CDP-ethanolamine

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanol-amine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PP, hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26). 

FIGURE 25.19 Diacylglycerol and CDP-diacylglycerol are the principal precursors of glycerolipids in eukaryotes. Phosphatidylethanolamine and phosphatidylcholine are formed by reaction of diacylglycerol with CDP-ethanolamine or CDP-choline, respectively. 

Figure 1. Synthetic pathway for PS and PE in mammalian cells. The major steps occuring in the synthesis and interconversion of PS and PE are shown. The PS synthases condense serine with a phosphatidyl moiety derived from PC and PE. The nascent PS can be converted to PE by decarboxylation. PE can also be formed by transfer of a phosphoethanolamine moiety from CDP-ethanolamine to diacylglycerol via the Kennedy pathway. The abbreviations used are PC, phosphatidylcholine PS, phosphatidylserine PE, phosphatidylethanolamine DG, diacylglycerol PSD, phosphatidylserine decarboxylase PSS, PS synthase.

Finally PC is made by releasing the CMP group in the process of fusing phosphocholine to diacylglycerol (DAG) by the enzyme CDP-choline 1,2-diacylglycerol choUnephosphotransferase (CPT McMaster and Bell, 1997). One cloned isoform of CPT seems specific for CDP-choUne, whereas another CPT also can synthesize PE from CDP-ethanolamine and DAG (Henneberry and McMaster, 1999 Heimeberry et al., 2000). 

PE can be generated by several mechanisms. The CDP-ethanolamine route may be important under physiological conditions in vivo when an abundant supply of ethanolamine is present. The CDP-ethanolamine pathway is very similar to the CDP-choline pathway described above (Tijburg et al. 1989 Bladergroen and Van Golde, 1997). The pathway comprises (i) an ethanolamine kinase (EK) which converts ethanolamine to phosphoethanolamine, (ii) an CTP phosphoethanolamine cytidyltransferase... 

ET), which catalyzes the formation of CDP-ethanolamine, and (iii) an ethanolaminephosphotransferase (EPT), which finally synthesizes PE from DAG and CDP-ethanolamine. As discussed rmder PC synthesis, four enzymes have been cloned that can phosphorylate ethanolamine, two of which preferentially use ethanolamine as a substrate, and two which are more specific for choline. Only one isoform of ET has been cloned, which contains two active sites, but seems to be not as strictly regulated compared to its counterpart CT (Bladergroen et al, 1999a). 

H]-ethanolamine into PE was inhibited by C6-ceramide in a dose and time-dependent manner (b) delayed disappearance of label from CDP-choline and CDP-ethanolamine in pulse-chase experiments indicated impaired conversion of these CDP-metaboUtes to PC and PE by CPT and EPT, respectively (c) the activities of CPT and EPT are decreased upon C6-ceramide treatment (see Eigure 2 adapted from Bladergroen et al. 1999b). In contrast to BHK cells the activity of CT was not affected significantly in rat-2 fibroblasts by short-chain ceramides. 

Figure 2. Effect of Ca-ceramide on phosphatidylcholine and phosphatidylethanolamine synthesis in rat-2 fibroblasts. Cells were treated for 2 h in the absence (open bars) or presence (hatched bars) of 25pM C6-ceramide and the following parameters were determined i) the incorporation of [ H]choline and [ H]ethanolamine into phosphatidylcholine (PC) and phosphatidylethanolamine (PE), respectively (panel A) and in CDP-choline and CDP-ethanolamine, respectively (panel B) and ii) the in vitro activity of choline- and ethanolaminephosphotransferase (CPT and EPT) (panel C).

Treatment of Rat-2 fibroblasts with various cell-permeable ceramides resulted, as discussed above, in a rapid inhibition of phospholipid synthesis. An intriguing observation was that C2-ceramides inhibited PC biosynthesis without affecting the synthesis of PE via the CDP-ethanolamine route. C2-ceramide are well known inducers of apoptosis and inhibit DNA- and protein synthesis (Obeid et al, 1993 Jayadev et al, 1995). C2-ceramide induced apoptosis in the rat-2 fibroblasts (Houweling et al, unpublished data), suggesting that inhibition of PE biosynthesis is not a prerequisite for apoptosis. 

