存在争议。已知κ-酪蛋白可维持酪蛋白胶束悬浮液的胶体稳定性，但酪蛋白组分与盐的空间组织仍未明确，多种结构模型并存。小角散射是原位探测生物胶体的优选方法，但由于酪蛋白胶束的层级结构，散射数据的解读仍具挑战性。本研究通过探测凝乳酶诱导的酶促胶凝过程中的胶束结构解决该问题，该过程会切割κ-酪蛋白并触发聚集。利用时间分辨SAXS，我们在3纳米至3微米的长度尺度上探测溶胶-凝胶转变全过程的结构变化。首先，我们发现与直觉相反，酶驱动的聚集可揭示酪蛋白胶束的内部组织信息：胶凝过程中比表面积的减小，使此前在对比度匹配条件下观测到的高q结构峰得以显现。其次，我们报告κ-酪蛋白的切割会导致中间尺度结构特征逐渐消失。对消失动力学的分析及与结构模型的对比显示，κ-酪蛋白的切割会诱导胶体多孔亚结构发生渐进式弛豫，为其对胶束组织的贡献提供直接证据。更广泛而言，这些结果表明胶凝过程为获取生物胶体的内部结构提供了独特途径，并为解读复杂软物质体系中的散射数据提供了新视角。
英文摘要：
Casein micelles, one of the most studied natural association colloids, are supramolecular assemblies of caseins and colloidal calcium phosphate that constitute the fundamental building blocks of dairy matrices. Despite extensive investigation, the internal structure of casein micelles remains debated. While $κ$-casein is known to ensure colloidal stability of casein micelle suspension, the spatial organization of casein fractions and salts is still unresolved, and several structural models coexist. Small-angle scattering is a method of choice to probe biological colloids \emph{in situ}, yet interpretation of scattering data remains challenging due to the hierarchical nature of casein micelles. Here we address this issue by probing micelle structure during enzymatic gelation induced by chymosin, which cleaves $κ$-casein and triggers aggregation. Using time-resolved SAXS, we probe structural changes throughout the sol-gel transition over length scales from 3 nm to 3 $μ$m. First, we show that enzyme-driven aggregation, counterintuitively, reveals information about the internal organization of casein micelles: the reduction of specific surface area during gelation unmasks a high-q structural peak previously observed under contrast-matching conditions. Second, we report that $κ$-casein cleavage leads to a gradual disappearance of the structural feature at intermediate scales. Analysis of the disappearance kinetics and comparison to structural models reveal that $κ$-casein cleavage induces a progressive relaxation of the colloidal porous substructure, providing direct evidence for its contribution to micellar organization. More broadly, these results demonstrate that the gelation process provides unique access to the internal structure of biological colloids and offers new perspectives for interpreting scattering data in complex soft-matter systems.
