Fatty Acids as Components of Complex Lipids

Fatty acids are important building blocks for various cellular complex lipids (Figure 7). For simplicity, the pathways for incorporation of fatty acids into these lipids are outlined only briefly. More details can be found in any good biochemistry text. In most cases, fatty acyl-CoA and not free fatty acid participates in these biosynthetic reactions. Nearly all cells synthesize phospholipids, which are essential membrane constituents. Phos-pholipid synthesis takes place in the endoplasmic reticulum. It begins by fatty acylating the two free hydroxyl groups in a-glycerophosphate, a triose derived from glycolytic intermediates, yielding phosphatidic acid. Various head groups (e.g., cho-line, ethanolamine, inositol, or serine) can then be linked to the phosphate group. For synthesis of triacylglycerol, this phosphate moiety is removed, yielding diacylglycerol, and a third fatty acyl group is esterified to the free hydroxyl group.

Another type of lipid, the ether-linked phospholi-pids (e.g., plasmalogens), comprises about 20% of membrane phospholipids (Figure 7). Plasmalogen synthesis requires enzymes present in both peroxi-somes and the endoplasmic reticulum. These lipids are thought to be part of the cellular defense mechanism against oxidative injury.

Fatty acids are also found esterified to the 3-hydroxyl group of cholesterol (cholesterol esters; ChE). ChE, which are more hydrophobic than free cholesterol, are a transport and storage form of cholesterol. ChE are found in high concentrations in low-density lipoproteins. Intracellular lipid droplets containing ChE are found in steroidogenic tissues and are thought to be a reservoir of cholesterol for steroid-hormone synthesis. The fatty acid most commonly found in ChE is 18:1. It must be activated to its CoA derivative before transfer to cholesterol in a reaction catalyzed by acyl-CoA cholesterol acyltransferase. ChE are also formed within lipoproteins by the transfer of one fatty acyl chain from phosphatidyl choline to cholesterol, a reaction catalyzed by circulating lecithin: cholesterol acyltransferase.

Synthesis of sphingolipids, which include sphingo-myelin, ceramides, cerebrosides, and gangliosides, begins by the condensation of palmitoyl-CoA (16:0-CoA) with serine. The amino group of serine is then acylated by a second fatty acyl-CoA to form ceramide; the chain length of the second fatty acid can be variable. Transfer of phosphorylcholine (from the phospholipid phosphatidyl choline) to the hydroxyl group of ceramide yields

-ch2

h2c-

CH2OH

Triacylglycerol

Diacylglycerol

H2C-

h2c-

OH I

Phosphatidic acid

Phospholipid

Plasmalogen

Cholesterol ester

Cholesterol ester

Ceramide

Figure 7 Fatty acids form the basis of most complex lipids. The part of the molecule derived from fatty acids is shown in black, and the part derived from other sources is shown in grey. For phospholipids and plasmalogens, R = choline, ethanolamine, inositol, serine, or a similar head group.

h2c sphingomyelin. Alternatively, sugars (from sugar nucleotide donors) are added to produce the cere-brosides, gangliosides, and related lipids.

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