Lipids

Biological lipids are a chemically diverse group of compounds, the common and defining feature of which is their insolubility in water. The biological functions of the lipids are equally diverse. Fats and oils are the principal stored forms of energy in many organisms, and phospholipids and sterols make up about half the mass of biological membranes. Other lipids, although present in relatively small quantities, play crucial roles as enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchors, emulsifying agents, hormones, and intracellular messengers. This chapter introduces representative lipids of each type, with emphasis on their chemical structure and physical properties.

Storage Lipids

The fats and oils used almost universally as stored forms of energy in living organisms are highly reduced compounds, derivatives of fatty acids. The fatty acids are hydrocarbon derivatives, at about the same low oxidation state (that is, as highly reduced) as the hydrocarbons in fossil fuels. The complete oxidation of fatty acids (to CO2 and H2O) in cells, like the explosive oxidation of fossil fuels in internal combustion engines, is highly exergonic. We will introduce here the structure and nomenclature of the fatty acids most commonly found in living organisms. 'Iwo types of fatty acid-containing compounds, triacylglycerols and waxes, are described to illustrate the diversity of structure and physical properties in this family of compounds.

Fatty Acids Are Hydrocarbon Derivatives

Fatty acids are carboxylic acids with hydrocarbon chains of 4 to 36 carbons. In some fatty acids, this chain is fully saturated (contains no double bonds) and unbranched; others contain one or more double bonds (Table 9-1). A few contain three-carbon rings or hydroxyl groups. A simplified nomenclature for these compounds specifies the chain length and number of double bonds, separated by a colon; the 16-carbon saturated palmitic acid is abbreviated 16:0, and the 18carbon oleic acid, with one double bond, is 18:1. The positions of any double bonds are specified by superscript numbers following Δ(delta); a 20-carbon fatty acid with one double bond between C-9 and C-10 (C-1being the carboxyl carbon), and another between C-12 and C-13, is designated 20:2(Δ9'12), for example. The most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons (Table 9-1). As we shall see in Chapter 20, the even number of carbons results from the mode of synthesis of these compounds, which involves condensation of acetate (two-carbon) units
The position of double bonds is also regular; in most monounsaturated fatty acids the double bond is between C-9 and C-10 (Δ9), and the other double bonds of polyunsaturated fatty acids are generally Δ12 and Δ15 (Table 9-1). The double bonds of polyunsaturated fatty acids are almost never conjugated (alternating single and double bonds, as in -CH=CH-CH=CH-), but are separated by a methylene group (-CH=CH-CH2-CH=CH-). The double bonds of almost all naturally occurring unsaturated fatty acids are in the cis configuration.
The physical properties of the fatty acids, and of compounds that contain them, are largely determined by the length and degree of unsaturation of the hydrocarbon chain. The nonpolar hydrocarbon chain accounts for the poor solubility of fatty acids in water. Lauric acid (12:0, M_r 200), for example, has a solubility of 0.063 mg/g of watermuch less than that of glucose (M_r 180), which is 1,100 mg/g of water. The longer the fatty acyl chain and the fewer the double bonds, the lower the solubility in water (Table 9-1). The carboxylic acid group is polar (and ionized at neutral pH), and accounts for the slight solubility of short-chain fatty acids in water.
The melting points of fatty acids and of compounds that contain them are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain (Table 9-1). At room temperature (25 °C), the saturated fatty acids from 12:0 to 24:0 have a waxy consistency, whereas unsaturated fatty acids of these lengths are oily liquids. In the fully saturated compounds, free rotation around each of the carbon-carbon bonds gives the hydrocarbon chain great flexibility; the most stable conformation is this fully extended form ( Fig. 9-la ), in which the steric hindrance of neighboring atoms is minimized. These molecules can pack together tightly in nearly crystalline arrays, with atoms all along their lengths in van der Waals contact with the atoms of neighboring molecules (Fig. 9-le). A cis double bond forces a kink in the hydrocarbon chain (Fig. 9-lb). Fatty acids with one or several such ki.nks cannot pack together as tightly as fully saturated fatty acids (Fig. 9-lc), and their interactions with each other are therefore weaker. Because it takes less thermal energy to disorder these poorly ordered arrays of unsaturated fatty acids, they have lower melting points than saturated fatty acids of the same chain length (Table 9-1).
In vertebrate animals, free fatty acids ( having a free carboxylate group) circulate in the blood bound to a protein carrier, serum albumin. However, fatty acids are present mostly as carboxylic acid derivatives such as esters or amides. Lacking the charged carboxylate group, these fatty acid derivatives are generally even less soluble in water than are the free carboxylic acids.

Triacylglycerols Are Fatty Acid Esters of Glycerol

The simplest lipids constructed from fatty acids are the triacylglycerols, also referred to as triglycerides, fats, or neutral fats. l~iacylglycerols are composed of three fatty acids each in ester linkage with a single glycerol (Fig. 9-2). Those containing the same kind of fatty acid in all three positions are called simple triacylglycerols, and are named after the fatty acid they contain. Simple triacylglycerols of 16:0, 18:0, and 18: l, for example, are tristearin, tripalmitin, and triolein, respectively. Mixed triacylglycerols contain two or more different fatty acids; to name these compounds unambiguously, the name and position of each fatty acid must be specified.

Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules, essentially insoluble in water. This explains why oil-water mixtures (oil-and-vinegar salad dressing, for example) have two phases. Because lipids have lower specific gravities than water, the oil floats on the aqueous phase.

Triacylglycerols Provide Stored Energy and Insulation.

In most eukaryotic cells, triacylglycerols form a separate phase of microscopic, oily droplets in the aqueous cytosol, serving as depots of metabolic fuel. Specialized cells in vertebrate animals, called adipocytes, or fat cells, store large amounts of triacylglycerols as fat droplets, which nearly fill the cell (Fig. 9-3). Triacylglycerols are also stored in the seeds of many types of plants, providing energy and biosynthetic precursors when seed germination occurs. As stored fuels, triacylglycerols have two significant advantages over polysaccharides such as glycogen and starch. The carbon atoms of fatty acids are more reduced than those of sugars, and oxidation of triacylglycerols yields more than twice as much energy, gram for gram, as that of carbohydrates. Furthermore, because triacylglycerols are hydrophobic and therefore unhydrated, the organism that carries fat as fuel does not have to carry the extra weight of water of hydration that is associated with stored polysaccharides. In humans, fat tissue, which is composed primarily of adipocytes, occurs under the skin, in the abdominal cavity, and in the mammary glands. Obese people may have 15 or 20 kg of triacylglycerols deposited in their adipocytes, sufficient to supply energy needs for months. In contrast, the human body can store less than a day's energy supply in the form of glycogen. Carbohydrates such as glucose and glycogen do offer certain advantages as quick sources of metabolic energy, one of which is their ready solubility in water.

In some animals, triacylglycerols stored under the skin serve not only as energy stores but as insulation against very low temperatures. Seals, walruses, penguins, and other warm-blooded polar animals are amply padded with triacylglycerols. In hibernating animals (bears, for example) the huge fat reserves accumulated before hibernation also serve as energy stores (see Box 16-1). The low density of triacylglycerols is the basis for another remarkable function of these compounds. In sperm whales, a store of triacylglycerols allows the animals to match the buoyancy of their bodies to that of their surroundings during deep dives in cold water (Box 9-1).

Many Foods Contain Triacylglycerols

Most natural fats, such as those in vegetable oils, dairy products, and animal fat, are complex mixtures of simple and mixed triacylglycerols. These contain a variety of fatty acids differing in chain length and degree of saturation (Table 9-2). Vegetable oils such as corn and olive oil are composed largely of triacylglycerols with unsaturated fatty acids, and thus are liquids at room temperature. They are converted industrially into solid fats by catalytic hydrogenation, which reduces some of their double bonds to single bonds. Triacylglycerols containing only saturated fatty acids, such as tristearin, the major component of beef fat, are white, greasy solids at room temperature.

When lipid-rich foods are exposed too long to the oxygen in air, they may spoil and become rancid. The unpleasant taste and smell associated with rancidity result from the oxidative cleavage of the double bonds in unsaturated fatty acids to produce aldehydes and carboxylic acids of shorter chain length and therefore higher volatility.

Hydrolysis of Triacylglycerols Produces Soaps

The ester linkages of triacylglycerols are susceptible to hydrolysis by either acid or alkali. Heating animal fats with NaOH or KOH produces glycerol and the Na + or K+ salts of the fatty acids, known as soaps (Fig. 9-4). The usefulness of soaps is in their ability to solubilize or disperse water-insoluble materials by forming microscopic aggregates (micelles). When used in "hard" water (having high concentrations of Ca2+ and Mg2+ ), soaps are converted into their insoluble calcium or magnesium salts, forming a residue. Synthetic detergents such as sodium dodecylsulfate (SDS; see p. 141) are less prone to precipitation in hard water, and have largely replaced natural soaps in many industrial applications. At neutral pH, a variety of lipases catalyze the enzymatic hydrolysis of triacylglycerols. Lipases in the intestine aid in the digestion and absorption of dietary fats. Adipocytes and germinating seeds contain lipases that break down stored triacylglycerols, releasing fatty acids for export to other tissues where they are required as fuel

Waxes Serve as Energy Stores and Water-Impermeable Coatings

Biological waxes are esters of long-chain saturated and unsaturated fatty acids (having 14 to 36 carbon atoms) with long-chain alcohols (having 16 to 30 carbon atoms) (Fig. 9-5). Their melting points (60 to 100 °C) are generally higher than those of triacylglycerols. In marine organisms that constitute the plankton, waxes are the chief storage form of metabolic fuel.

Waxes also serve a diversity of other functions in nature, related to their water-repellent properties and their firm consistency. Certain skin glands of vertebrates secrete waxes to protect the hair and skin and to keep them pliable, lubricated, and waterproof. Birds, particularly waterfowl, secrete waxes from their preen glands to make their feathers water-repellent. The shiny leaves of holly, rhododendrons, poison ivy, and many tropical plants are coated with a layer of waxes, which protects against parasites and prevents excessive evaporation of water. Biological waxes find a variety of applications in the pharmaceutical, cosmetic, and other industries. Lanolin (from lamb's wool), beeswax (Fig. 9-5), carnauba wax (from a Brazilian palm tree), and spermaceti oil (from whales) are widely used in the manufacture of lotions, ointments, and polishes.