In Fig. 7.1 it was seen that phosphatidic acid can be converted into CDP-diacylglycerol (CDP-DAG) using another cytidylyltransferase (CDP-di-acylglycerol synthase, CDP-DAG synthase). The CDP-DAG is an important intermediate for the acidic phosphoglycerides, phosphatidylinositol, phosphatidylglycerol and diphosphatidylglycerol in animals and plants. In yeast it also has a role in phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine production, whereas in E. coli, all phosphoglycerides are formed via CDP-diacylglycerol.
We will begin by considering the situation in animals and plants for which the main reactions are shown in Fig. 7.3. The CDP-diacylglycerol intermediate can react with either myo-inositol to give rise to the various inositol-containing phospholipids or with glycerol 3-phosphate. Phosphatidylinositol is the main inositol-containing phospholipid in all eukaryotes. It can be further phosphorylated and the
4-kinase and 5-kinase enzymes are the most important quantitatively (Fig. 7.3). Another important phosphorylation is that catalysed by a 3-kinase (Section 7.10) where a series of inositol-lipids, important for signalling, is produced.
The reaction of glycerol 3-phosphate with CDP-diacylglycerol forms an intermediate that does not accumulate because it rapidly loses a phosphate to yield phosphatidylglycerol (Fig. 7.3). In animals, almost all of the phosphatidylglycerol is converted into diphosphatidylglycerol (cardiolipin), which accumulates as an important membrane constituent of the inner mitochondrial membrane. An exception is the epithelial cell of the lung that produces pulmonary surfactant (Section 7.7) containing significant amounts of phosphatidylglycerol.
The formation of phosphatidylglycerol in plants and algae is most active in the chloroplast where, of course, phosphatidylglycerol is the only significant phosphoglyceride (Section 6.2.1).
In yeast, the CDP-diacylglycerol pathway is not only important for acidic phosphoglyceride formation but is also the main way in which the zwitterionic phospholipids are made. A key reaction in the latter connection is the reaction of CDP-diacylglycerol with serine to produce phosphati-dylserine. The overall scheme for phosphoglyceride synthesis in yeasts is shown in Fig. 7.4.
After CDP-diacylglycerol has been converted into phosphatidylserine, a decarboxylation yields phosphatidylethanolamine, which can then be methylated three times to produce phosphati-dylcholine. These reactions are described in the following section in more detail and are typical of bacterial systems. Indeed, phospholipid synthesis in yeast can be regarded as a more complex version of E. coli metabolism, but where the modifications have not gone quite as far as in animals or plants.
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