Phosphatidylethanolamine methylation

Kodaki, T., Hosaka, K., Nikawa, J.-I., and Yamashita, S., 1991a, Identification of the upstream activation sequences responsible for the expression and regulation of the PEM1 and PEM2 genes encoding the enzymes of the phosphatidylethanolamine methylation pathway in Saccharomyces cerevisiae. J. Biochem. 109 276-287. 

Kodaki, T., and Yamashita, S., 1987, Yeast phosphatidylethanolamine methylation pathway. J. Biol. Chem. 262 15428-15435. 

Phosphatidylcholine can also be synthesized by the stepwise methylation of phosphatidylethanolamine. Methyl groups are transferred from 5-adenosyl-L-methionine and this pathway is a minor one in animals (20% of liver phosphatidylcholine synthesis but undetectable in other tissues Bell and Coleman, 1980) and plants (Moore, 1982). In those bacteria which contain phosphatidylcholine, it is made by stepwise methylation of phosphatidylethanolamine (Lennarz, 1970). 

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). 

Mason JT, O Leary TJ. Effects of headgroup methylation and acyl chain length on the volume of melting of phosphatidylethanolamines. Biophys. J. 1990 58 277-281. 

A minor pathway to synthesize PC, which is mainly active in liver cells, utilizes the enzyme phosphatidylethanolamine-A-methyltransferase (PEMT), which converts phosphatidylethanolamine (PE) to PC by the subsequent transfer of three methyl groups from S-adenosylmethionine (Vance et al, 1997). The PEMT pathway, which links PE synthesis to PC, was found to be critical for PC homeostasis in the Uver dining choline deficiency (Walkey et al, 1997). 

Phosphatidylcholine can also be produced from phosphatidylethanolamine by methylation which involves the methylating agent S-adenosylmethionine as follows ... 

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. 

Yeast pathways for the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylglycerol are similar to those in bacteria phosphatidylcholine is formed by methylation of phosphatidylethanolamine. 

Hay and Morrison (1971) later presented additional data on the fatty acid composition and structure of milk phosphatidylethanolamine and -choline. Additionally, phytanic acid was found only in the 1-position of the two phospholipids. The steric hindrance presented by the four methyl branches apparently prevents acylation at the 2-position. The fairly even distribution of monoenoic acids between the two positions is altered when the trans isomers are considered, as a marked asymmetry appears with 18 1 between the 1- and 2-positions of phosphatidylethanolamine, but not of phosphatidylcholine. Biologically, the trans isomers are apparently handled the same as the equivalent saturates because the latter have almost the same distribution. There are no appreciable differences in distribution of cis or trans positional isomers between positions 1 and 2 in either phospholipid. Another structural asymmetry observed is where cis, cis nonconjugated 18 2s are located mostly in the 2-position in both phospholipids. It appears that one or more trans double bonds in the 18 2s hinders the acylation of these acids to the 2-position. 

Phosphatidylcholine, which is rarely present in bacteria, is formed in eukaryotes from phosphatidylethanolamine by three consecutive steps of methylation... 

As before, the phosphatidylethanolamine sample in chloroform-methanol (1 10, v/v) is treated with 0.5 N NaOH in methanol for 20 min at room temperature. The reaction is stopped by the addition of an equivalent amount of 6 N HCI. Subsequent extraction of the mixture by the Bligh-Dyer method will ultimately yield a chloroform soluble fraction (containing the methyl esters) and a water-soluble phase (containing the glycerophosphoethanola-mine). Under these conditions, the reaction is usually quantitative this can be... 

Phospholipase A2 Action. As in the case of phosphatidylcholine, the above-mentioned phospholipases will attack only the sn-3 form of naturally occurring (as well as synthetic) phosphatidylethanolamine. The products are, of course, lysophosphatidylethanolamine (1 -6>-acyl-2-lyso-.rn-glycero-3-phosphoethanolamine) and the fatty acids (liberated from the sn-2 position). The latter can be analyzed for composition and structure, as the methyl esters, by gas-liquid chromatography coupled with mass spectrometry. Usually these acyl groups are largely the unsaturated types. 

