Synthesis of Phospholipids and Sphingolipids

Phosphatidylcholine, a major phospholipid constituent of membranes and lipoproteins, is synthesized de novo in liver cells. The synthesis occurs on the ER and is linked, through 1,2-diacylglycerol, with the synthesis of triacylglycerol. Three compounds specifically involved in the synthesis of phosphatidylcholine are  

Note that the enzymes CDP-choline 1,2-diacylglycerol transferase in phospholipid synthesis and acyl-CoA transferase in triglyceride synthesis have the common substrate 1,2-diacylglycerol thus we have this two-way synthesis shown in Fig. 13-13. 

Phosphatidylethanolamine is synthesized de novo in a similar way to phosphatidylcholine. Phosphatidylserine and phosphatidylcholine may then be formed from phosphatidylethanolamine. 

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). 

Sphingolipids comprise glycolipids (gangliosides and cerebrosides) and the phospholipid, sphingomyelin. These compounds, too, are important membrane constituents. The biosynthesis of sphingolipids involves the common intermediate ceramide (Fig. 13-15). 

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