CUBIC PHASES OF LIPID-CONTAINING SYSTEMS - ELEMENTS OF A THEORY AND BIOLOGICAL CONNOTATIONS
CUBIC PHASES OF LIPID-CONTAINING SYSTEMS - ELEMENTS OF A THEORY AND BIOLOGICAL CONNOTATIONS
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DOI：
10.1006/jmbi.1993.1053
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发表时间：
1993-01-20
影响因子：
5.6
通讯作者：
DELACROIX, H
中科院分区：
文献类型：
--
作者：
LUZZATI, V;VARGAS, R;DELACROIX, H
最近的研究表明，在含脂质的体系中，六种立方相中有两种是胶束结构，一种（Q223）是I型，另一种（Q227）是II型。这两个阶段的胶束属于两个不同的类，这些每一类被集中在空间群的特殊位置之一。从化学角度来看，相Q227似乎需要水混溶性和水不混溶性脂质的非均质混合物，而相Q223则是用化学纯脂质观察到的。此外，在极性/非极性界面处测量的面积/体积比在相Q223的两种类型的胶束中取相同的值，在那些相Q227中取不同的值，这与面积/分子比与脂质组分的化学活性密切相关的概念一致。胶束相的拓扑性质与双连续相的拓扑性质有很大的不同。双连续立方相（Q230，Q224，Q229）常被看作是无限周期极小曲面（IPMS）的典范。一些作者已经概括了这一概念，并在IPMS中寻求一个统一的理论，作为整个脂质多态性领域的基础。这些类比在表面的数学概念和界面的物理概念之间产生了一些混淆。几个电子密度图的文件，从IPMS的极性/非极性界面的距离。地图还显示，一些几何奇点（点，线，表面）的结构与CH 3的轨迹，链的末端和水矩阵的中心，即与短程无序最高的区域相一致。我们引入混沌区来表示这些区域。在所有的脂质相的混沌区被发现占据特殊的几何位置，无论是相关的对称元素或IPMS。因此，似乎采用短程无序的有序处置比使极性/非极性界面的面积最小化在能量上更有利。最后，关于脂质多态性可能的生物学意义，强调的一点是，在与过量水平衡中观察到的相（这些相也是最有可能的生物学作用的候选者）中，那些具有立方对称性的相值得特别注意。我们以前曾涉及的双连续相（Q224）的脂肪的消化和嗜热嗜酸古细菌的外膜结构的猜测。其中一种胶束立方相（Q227）也很有趣：一方面，它的化学组成与脂质在生物膜中可能发生的酶促降解有关;另一方面，它的物理结构使该相不透水。由于相Q224的结构与相Q227的结构密切相关，因此可以想象“修补穿孔”过程，其中脂质的酶攻击可能导致膜泄漏，也可能诱导局部转变为水密结构，最终阻止泄漏。这一过程可以在各种生物学意义上的情况下发挥作用。
It has recently been shown that the structure of two of the six cubic phases so far identified in lipid-containing systems is micellar, one (Q223) of type I, the other (Q227) of type II. The micelles of both phases belong to two distinct classes, those of each class being centred at one of the special positions of the space group. From the chemical viewpoint, phase Q227seems to require a heterogenous mixture of water-miscible and water-immiscible lipids, whereas phase Q223has been observed with chemically pure lipids. Also, the area/volume ratio measured at the polar/apolar interface takes the same value in the two types of micelles of phase Q223, different values in those phase Q227, in keeping with the notion that the area/molecule ratio is closely related to the chemical activity of the lipid components. The topological properties of the micellar phases are profoundly different from those of the bicontinuous phases. The bicontinuous cubic phases (Q230, Q224, Q229) are often presented as paradigms of the infinite periodic minimal surfaces (IPMS). Some authors have generalized that notion and sought in the IPMS a unified theory underlying the entire field of lipid polymorphism. These analogies entertain some confusion between the mathematical concept of surface and the physical notion of interface. A few electron density maps are presented to document the distance that separates the polar/apolar interfaces from the IPMS. The maps also show that some of the geometric singularities (points, lines, surfaces) of the structures coincide with the locus of the CH3, ends of the chains and with the very centre of the water matrix, i.e. with the regions where the short-range disorder is highest. We introduce the expressionchaotic zonesto designate these regions. In all the lipid phases the chaotic zones are found to occupy special geometric positions, either related to the symmetry elements or to the IPMS. It thus appears that it is energetically more advantageous to adopt an orderly disposal of the disposal of the short-range disorder than to minimize the area of the polar/apolar interfaces. Finally, regarding the possible biological significance of lipid polymorphism, the point is stressed that among the phases that are observed in equilibrium with excess water (these phases are also the most likely candidates for a biological role) those with a cubic symmetry deserve special attention. We have previously involved one of the bicontinuous phases (Q224) in speculations regarding the digestion of fats and the structure of the outer membrane of thermoacidophilic archaebacteria. One of the micellar cubic phases (Q227) is also interesting: on the one hand, its chemical composition is related to the enzymatic degradations that lipids may undergo in biological membranes; its physical structure, on the other hand, makes that phase impervious to water. Since the structure of phase Q224is closely related to that of phase Q227, a "patch-the-puncture" process can be imagined whereby an enzymatic attack of the lipid, leading potentially to a leak in the membrane, might also induce a local transition to a water-tight structure, eventually stopping the leak. That process could play a role in a variety of circumstances of biological interest.