Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Lipid Biosynthesis
Phosphatidylcholine is formed via two different pathways

One pathway of phosphatidylcholine Biosynthesis is referred to as the de novo synthesis ("from the beginning"), since it does not require a preformed Choline as a precursor. In de novo synthesis (Fig. 21-15), the choline HEAD group of the phosphatidylcholine molecule is not incorporated into the molecule as such, but is instead formed from ethanolamine, which is part of phosphatidylethanolamine, through a threefold methylation. S-Adenosylmethionine (SAM; Fig. 21-18) serves as the methyl group donor—an activated form of Methionine in which the methyl group exhibits enhanced reactivity. The reactions proceed in the following sequence:

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Fig. 21-18. S-Adenosylmethionine (SAM) and the product formed upon its demethylation, S-adenosylhomocysteine.

Another pathway for phosphatidylcholine synthesis is known as the salvage pathway, because the choline produced during the metabolic breakdown of phosphatidylcholine is, in a sense, rescued from degradation and reused directly in its ready-made form to build phosphatidylcholine. This pathway closely resembles The biosynthesis of phosphatidylethanolamine (Fig. 21-15). In The First stage of synthesis, free choline is activated by ATP in a reaction catalyzed by choline kinase to yield phosphocholine:

Phosphocholine then reacts with CTP to form cytidine diphosphate choline:

CDP-choline interacts with 1,2-diacylglycerol, resulting in The formation of phosphatidylcholine:

The salvage pathway of phosphatidylcholine synthesis is essential for many animals, as their capacity to synthesize this compound de novo is limited. This is due to the fact that the methyl groups in the form of S-adenosylmethionine required for such synthesis are generated from the essential amino acid methionine. When dietary methionine is deficient, the methylation of phosphatidylethanolamine and other methyl group acceptors becomes restricted. Under these conditions, the Organism strives to "salvage" and reuse already methylated free choline. In fact, a dietary deficiency of methionine in animals can be at least partially compensated for by dietary choline. Thus, in certain cases, choline can function as an accessory vitamin (Ch. 26).

Other Phospholipids found in membranes (Table 21-2)—phosphatidylserine, phosphatidylinositol, and cardiolipin (Section 12.5)—are synthesized from diacylglycerols and CDP derivatives via reactions analogous to those described above.

Table 21-2. Major Membrane Lipids found in animal Cells (see Ch. 12)