Wieslander A, Christiansson A, Rilfors L, Khan A, Johansson L B, Lindblom G
Rev Infect Dis. 1982 May-Jun;4 Suppl:S43-9. doi: 10.1093/clinids/4.supplement_1.s43.
In Acholeplasma laidlawii membranes the ratio between the dominating lipids, monoglucosyldiglyceride (MGDG) and diglucosyldiglyceride (DGDG), depends on temperature, configuration of incorporated fatty acids, and membrane cholesterol content, which affect the molecular geometry of the lipids. MGDG and DGDG have wedge- and rod-like molecular shapes, respectively, that are modifiable. The packing constraints of lipids in amphiphilic aggregates, i.e., the area of the hydrocarbon-water interface and the volume and length of the hydrocarbon chains, are important in determining the aggregate structure. Pure MGDG forms a reversed hexagonal- (HII) phase structure with different acyl chain contents, while DGDG forms a lamellar phase. Depending on the unsaturated acyl chain content in the lipids, an in vitro mixture of MGDG and DGDG forms lamellar or cubic phases at physiologic temperatures. A high degree of cis-unsaturation, large amounts of MGDG and high temperatures favor formation of the cubic phase. Addition of cholesterol corresponding to the maximal amount incorporable into A. laidlawii induces a transition from a lamellar or a cubic phase to a reversed hexagonal phase. Lipid mixtures containing only unsaturated acyl chains are more sensitive to the bilayer-destabilizing effect of cholesterol than are mixtures with equal amounts of saturated and unsaturated acyl chains. The lamellar phase is the only one compatible with a functional biological membrane. Consequently, the balance between lipids that form lamellar and other mesophase structures must keep within certain limits. The cubic and reversed hexagonal structures were discovered under conditions not existing in the living Acholeplasma cell. Thus, the response of A. laidlawii lipid metabolism to external and internal stimuli can be predicted on the basis of molecular shapes and is necessary to the maintenance of optimal membranes stability. The reduced capacity of Acholeplasma membranes to incorporate cholesterol is a consequence of this regulation.
在莱氏无胆甾原体细胞膜中，主要脂质单葡萄糖二甘油酯（MGDG）和双葡萄糖二甘油酯（DGDG）之间的比例取决于温度、所含脂肪酸的构型以及膜胆固醇含量，这些因素会影响脂质的分子几何形状。MGDG和DGDG分别具有楔形和棒状的分子形状，且这些形状是可改变的。脂质在两亲性聚集体中的堆积限制，即烃 - 水界面的面积以及烃链的体积和长度，对于确定聚集体结构很重要。纯MGDG在不同酰基链含量下形成反相六方（HII）相结构，而DGDG形成层状相。根据脂质中不饱和酰基链的含量，MGDG和DGDG的体外混合物在生理温度下形成层状或立方相。高度的顺式不饱和、大量的MGDG和高温有利于立方相的形成。添加与莱氏无胆甾原体中可掺入的最大量相对应的胆固醇会诱导从层状相或立方相转变为反相六方相。仅含不饱和酰基链的脂质混合物比含有等量饱和与不饱和酰基链的混合物对胆固醇的双层去稳定作用更敏感。层状相是唯一与功能性生物膜相容的相。因此，形成层状相和其他中间相结构的脂质之间的平衡必须保持在一定限度内。立方相和反相六方相结构是在莱氏无胆甾原体活细胞不存在的条件下发现的。因此，基于分子形状可以预测莱氏无胆甾原体脂质代谢对外部和内部刺激的反应，这对于维持最佳膜稳定性是必要的。莱氏无胆甾原体细胞膜掺入胆固醇能力的降低是这种调节的结果。
