Lipids: Structure, Function, and Composition, Lecture notes of Biochemistry

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Chap. 10A. Lipids

Storage Lipids

• Structural Lipids in Membranes

• Lipids as Signals, Cofactors, and Pigments

• Working with Lipids

Fig. 10-4a. Fat droplets in human adipose tissue cells.

Intro. to Lipids

Biological lipids are a chemically diverse group of compounds whose common and defining feature is their insolubility in water. The biological functions of lipids are as diverse as their chemistry. Fats and oils are the principal stored forms of energy in many organisms. Phospholipids and sterols are major structural components of biological membranes. Other lipids play crucial roles as enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchors for proteins, chaperones that help membrane proteins fold, emulsifying agents in the digestive tract, hormones, and intracellular messengers. The first group of lipids that will be presented are the storage lipids. Storage lipids, e.g., triacylglycerols, make up the fats and oils used by most organisms as stored forms of energy. These compounds contain fatty acids. Fatty acids are hydrocarbon derivatives and are highly reduced and have about the same oxidation state as hydrocarbon fossil fuels. The burning (oxidation) of fatty acids is highly exergonic. The structures and other properties of the fatty acids most commonly found in living organisms are covered in the next few slides.

Fatty Acids (II)

The most commonly occurring fatty acids in animals have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons (Table 10-1). There is a common pattern in the location of double bonds. In most monounsaturated fatty acids, the double bond is between C-9 and C- 10 (∆ 9 ), and the other double bonds of polyunsaturated fatty acids are generally ∆ 12 and ∆ 15 . Arachidonate [20:4(∆ 5,8,11, )] is an exception to this generalization. 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-CH 2 -CH=CH-). In nearly all naturally occurring unsaturated fatty acids in animals, the double bonds are in the cis configuration. Trans fatty acids are synthesized naturally by fermentation in the rumens of dairy animals and are obtained from dairy products and meat.

Common Saturated Fatty Acids

Common saturated fatty acids found in animal cells are listed in Table 10-1. Palmitic acid (16:0) and stearic acid (18:0) are particularly prevalent.

Physical Properties of Fatty Acids (I)

The physical properties of the fatty acids, and of compounds that contain them, are largely determined by the length and degree of unsaturation of their hydrocarbon chains. The hydrocarbon chain accounts for the poor solubility of fatty acids in water. The longer the fatty acid chain and the fewer the double bonds, the lower is the solubility in water. The carboxylic acid group is polar (and ionized at neutral pH) and accounts for the slight solubility of short- chain fatty acids in water. Melting points are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain. At 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. The differences in melting points is due to differences in the packing abilities of the fatty acid chains (Fig. 10-2). (Continued on the next slide).

Physical Properties of Fatty Acids (II)

In the fully saturated fatty acids, free rotation around each carbon-carbon bond gives the hydrocarbon chain great flexibility. The most stable conformation is the fully extended form, 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. In unsaturated fatty acids, a cis double bond forces a kink in the hydrocarbon chain. Fatty acids with one or several such kinks cannot pack together as tightly as fully saturated fatty acids, and their interactions with each other are therefore weaker. Because less thermal energy in needed to disorder these poorly ordered arrays of unsaturated fatty acids, they have markedly lower melting points than saturated fatty acids of the same chain length (Table 10-1).

Fat Stores in Cells

In most eukaryotic cells, triacylglycerols form microscopic, oily droplets in the aqueous cytosol, serving as metabolic fuel. In vertebrates, specialized cells called adipocytes or fat cells, store large amounts of triacylglycerols as fat droplets that nearly fill the cell (Fig. 10-4a). Triacylglycerols are also stored in the seeds of many types of plants, providing energy and biosynthetic precursors during seed germination (Fig. 10-4b). Adipocytes and germinating seeds contain enzymes known as lipases that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuel. Triacylglycerols contain more energy per gram than do polysaccharides such as glycogen (9 cal/g vs 5 cal/g). In addition, they are unhydrated, and the organism does not have to carry extra weight in the form of hydrated water as with stored polysaccharides. In some animals, such as seals, penguins, and bears, fat stores under the skin also serve as insulation against cold temperatures.

Fatty Acid Compositions of Food Fats

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 (Fig. 10-5). The melting points of these fats and hence their physical state at room temperature are a direct function of their fatty acid compositions. Olive oil has a high proportion of long-chain (C 16 and C 18 ) unsaturated fatty acids, which accounts for its liquid state at 25˚C. The higher proportion of long-chain (C 16 and C 18 ) saturated fatty acids in butter increases its melting point, so butter is a soft solid at room temperature. Beef fat, with an even higher proportion of long-chain saturated fatty acids is a hard solid.

