Pathways for Biosynthesis of Phosphatidylserine and Phosphatidylethanolamine

In 1884 Thudicum isolated kephalin and described it as a lipid that contained nitrogen and phosphorus but was different from lecithin (PC).

In 1930 PE was purified from kephalin, thereby setting the stage for elucidation of the structure of PE in 1952 by Baer et al. (18). PS was another component of kephalin but not until 1941 did Folch discover that PS was a distinct entity (19). The structure of PS was confirmed by chemical synthesis by Baer and Maurukas in 1955 (20).

In mammalian cells, PS is synthesized by an exchange of L-serine for the ethanolamine or choline head-group of a PE or PC molecule, respectively (Fig. 1B) (21). In contrast, in prokaryotes and yeast, PS is synthesized by reaction of CDP-diacylglycerol with L-serine (Fig. 1A) (22-24). The latter biosynthetic pathway has not been detected in mammalian cells, but recent evidence has revealed that plants have the capacity to make PS by this route, as well as by the base-exchange reaction (25).

The biosynthesis of PE in eukaryotic cells occurs by two major routes— from the CDP-ethanolamine pathway, as originally described by Kennedy and Weiss in 1956 (26), and from the decarboxylation of PS (27) (Fig. 2). PE can also be synthesized from the acylation of lyso-PE, and from a base-exchange reaction in which ethanolamine is exchanged for the serine head-group of PS (Fig. 1B). The latter two pathways are thought to be the minor contributors to PE synthesis (28). The relative importance of the two major pathways for PE synthesis in mammalian cells has not been unambiguously established. However, in many types of cultured mammalian cells, such as BHK cells (29) and Chinese hamster ovary (CHO) cells (30,31), the PS decarboxylation

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