Transfer of a phosphocholine residue to the free OH group gives rise to phosphatidylcholine (lecithin enzyme l-alkyl-2-acetyl-glycerolcholine phosphotransferase 2.7.8.16). The phosphocholine residue is derived from the precursor CDP-choline (see p. 110). Phos-phatidylethanolamine is similarly formed from CDP-ethanolamine and DAG. By contrast, phosphatidylserine is derived from phosphatidylethanolamine by an exchange of the amino alcohol. Further reactions serve to interconvert the phospholipids—e.g., phosphatidylserine can be converted into phosphatidylethanolamine by decarboxylation, and the latter can then be converted into phosphatidylcholine by methylation with S-adenosyl methionine (not shown see also p. 409). The biosynthesis of phosphatidylino-sitol starts from phosphatidate rather than DAG. 

Phosphatidylcholine (l Ethanolamine CDP-ethanolamine Phosphatidylethanolamine (l Palmitoyl CoA Phosphatidylserine Sphingosine Serine... 

This enzyme [EC 2.7.8.1] converts CDP-ethanolamine and 1,2-diacylglycerol into CMP and phosphatidyletha-nolamine. 

Synthesis of PE and PC from preexisting choline and ethanolamine These synthetic pathways involve the phosphorylation of choline or ethanolamine by kinases, followed by conversion to the activated form, CDP-choline or CDP-ethanolamine. Finally, choline-phosphate or ethanolamine-phosphate is transferred from the nucleotide (leaving CMP) to a molecule of diacylglycerol (see Figure 17.5). 

Choline and ethanolamine are activated in much the same way as are sugars. For example, choline can be phosphorylated using ATP (Eq. 17-58, step a) and the phosphocholine formed can be further converted (Eq. 17-58, step b) to cytidine diphosphate choline. Phosphocholine is transferred from the latter onto a suitable acceptor to form the final product (Eq. 17-58, step c). Tire polymerization pattern differs from that for polysaccharide synthesis. When the sugar nucleotides react, the entire nucleoside diphosphate is eliminated (Eq. 17-56), but CDP-choline and CDP-ethanolamine react with elimination of CMP (Eq. 

Fig. 21-5, are also used for formation of both phosphatidylcholine and phosphatidylethanolamine. In both cases, the free base, choline, or ethanolamine180a b is phosphorylated with ATP. Choline phosphate formed in this manner is then converted by reaction with CTP to CDP-choline (Eq. 17-58).181 Phosphatidylcholine is formed from this intermediate1813/b while CDP-ethanolamine is used to form phosphatidylethanolamine (Fig. 21-5). These synthetic reactions occur within cell nuclei as well as on surfaces of cytoplasmic membranes.1810...

Additional regulation of phosphatidylcholine and phosphatidylethanolamine biosynthesis occurs at the second step in the biosynthetic sequence (see fig. 19.4) where either CDP-choline or CDP-ethanolamine are made. For phosphatidylcholine biosynthesis, the activity of CTP phos-phocholine cytidylyltransferase (which makes CDP-choline) is governed by an unusual mechanism. The enzyme... 

Synthesis of phosphatidylcholine and phosphatidylethanolamine begins with activation of choline (or ethanolamine) with ATP via choline kinase to yield phosphocholine (phosphoethanolamine) + ADP the activated base is transferred via CTP and phosphocholine cytidyl transferase to form CDP-choline (CDP-ethanolamine) and PPi. The base is then transferred to the sn-3 of diacylglycerol via phosphocholine diacylglycerol transferase to yield phosphatidylcholine (phosphatidylethanolamine) + CMP. The cytidyl transferase is believed to be the rate-limiting or regulatory step in the pathway. Phospha-tidylserine is formed by a direct transfer and substitution of serine for ethanolamine in phosphatidylethanolamine. Phosphatidyl serine can be dec-arboxylated to form phosphatidylethanolamine. 

In mammals, phosphatidyl ethanolamine can also be synthesized from ethanolamine through the formation of CDP-ethanolamine. In this case, the alcohol ethanolamine is phosphorylated by ATP to form the precursor, phosphorylethanolamine. This precursor then reacts with CTP to form the activated alcohol, CDP-ethanolamine. The phosphorylethanolamine unit of CDP-ethanolamine is then transferred to a diacylglycerol to form phosphatidyl ethanolamine. 

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