Phosphatidyl serine, Biosynthesis

Membrane proteins have a unique orientation because they are synthesized and inserted into the membrane in an asymmetric manner. This absolute asymmetry is preserved because membrane proteins do not rotate from one side of the membrane to the other and because membranes are always synthesized by the growth of preexisting membranes. Lipids, too, are asymmetrically distributed as a consequence of their mode of biosynthesis, but this asymmetry is usually not absolute, except for glycolipids. In the red-blood-cell membrane, sphingomyelin and phosphatidyl choline are preferentially located in the outer leaflet of the bilayer, whereas phosphatidyl ethanolamine and phosphatidyl serine are located mainly in the inner leaflet. Large amounts of cholesterol are present in both leaflets. 

Recent work in the general field of lipid biochemistry has revealed that although the carbon skeleton of serine is extensively used in the biosynthesis of ethanolamine (and hence choline and acetylcholine), decarboxylation of free serine does not occurnor is the ethanolamine formed from serine found free. It appears that the hydroxyl group of serine is first esterified with phosphatidic acid by reaction with cytidine diphosphate diglyceride (Figure 21). Phosphatidyl... 

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