Waxes

Biological waxes are esters of long-chain (C 14 to C 36 ) saturated and unsaturated fatty acids with long-chain (C 16 to C 30 ) alcohols. The structure of the wax, triacontanoylpalmitate, which is the major component of the beeswax used in honeycomb construction, is shown in Fig. 10.6. The melting points of waxes (60 to 100˚C) are generally higher than those of triacylglycerols. In plankton, waxes are the chief storage form of metabolic fuel. For many other organisms, waxes function as water-repellants (e.g., waterfowl, plant leaves). Biological waxes also find a variety of applications in the pharmaceutical, cosmetic, and other industries, where they are used in the manufacture of lotions, ointments, and polishes.

Intro. to Membrane Lipids

The central architectural feature of biological membranes is a double layer of lipids, which acts as a barrier to the passage of polar molecules and ions. Membrane lipids are amphipathic: one end of the molecule is hydrophobic, the other hydrophilic. Their hydrophobic interactions with each other, and their hydrophilic interactions with water direct their packing into sheets called membrane bilayers. The compositions of some common types of membrane lipids are summarized in Fig. 10-7. All of the lipid types shown have either glycerol or sphingosine as the backbone (light red shading), to which are attached one or more long-chain alkyl groups (yellow) and a polar head group (blue). The detailed structures of these membrane lipids are covered in the next few slides.

Glycerophospholipids (II)

Glycerophospholipids are named as derivatives of the parent compound, phosphatidic acid (Fig. 10-9), according to the polar alcohol in the head group. Phosphatidylcholine and phosphatidylethanolamine, for example, have choline and ethanolamine as their polar head groups. In all these compounds, the head group is joined to glycerol through a phosphodiester bond, in which the phosphate group bears a negative charge at neutral pH. The polar alcohol may be negatively charged (phosphatidylinositol 4,5-bisphosphate), neutral (phosphatidylserine), or positively charged (phosphatidylcholine, phosphatidylethanolamine). The charges of the head groups contribute greatly to the surface properties of membranes.

Glycerophospholipids (III)

A wide variety of fatty acids occur in glycerophospholipids, and a given phospholipid such as phosphatidylcholine may consist of several molecular species, each with its unique complement of fatty acids. The distribution of molecular species is specific for different organisms, different tissues of the same organism, and different glycerophospholipids in the same cell or tissue. In general, animal glycerophospholipids contain a C 16 or C 18 saturated fatty acid at C-1 of glycerol, and a C 18 or C 20 unsaturated fatty acid at C-2. The biological significance of the variation in fatty acids and head groups is not yet understood.

Galactolipids

Galactolipids contain one or two galactose residues connected via an  glycosidic bond to C- of a 1,2-diacylglycerol (Fig. 10-11). Galactolipids are particularly abundant in plant cells where they are localized mostly to the thylakoid membranes (internal membranes) of chloroplasts. They make up 70% to 80% of the total membrane lipids of a vascular plant and therefore are probably the most abundant membrane lipids in the biosphere. Phosphate is often a growth-limiting nutrient in the soil, and perhaps the evolutionary pressure to conserve phosphate for more critical roles favored plants that made phosphate-free lipids. Plant membranes also contain sulfolipids, in which a sulfonated glucose residue is joined to a diacylglycerol in glycosidic linkage. The sulfonate group bears a negative charge like that of the phosphate group in phospholipids.

Archaean Membrane Lipids

Some archaea that live in extreme conditions--high temperatures (boiling water), low pH, high ionic strength, for example--have membrane lipids containing long-chain (32 carbons) branched hydrocarbons (phytanal groups) linked at each end to glycerol (Fig. 10-12). These linkages are through ether bonds, which are more stable to hydrolysis at low pH and high temperatures than are the ester bonds found in the membrane lipids of bacteria and eukaryotes. In the diphytanyl tetraether lipid shown in the diagram, the diphytanyl moieties (yellow) are long hydrocarbons composed of eight, five-carbon isoprene groups condensed end-to-end. In this extended form, the diphytanyl groups are about twice the length of a 16-carbon fatty acid, and therefore one of these lipids spans the membrane. The glycerol moieties in the archaeal lipids are in the R configuration, in contrast to those of bacteria and eukaryotes, which have the S configuration. Archaeal lipids differ in the substituents on the glycerols. In the molecule shown here, one glycerol is linked to the disaccharide -glucopyranosyl-(12)-ß-galactofuranose. The other glycerol is linked to a glycerol phosphate head group.