The major (salvage) pathways for the formation of phosphatidylcholine and ethanolamine are illustrated in Figure 19.16. Free (dietary) choline and etha-nolamine are converted to their CDP derivatives, which then react with diacyl-glycerol to form phosphatidylcholine and ethanolamine. In the lungs, another pathway forms dipalmitoyl phosphatidylcholine, a powerful surfactant. Phos-phatidylethanolamine may be methylated by S-adenosylmethionine (SAM see Chapter 20) to yield phosphatidylcholine. The reaction is catalyzed by two enzymes the first methyl group is transferred via phosphatidylethanolamine N-methyltransferase I. The other two methyl groups are transferred by phosphatidylethanolamine N-methyltransferase II. Some authorities believe that the two enzymes are identical. It has also been proposed that methylation of phospha-... 

Methionine is intimately related to lipid metabolism in the liver. Methionine deficiency is one of the causes of the fatty liver syndrome. Lack of methionine prevents the methylation of phosphatidylethanolamine to phosphatidylcholine, resulting in an ability by the liver to build and export very low density lipoprotein. The syndrome can be treated by the administration of choline, and for this reason, choline has often been referred to as the lipotropic factor. 

Phosphatidylserine arises by an exchange of the ethanolamine residue of phosphatidylethanolamine for a seryl group. Decarboxylation of the serine of phosphatidylserine reforms phosphatidylethanolamine. Three successive methylation reactions convert phosphatidylethanolamine to phosphatidylcholine. 5-Adenosyl-methionine is the methyl-group donor (Chap. 15) (see Fig. 13-14). 

All the cosubstrates that occur in drug conjugation (Figure 2.28) have other roles in metabolism e.g., UDP-glucuron-ic acid and PAPS provide acidic groups for the S5mthesis of mucopolysaccharides, whereas S-adenosylmethionine provides methyl groups for the synthesis of phosphatidylcholine from phosphatidylethanolamine. 

Phosphatidylcholine can be synthesized by the pathway shown in Figure 14.3. Decarboxylation of phosphatidylserine to phosphatidylethanolamine (cephalin) is followed by methylation in which S-adenosylmethionine is the methyl donor to yield successively the relatively rare mono- and dimethyl derivatives, then phosphatidylcholine. 

PE (trivial name cephalin ) also has a net neutral charge (Fig. 1). PE is widespread and usually the second most abundant phospholipid in animal and plant membranes. It is also the main lipid component of microbial membranes. In animal tissues, phosphatidylethanolamine may exist in diacyl, aUcylacyl and alkenylacyl forms. Moreover, animal phosphatidylethanolamine usually contains higher levels of arachidonic and docosahex-aenoic acids in comparison with the other zwitterionic phospholipid, PC. The partly methylated derivatives of PE (phos-phatidyl-A -methyl-ethanolamine, phosphatidyl-Af -dimethyl-... 

FIGURE 6.2 Phosphatidylcholine (PC) synthesis by the CDP-ethanolamine pathway. Structures of ethanolamine and choline. The nucleotide moiety S-adenosyl methionine (SAM) is requiivct to transfer the methyl group of methionine to phosphatidylethanolamine (J E) to form PC. In the process, SAM is converted to S-adenosyl homocysteine (SAH). The nitrogen atom of PC has four covalent bonds and is called a quaternary amine. It bears a positive charge that is not influenced by changes in the pH of the suitoimding fluids. The PE methyltran.sferase pathway of I C synthesis occurs only in the liver. 

A transition induced in the lipid bilayer will result in a conformation "pressure" on the protein so tiiat it can adapt to the changed bilayer structure. A consequence of this is the possibility of controlling lipid composition by an on/off switch of enzymes responsible for lipid modifications. For example the methyl transferase enzymes that convert phosphatidylethanolamines into phosphatidylcholines might be controlled in this way. (PE favours whereas PC favours the conformation and should thus be expected to switch on and switch off the enzyme activity). 

D. Phosphatidylcholine may also be produced when S-adenosylmethionine donates methyl groups to phosphatidylethanolamine. 

D. In the absence of dietary choline, PC can be synthesized de novo from glucose. The last step in this pathway involves methylation of phosphatidylethanolamine. 

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