Self-assembly of bovine β-casein below the isoelectric pH
Self-assembly of bovine β-casein below the isoelectric pH
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DOI：
10.1021/jf072630r
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发表时间：
2008-03-26
影响因子：
6.1
通讯作者：
Danino, Dganit
中科院分区：
文献类型：
--
作者：
Portnaya, Irina;Ben-Shoshan, Einav;Danino, Dganit
β-酪蛋白是一种本质上非结构化的两亲性蛋白质，在中性pH下自组装成胶束。本文报道了β-酪蛋白在酸性条件下也自组装成胶束。的蛋白质缔合行为和胶束特性在pH 2.6，远低于PL，提出。发现pH强烈影响胶束的形状和尺寸。低温透射电子显微镜（cryo-TEM）实验显示，在酸性条件下，盘状胶束的长度为20-25 nm，高度为3.5 nm。在这些条件下通过沉降平衡测定聚集数为6。等温滴定量热实验验证了低于PL的关联，并允许确定胶束化焓，临界胶束浓度，和胶束化相对协同性（MR）。在低于临界胶束化浓度（CIVIC）的浓度下的小角X-射线散射结果表明，单体蛋白质可能是在一个premelt小球状态在低pH值。在酸性和中性pH值的蛋白质电荷的计算揭示了类似的高净电荷，但相当大的差异，电荷分布沿着的蛋白质骨架。总的来说，结果表明，β-酪蛋白在低pH下是两亲性的，但是电荷沿着蛋白质链的分布产生了影响胶束组织的堆积约束，导致在高于CIVIC的浓度下形成盘状胶束。
beta-Casein is an intrinsically unstructured amphiphilic protein that self-assembles into micelles at neutral pH. This paper reports that beta-casein self-organizes into micelles also under acidic conditions. The protein association behavior and micelle characteristics at pH 2.6, well below the pl, are presented. The pH was found to strongly affect the micelle shape and dimensions. Cryogenic transmission electron microscopy (cryo-TEM) experiments revealed disk-like micelles of 20-25 nm in length and similar to 3.5 nm in height in acidic conditions. An aggregation number of 6 was determined by sedimentation equilibrium under these conditions. Isothermal titration calorimetry experiments verified the association below the pl and allowed determination of the micellization enthalpy, the critical micellar concentration, and the micellization relative cooperativity (MR). Small-angle X-ray scattering results at concentrations below the critical micellization concentration (CIVIC) suggest that the monomeric protein is likely in a premolten globule state at low pH. Calculations of the protein charge at acidic and neutral pH reveal a similar high net charge but considerable differences in the charge distribution along the protein backbone. Overall the results show that beta-casein is amphiphilic at low pH, but the distribution of charge along the protein chain creates packing constraints that affect the micelle organization, leading at concentrations above the CIVIC to the formation of disk micelles.