Phospholipid biosynthesis eukaryotic

Kent, C., 1995, Eukaryotic phospholipid biosynthesis, Anrar. Rev. Biochem. 64 315-343. 

The majority of information on the chemical and physical properties of lipids comes from studies on the major phospholipid classes of eubacteria and eukaryotes with only limited information on the lipids from archaebacteria. The biosynthetic pathways and the genetics of lipid metabolism have also been extensively studied in eubacteria (Chapter 3) and eukaryotes (Chapter 8). Clearly, the archaeol lipids confer some advantage with respect to the environment of archaebacteria. Interestingly, the pathways for phospholipid biosynthesis in eubacteria and archaebacteria are very similar even though their lipids differ in chirality of the glycerol backbone. Many of these organisms exist in harsh environments that call for more chemically stable lipid bilayers that are afforded by the above lipids. How the physical properties of the more commonly studied lipids change with environment will be discussed later. 

This chapter provides an overview of eukaryotic phospholipid biosynthesis. A discussion of phospholipid synthesis in mammalian cells is emphasized but the synthesis of phospholipids in yeast is also discussed. Phospholipid synthesis in plants is considered in Chapter 4 and in bacteria in Chapter 3. Phospholipids make up the essential milieu of cellular membranes and act as a barrier for entry of compounds into cells. Phospholipids also function as precursors of second messengers such as diacylglycerol (DG) and inositol-1,4,5-P3. A third, and usually overlooked function of phospholipids, is storage of energy in the form of fatty acyl components. This function is probably quantitatively important only under extreme conditions such as starvation. 

A large number of cellular processes and biosythetic pathways of eukaryotic cells are compartmentalized and restricted to specific membranes. Mitochondria and chloroplasts are two cases in point. In prokaryotic cells, many of the same functions are performed by a single membrane. The transport of metabolites and ions, oxidative phosphorylation, photosynthesis, phospholipid biosynthesis, and the synthesis of cell-wall constituents are a few examples of processes carried out by enzyme systems localized in the bacterial plasma membrane (Chapters... 

Figure 21-4 Biosynthesis of triacylglycerols, glycol ipids, and major phospholipids that are formed both in prokaryotes and eukaryotes. More complete schemes of phospholipid synthesis are shown in Figs. 21-3 and 21-5. Green arrow pathway occurring only in eukaryotes.

Figure 21-5 A more complete outline of the biosynthesis of triacylglycerols, glycolipids, and phospholipids including characteristic eukaryotic pathways. Green lines indicate pathways utilized by both bacteria and eukaryotes. Structures of some of the compounds are shown in Fig. 21-4. The gray arrows show the formation of phosphatidylserine by exchange with ethano-lamine (Eq. 21-10).

In the first phase of phospholipid synthesis from glyc-erol-3-phosphate to phosphatidic acid, the pathways in E. coli and eukaryotes are very similar (see fig. 19.2). The major difference is that one additional pathway exists for generation of phosphatidic acid from dihydroxyacetone phosphate, an intermediate in glycolysis. Once phosphatidic acid is made, it is rapidly converted to diacylglycerol or CDP-diacylglycerol (see fig. 19.2) both of which are intermediates for the biosynthesis of eukaryotic phospholipids. 

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