Andersson A S, Rilfors L, Bergqvist M, Persson S, Lindblom G
Department of Physical Chemistry, Umea University, Sweden.
Biochemistry. 1996 Aug 27;35(34):11119-30. doi: 10.1021/bi960561w.
A new membrane lipid, monoacyldiglucosyldiacylglycerol (MADGlcDAG), was recently discovered in Acholeplasma laidlawii strain A-EF22, demanding a new study of the biosynthetic regulation, and the phase behavior, of the glucolipids in this organism. The only liquid-crystalline phase formed by MADGlcDAG is a reversed hexagonal phase. A. laidlawii A-EF22 synthesizes four lipids that have the ability to induce the formation of reversed nonlamellar phases: MADGlcDAG, monoglucosyldiacylglycerol (MGlcDAG), monoacylmonoglucosyldiacylglycerol (MAMGlcDAG), and diacylglycerol (DAG). A Cn value of approximately 16 seems to be a critical value for the fractions of these lipids in the membrane: the fractions of MADGlcDAG and MGlcDAG are largest when the Cn values are lower than 16, while the fractions of MAMGlcDAG and DAG are largest when the Cn values are higher than 16. The fraction of nonlamellar-forming lipids was 55 mol% when the Cn value was 14.8 and the degree of unsaturation was 33 mol%. This fraction was reduced to 7 mol% when the Cn value and the degree of unsaturation were increased to 17.8 and 92 mol%, respectively, i.e., at conditions that markedly favor the formation of reversed nonlamellar phases. These observations convincingly show that a balance between lamellar- and nonlamellar-forming lipids is maintained in the membrane and strongly support the validity of the lipid regulation model proposed by us. From earlier biochemical data, obtained with short acyl chains, that were difficult to reconcile with our regulation model, it could be predicted that a lipid ought to be synthesized that assists MGlcDAG to maintain the nonlamellar-forming properties with the short chains. It is shown in the present work that this lipid is MADGlcDAG and that the regulation of the balance between lamellar- and nonlamellar-forming lipids is even more complex and sophisticated in A. laidlawii A-EF22 than previously proposed.
最近在莱氏无胆甾原体A-EF22菌株中发现了一种新的膜脂，单酰基二葡糖基二酰甘油（MADGlcDAG），这需要对该生物体中糖脂的生物合成调控和相行为进行新的研究。MADGlcDAG形成的唯一液晶相是反相六角相。莱氏无胆甾原体A-EF22合成四种具有诱导反相非层状相形成能力的脂质：MADGlcDAG、单葡糖基二酰甘油（MGlcDAG）、单酰基单葡糖基二酰甘油（MAMGlcDAG）和二酰甘油（DAG）。对于这些脂质在膜中的比例而言，约16的Cn值似乎是一个临界值：当Cn值低于16时，MADGlcDAG和MGlcDAG的比例最大，而当Cn值高于16时，MAMGlcDAG和DAG的比例最大。当Cn值为14.8且不饱和度为33 mol%时，非层状相形成脂质的比例为55 mol%。当Cn值和不饱和度分别增加到17.8和92 mol%时，即处于明显有利于反相非层状相形成的条件下，该比例降至7 mol%。这些观察结果令人信服地表明，膜中维持了层状相形成脂质和非层状相形成脂质之间的平衡，并有力地支持了我们提出的脂质调控模型的有效性。从早期使用短酰基链获得的难以与我们的调控模型相协调的生化数据可以预测，应该合成一种脂质来协助MGlcDAG在短链情况下维持非层状相形成特性。本研究表明，这种脂质就是MADGlcDAG，并且在莱氏无胆甾原体A-EF22中，层状相形成脂质和非层状相形成脂质之间平衡的调控比之前提出的更为复杂和精细。
