A theoretical and experimental investigation of the effect of sodium dodecyl sulfate on the structural and conformational properties of bovine β-casein
A theoretical and experimental investigation of the effect of sodium dodecyl sulfate on the structural and conformational properties of bovine β-casein
复制标题
十二烷基硫酸钠对牛β-酪蛋白结构和构象特性影响的理论和实验研究
作者：
Zhou Meng;Xia Yuanyuan;Cao Feng;Li Na;Hemar Yacine;Tang Shangwen;Sun Yang
利用MODELLER和I-TASSER程序，采用同源模建方法预测了牛β-酪蛋白的三维结构，并通过立体化学性质和小角X射线散射分析评价了模型的有效性和可靠性。通过比较两个模型的计算结果，选择了一个最优的β-酪蛋白结构模型，并将其用于后续的分子动力学（MD）分析。对β-酪蛋白在300 K下的水溶液和不同SDS浓度下进行了300 ns的分子动力学模拟。分子动力学模拟结果表明，SDS分子在300 K时对β-酪蛋白的构象具有双重修饰作用。低于CMC（1 mM）的SDS浓度（此时仅存在单体形式的SDS）可诱导β-酪蛋白通过将螺旋转化为无规卷曲而失去其二级结构;但复合物的构象仍与天然β-酪蛋白相当。在10 mM SDS（高于CMC）存在下，β-酪蛋白的螺旋含量沿着增加，无规卷曲减少，结构重排导致构象更加紧凑。后一种变化可能与疏水相互作用有关，疏水相互作用主导C-末端区域的结合，沿着SDS的硫酸酯基团通过静电吸引锚定在带正电荷的N-末端部分上。氢键作用对SDS诱导的β-酪蛋白稳定化起到了补充作用。提出了β-酪蛋白-SDS复合物的“项链-珠子”模型，即胶束在蛋白质疏水位点上成核。
A predicted three-dimensional structure of bovine β-casein was constructed using homology modeling with the aid of MODELLER and I-TASSER programs, with the validity and reliability of the models evaluated according to stereochemical qualities and small angle X-ray scattering. By comparing the results obtained from the two models using the CRYSOL program, an optimal model of the β-casein structure derived from I-TASSER was selected and used in subsequent molecular dynamics (MD) analysis. 300 ns MD simulations of β-casein in water and in the presence of different SDS concentrations at 300 K were performed. The results of the MD simulations indicated that SDS molecules played a dual role in modifying the conformation of β-casein at 300 K. Concentrations of SDS below its CMC (1 mM), at which only the monomer form of SDS was present, induced β-casein to lose its secondary structure by converting helices into random coils; however the conformation of the complex was still comparable with that of native β-casein. In the presence of 10 mM SDS (above its CMC), the helical content of β-casein was increased along with reduced random coils, and the structural rearrangement led to a more compact conformation. The latter change is likely related to the hydrophobic interactions that dominate the binding of the C-terminal region, along with the anchoring of sulfate groups of SDS on the positively charged N-terminal portion via electrostatic attraction. Hydrogen bonding supplemented the SDS-induced stabilization of β-casein. A correlated “necklace and bead” model, in which the micelles nucleate on the protein hydrophobic sites, was proposed for the structure of β-casein–SDS complexes.