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Lipids are hydrophobic substances or their derivatives that are poorly soluble in water but much more soluble in organic solvents. Triacylglycerols have long been known as a means of storage of metabolic energy and amphiphilic lipids, for example phospholipids, glycolipids, and cholesterol, have been recognized as constituents of biological membranes. More recently, lipid-based signaling systems and the function of lipid moieties co-valently bound to proteins have been discovered. The regulation of membrane protein function by their lipid surroundings will be an important area of research in the future.

Selected Lipid Structures

• Fatty acids are long-chain monocarboxylic acids, for example palmitic acid (H3C(CH2)14COOH) and stearic acid (H3C(CH2)16COOH). They are most often found as building blocks of complex lipids and can also be branched or hydroxylated. Many fatty acids found in lipids, for example oleic acid, (Z)-H3C(CH2)7HC=CH(CH2)7COOH, contain Z-configured double bonds. Because of their detergent-like properties free fatty acids are not usually found in living cells, only bound to binding proteins.

• Fatty alcohols are found as components of waxes and, e.g., as pheromones in insects (cf. other lipids).

Eicosanoids, e.g. prostaglandins, prostacyclins, and the leukotrienes are signaling substances formed from arachidonic acid which, in turn, is released from phospholipids in response to different stimuli.

• Terpenes and steroids are biosynthetically derived from mevalonate. They can be hydrocarbons, e.g. carotene, alcohols, e.g. dolichol, esters, e.g. do-licholphosphate, or cholesterol esters, and others. Cholesterol is essential for membrane function in higher animals.

• Esters and amides. Waxes are esters of fatty acids with long chain alcohols. Together with other hydrophobic substances, they occur on exterior surfaces of plants. Fats and oils are triesters of glycerol and serve as fuels. Other esters and amides are signaling substances in insects or higher animals; e.g. N-Arachidonoylethanolamine is an endogenous ligand of the cannabinoid-receptor in the human brain.

Glycerolipids are glycerol derivatives with one, two, or three hydrocarbon chains. The lipid bilayers found in most biological membranes are essentially composed of derivatives of phosphatidic acid, a diacylglycerol phosphate. Attachment of hydrophilic ''head groups'' by ester linkages to phosphatidic acid give rise to many phospholipids (Figures B.4.1 and B.4.2). Phosphatidylcholine (lecithin) is the main glycerolipid found in eukaryotic membranes; phosphatidylethanolamine is abundant in bacterial membranes. More than 1500 different molecular phospholipid species have been detected in biological membranes, but little is known about their precise molecular function.

Sphingolipids are derivatives of ceramide (N-acylsphingosine, compare Chapter 1.4). Most eukaryotic glycolipids and the phospholipid sphingo-myelin contain ceramide as the hydrophobic backbone.

Fig. B.4.1. Schematic representation of membrane phospholipids such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol.

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