Désormeaux A, Laroche G, Bougis P E, Pézolet M
Centre de Recherche en Sciences et en Ingénierie des Macromolécules, Département de Chimie, Université Laval, Québec, Canada.
Biochemistry. 1992 Dec 8;31(48):12173-82. doi: 10.1021/bi00163a029.
The effect of cardiotoxin IIa from Naja mossambica mossambica, a small basic protein extracted from snake venom, on dimyristoylphosphatidic acid (DMPA) and on equimolar mixtures of DMPA and dimyristoylphosphatidylcholine (DMPC) has been studied by Fourier transform infrared spectroscopy. The interaction of cardiotoxin with DMPA dispersions decreases both the cooperativity of the phase transition of the lipid and the molecular order of the lipid acyl chains in the gel phase. This effect increases with the proportion of the toxin in the complexes and leads to the total abolition of the phase transition of DMPA at a lipid-to-protein molar ratio of 5. Small-angle X-ray results demonstrate that the structure of the lipid-protein complexes is poorly ordered and gives rise to broad diffusion peaks rather than to well-resolved diffraction patterns. Infrared spectra of oriented cardiotoxin-DMPA films show that the protein is not homogeneously oriented with respect to the bilayer surface. The destabilization of the gel-phase structure of DMPA by cardiotoxin also results in a deeper water penetration in the interfacial region of the lipid since more carbonyl ester groups appear to be hydrogen bonded in the presence of the toxin. The infrared results on the phosphate group vibrations also indicate clearly that the basic residues of cardiotoxin interact strongly with the phosphate group of DMPA that becomes partly ionized at a pH as low as 6.5. The results obtained on the interaction of cardiotoxin with an equimolar mixture of DMPA and DMPC clearly demonstrate the ability of this toxin to induce lateral phase separation in this mixture with one phase containing DMPA-rich domains perturbed by cardiotoxin while the second phase is composed of regions enriched in DMPC. Comparison of the results of the current study with those obtained on other basic proteins and polypeptides suggests that charge-induced phase separation occurs only when the charge density on certain regions of the protein structure is high enough to lead to efficient electrostatic interactions with anionic phospholipids. This condition occurs only when the conformation of the protein or polypeptide is well-ordered at the lipid interface.
对从莫桑比克喷毒眼镜蛇毒液中提取的一种小碱性蛋白——莫桑比克喷毒眼镜蛇心脏毒素IIa，通过傅里叶变换红外光谱研究了其对二肉豆蔻酰磷脂酸（DMPA）以及DMPA与二肉豆蔻酰磷脂酰胆碱（DMPC）等摩尔混合物的影响。心脏毒素与DMPA分散体的相互作用降低了脂质相变的协同性以及凝胶相中脂质酰链的分子有序性。这种效应随着复合物中毒素比例的增加而增强，并且在脂质与蛋白质的摩尔比为5时导致DMPA的相变完全消失。小角X射线结果表明，脂质 - 蛋白质复合物的结构有序性较差，产生的是宽扩散峰而非分辨率良好的衍射图案。取向的心脏毒素 - DMPA膜的红外光谱表明，蛋白质相对于双层表面并非均匀取向。心脏毒素对DMPA凝胶相结构的破坏还导致脂质界面区域的水渗透更深，因为在毒素存在下似乎有更多的羰基酯基团形成氢键。关于磷酸基团振动的红外结果也清楚地表明，心脏毒素中的碱性残基与DMPA的磷酸基团强烈相互作用，而DMPA的磷酸基团在低至6.5的pH下会部分电离。关于心脏毒素与DMPA和DMPC等摩尔混合物相互作用的结果清楚地表明，这种毒素能够在该混合物中诱导横向相分离，其中一个相包含受心脏毒素扰动的富含DMPA的区域，而第二个相由富含DMPC的区域组成。将本研究结果与其他碱性蛋白和多肽的结果进行比较表明，仅当蛋白质结构某些区域的电荷密度高到足以导致与阴离子磷脂发生有效静电相互作用时，才会发生电荷诱导的相分离。这种情况仅在蛋白质或多肽在脂质界面的构象有序时才会出现。
