Phosphoacylglycerols

Phosphoacylglycerols containing choline (Figure 14-8) are the most abundant phospholipids of the cell mem-... 

These are phosphoacylglycerols containing only one acyl radical, eg, lysophosphatidylcliolme (lysoleci-thin), important in the metabohsm and interconversion of phosphohpids (Figure 14—9).lt is also found in oxidized hpoproteins and has been imphcated in some of their effects in promoting atherosclerosis. 

Lipoproteins (Table 5.2) are macromolecular aggregates with varying proportions of triglycerides and cholesterol (with some phosphoacylglycerols) and apoproteins. The apoproteins act as recognition flags for receptor binding, for example apo B and apo E,... 

Phospholipids are hydrolyzable lipids that contain a phosphorus atom. There are two common types of phospholipids phosphoacylglycerols and sphingomyelins. Both classes are found almost exclusively in the cell membranes of plants and animals, as discussed in Section 3.7. 

There are two prominent types of phosphoacylglycerols. They differ in the identity of the R group in the phosphodiester. 

The phosphorus side chain of a phosphoacylglycerol makes it different from a triacylglycerol. The two fatty acid side chains form two nonpolar tails that lie parallel to each other, while the phosphodiester end of the molecule is a charged or polar head. A three-dimensional stmcture of a phosphoacylglycerol is shown in Figure 29.3. 

Problem 29.7 Phosphoacylglycerols should remind you of soaps (Section 3.6). In what ways are these compounds similar ... 

Sphingomyelins, the second major class of phospholipids, are derivatives of the amino alcohol sphingosine, in much the same way that triacylglycerols and phosphoacylglycerols are derivatives of glycerol. Other notable features of a sphingomyelin include ... 

A phosphoacylglycerol has two distinct regions two nonpolar tails due to the long-chain fatty acids, and a very polar head from the charged phosphodiester. 

Like phosphoacylglycerols, sphingomyelins are also a component of the lipid bilayer of cell membranes. The coating that surrounds and insulates nerve cells, the myelin sheath, is particularly rich in sphingomyelins, and is vital for proper nerve function. Deterioration of the myelin sheath as seen in multiple sclerosis leads to disabling neurological problems. 

Figure 29.4 compares the structural features of the most common hydrolyzable lipids a triacyl-glycerol, a phosphoacylglycerol, and a sphingomyelin. 

A phosphoacylglycerol has two nonpolar side chain tails and one ionic head. 

Cephalin (Section 29.4A) A phosphoacylglycerol in which the phosphodiester alkyl group is CH2CH2NH3. Cephalins are also called phosphatidylethanolamines. 

Phosphoacylglycerols (Section 29.4A) A lipid having a glycerol backbone with two of the hydroxy groups esterified with fatty acids and the third hydroxy group as part of a phosphodiester. 

Phosphatidic acids are the simplest phosphoacylglycerols and are present only in small amounts in membranes. The most common phosphoacylglycerols in membranes have a second phosphate ester linkage. The alcohols most commonly used to form this second ester group are ethanolamine, choline, and serine. Phosphatidylethanolamines are also called cephalins, and phosphatidylcholines are called lecithins. Used as emulsifying agents, lecithins are added to foods such as mayonnaise to prevent the aqueous and fat components from separating. 

Phosphoacylglycerols form membranes by arranging themselves in a lipid bilayer. The polar heads of the phosphoacylglycerols are on the outside of the bilayer, and the fatty acid chains form the interior of the bilayer. Cholesterol—a membrane lipid discussed in Section 26.9—is also found in the interior of the bilayer (Figure 26.1). A typical bilayer is about 50 A thick. [Compare the bilayer with the micelles formed by soap in aqueous solution (Section 17.13).]... 

Phosphoacylglycerols contain fatty acids esterified to position 1 and 2 of glycerol and a phosphate (alone or with a substituent) attached to carbon 3. If only a phosphate group is attached to carbon 3, the compound is phosphatidic acid (see Fig. 5.21). Phosphatidic acid is a precursor for the synthesis of the other phosphoacylglycerols. 

Fig. 5.21. Phosphoacylglycerols. Phospholipids found in membranes, such as phosphatidylcholine, have a polar group attached to the phosphate.

Fig. 10.3. Common phospholipids in the mammalian cell membrane. The polar head groups shown for ethanolamine and serine replace the choline in phosphatidylcholine to form phos-phatidylethanolamine and phosphatidylserine, respectively. Phosphatidylcholine, phos-phatidylethanolamine, and phosphatidylserine are phosphoacylglycerols. In contrast, sphingomyelin does not contain the glycerol backbone but has a sphingosine backbone and is a sphingolipid.

Cholesterol, which is interspersed between the phospholipids, maintains membrane fluidity. In the phosphoacylglycerols, unsaturated fatty acid chains bent into the cis conformation form a pocket for cholesterol, which binds with its hydroxyl group in the external hydrophilic region of the membrane and its hydrophobic steroid nucleus in the hydrophobic membrane core (Fig. 10.5). The presence of cholesterol and the cis unsaturated fatty acids in the membrane prevent the hydrophobic chains from packing too closely together. As a consequence, lipid and protein molecules that are not bound to external or internal structural proteins can rotate and move laterally in the plane of the leaflet. This movement enables the plasma membrane to partition between daughter cells during cell division, to... 

The arachidonic acid present in membrane phospholipids is released from the lipid bilayer as a consequence of the activation of membrane-bound phospholipase A2 or C (see Fig. 33.31 and Fig. 35.2). This activation occurs when a variety of stimuli (agonists), such as histamine and the cytokines, interact with a specific plasma membrane receptor on the target cell surface. Phospholipase A2 is specific for the sn-2 position of phosphoacylglycerols, the site of attachment of arachidonic acid to the glycerol moiety. Phospholipase C hydrolyzes phosphorylated inositol... 

Phosphatidyl esters are classed as phosphoacylglycerols. The natures of the fatty acids vary widely, as they do in triacylglycerols. As a result, the names of the types of lipids (such as triacylglycerols and phosphoacylglycerols) that contain fatty acids must be considered generic names. 

A phosphatidyl ester (phosphoacylglycerol). Glycerol is esterfied to two carboxylic acids, stearic acid and linoleic acid, as well as to phosphoric acid. Phosphoric acid, in turn, is esterified to a second alcohol, ROH. 

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