Lefèvre T, Subirade M
Centre de Recherches en Sciences et Technologie du Lait (STELA), Département d'Alimentation et de Nutrition, Faculté des Sciences de l'Agriculture et de l'Alimentation, Pavillon Paul Comtois, Université Laval, Sainte-Foy, QC, Canada G1K 7P4.
Biochim Biophys Acta. 2001 Sep 10;1549(1):37-50. doi: 10.1016/s0167-4838(01)00242-4.
Interactions between beta-lactoglobulin (beta-lg) and dimyristoylphosphatidylglycerol (DMPG) bilayers were studied using one- and two-dimensional infrared spectroscopy above (pD 7.4) and below (pD 4.4) the protein's (beta-lg's) isoelectric point (pI=5.2). The aim of the study was threefold: (1) gain a better understanding of beta-lg-phospholipid interaction; (2) provide information relative to the structure of beta-lg as it interacts with membranes; (3) determine whether the conformational modifications of the protein in the presence of lipids are strictly caused by thermal effects or whether they are modulated by the chain-melting phase transition. At pD 7.4, the lipid thermotropism, the acyl-chain order, and the membrane interfacial region were essentially unaffected by the presence of beta-lg, whereas the protein amide I region showed dramatic alterations. The results suggested the predominance of beta-sheets and alpha-helix elements, with a lost of structural integrity. At pD 4.4, beta-lg induced an approximately 2 degrees C downshift of the transition temperature, whereas the conformational order of the lipid chain decreased in the gel phase and increased in the liquid-crystalline phase. The hydration state of the DMPG C==O groups increased in the liquid-crystalline phase. The conformation of beta-lg at pD 4.4 in the presence of DMPG showed similarities with that observed at pD 7.4, but an increase in the alpha-helix content and a reduced thermal stability were noticed. In contrast to the protein alone, beta-lg aggregates in the presence of DMPG at pD 4.4 above 50 degrees C. At both pD values, the charged surface of the membrane seemed to be the main factor for inducing protein conformational changes by altering the intramolecular interactions that stabilize the native structure. However, protein incorporation within the membrane seemed to be involved at pD 4.4. The two-dimensional analysis performed with spectra recorded upon heating showed that spectral intensity changes at pD 4.4 and 7.4 occurred at the same frequencies in the amide I' region. The heat-induced structural changes of beta-lg were not correlated with the conformational modifications of the phospholipids along the phase transition, indicating that the thermal behavior of the protein was not modulated by the lipid chain melting, but rather represented the heat-induced protein rearrangement in the presence of DMPG.
利用一维红外光谱和二维红外光谱，研究了β-乳球蛋白（β-lg）与二肉豆蔻酰磷脂酰甘油（DMPG）双层膜在高于（pD 7.4）和低于（pD 4.4）蛋白质（β-lg）等电点（pI = 5.2）时的相互作用。该研究的目的有三个：（1）更好地理解β-lg-磷脂相互作用；（2）提供与β-lg与膜相互作用时的结构相关的信息；（3）确定在脂质存在下蛋白质的构象修饰是否严格由热效应引起，或者它们是否受链熔化相变的调节。在pD 7.4时，脂质的热致性、酰基链顺序和膜界面区域基本上不受β-lg存在的影响，而蛋白质的酰胺I区域则显示出显著变化。结果表明β-折叠和α-螺旋元件占主导地位，结构完整性丧失。在pD 4.4时，β-lg使转变温度下降约2℃，而脂质链的构象顺序在凝胶相中降低，在液晶相中增加。DMPG C==O基团的水合状态在液晶相中增加。在DMPG存在下，pD 4.4时β-lg的构象与pD 7.4时观察到的构象相似，但α-螺旋含量增加，热稳定性降低。与单独的蛋白质相比，在pD 4.4、50℃以上时，DMPG存在下β-lg会聚集。在两个pD值下，膜的带电表面似乎都是通过改变稳定天然结构的分子内相互作用来诱导蛋白质构象变化的主要因素。然而，在pD 4.4时，似乎涉及蛋白质掺入膜内。对加热时记录的光谱进行的二维分析表明，pD 4.4和7.4时酰胺I'区域的光谱强度变化发生在相同频率。β-lg的热诱导结构变化与相变过程中磷脂的构象修饰无关，这表明蛋白质的热行为不受脂质链熔化的调节，而是代表了在DMPG存在下热诱导的蛋白质重排。
