Phosphatidic acid , metabolic roles

These compounds are well established as important intermediates in the biosynthesis of phosphoglycerides, such as phosphatidylcholine. The fact that they are formed on agonist stimulation of a cell suggested a role in the cell signaling process (Exton, 1990). In addition, over a period of many years, there have been reports on the interrelation of phosphatidic acid formation and cellular responses (Ca2+ movements to agonists, for example). Thus, these compounds have emerged as important components of cellular metabolism and merit serious attention in any metabolic investigation. 

Phosphatidylinositol is uncommon in bacteria and is found in a few Gram-positive species only. In actinomycetes and a few other bacteria mannosides of phosphatidylinositol may be present. Other glycerophospholipids such as phosphatidic acid and phosphatidylserine have a widespread occurrence in bacteria but only in small amounts. Both of these lipids play an important role as metabolic intermediates (Finnerty, 1978 Raetz, 1978). 

The property of the glycerol-based lipids to form micelles even without saponification is especially pronounced in phospholipids. The basic structural unit of the phospholipid molecule is phosphatidic acid shown in the scheme below. The substituents R and R are long-chained alkane or alkene groups. The phosphate group can be bound in different structures such as choline or serine, molecules which play important roles in metabolism. 

The direct oxidation of amino acids (oxidative deamination) is insignificant compared to transamination. Decarboxylation is important only for some special metabolic processes. Serine, e.g., is transformed to ethanolamine, one of the major components of the phosphatides. Similarly, cysteine can become /3-mercaptoethyl-amine, a constituent of coenzyme A. Decarboxylation also plays a role in the formation of certain hormones (cf. Chapt. VIII-5). 

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