lipids - universidade federal de minas...

Biological lipids are a chemically diverse group of compounds, the commonand defining feature of which is their insolubility in water. The biologicalfunctions of the lipids are as diverse as their chemistry. Fats and oils are theprincipal stored forms of energy in many organisms. Phospholipids andsterols are major structural elements of biological membranes. Other lipids,although present in relatively small quantities, play crucial roles as enzymecofactors, electron carriers, light-absorbing pigments, hydrophobic an-chors, emulsifying agents, hormones, and intracellular messengers. Thischapter introduces representative lipids of each type, with emphasis ontheir chemical structure and physical properties.

Storage LipidsThe fats and oils used almost universally as stored forms of energy in livingorganisms are derivatives of fatty acids. The fatty acids are hydrocarbonderivatives, at about the same low oxidation state (that is, as highly re-duced) as the hydrocarbons in fossil fuels. The cellular oxidation of fattyacids (to CO2 and H2O), like the controlled, rapid burning of fossil fuels ininternal combustion engines, is highly exergonic.

We introduce here the structures and nomenclature of the fatty acids most commonly found in living organisms. Two types of fatty acid–containing compounds, triacylglycerols and waxes, are described to illus-trate the diversity of structure and physical properties in this family of compounds.

Fatty Acids Are Hydrocarbon DerivativesFatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to36 carbons long (C4 to C36). In some fatty acids, this chain is fully saturated(contains no double bonds) and unbranched; in others the chain containsone or more double bonds (Table 11–1). A few contain three-carbon rings,hydroxyl groups, or methyl-group branches. A simplified nomenclature forthese compounds specifies the chain length and number of double bonds,separated by a colon; the 16-carbon saturated palmitic acid is abbreviated16:0, and the 18-carbon oleic acid, with one double bond, is 18:1. The posi-tions of any double bonds are specified by superscript numbers following D(delta); a 20-carbon fatty acid with one double bond between C-9 and C-10(C-1 being the carboxyl carbon) and another between C-12 and C-13 is des-ignated 20:2(D9,12). The most commonly occurring fatty acids have evennumbers of carbon atoms in an unbranched chain of 12 to 24 carbons (Table11–1). As we shall see in Chapter 21, the even number of carbons results

363

Lipids

chapter

11

from the mode of synthesis of these compounds, which involves condensa-tion of acetate (two-carbon) units.

There is also a common pattern in the location of double bonds; in mostmonounsaturated fatty acids the double bond is between C-9 and C-10 (D9),and the other double bonds of polyunsaturated fatty acids are generally D12

and D15. (Arachidonic acid is an exception to this generalization.) The dou-ble bonds of polyunsaturated fatty acids are almost never conjugated (al-ternating single and double bonds, as in XCHUCHXCHUCHX), but areseparated by a methylene group (XCHUCHXCH2XCHUCHX). In nearlyall naturally occurring unsaturated fatty acids, the double bonds are in thecis configuration.

The physical properties of the fatty acids, and of compounds that con-tain them, are largely determined by the length and degree of unsaturationof the hydrocarbon chain. The nonpolar hydrocarbon chain accounts for thepoor solubility of fatty acids in water. Lauric acid (12:0, Mr 200), for exam-ple, has a solubility in water of 0.063 mg/g—much less than that of glucose

364 Part II Structure and Catalysis

Some Naturally Occurring Fatty Acids

Solubility at 30 �C(mg/g solvent)

Carbon Common name Meltingskeleton Structure* Systematic name† (derivation) point (�C) Water Benzene

12:0 CH3(CH2)10COOH n-Dodecanoic acid Lauric acid 44.2 0.063 2,600(Latin laurus,“laurel plant”)

14:0 CH3(CH2)12COOH n-Tetradecanoic acid Myristic acid 53.9 0.024 874(Latin Myristica,nutmeg genus)

16:0 CH3(CH2)14COOH n-Hexadecanoic acid Palmitic acid 63.1 0.0083 348(Latin palma,“palm tree”)

18:0 CH3(CH2)16COOH n-Octadecanoic acid Stearic acid 69.6 0.0034 124(Greek stear,“hard fat”)

20:0 CH3(CH2)18COOH n-Eicosanoic acid Arachidic acid 76.5(Latin Arachis,legume genus)

24:0 CH3(CH2)22COOH n-Tetracosanoic acid Lignoceric acid 86.0(Latin lignum,“wood” � cera, “wax”)

16:1(D9) CH3(CH2)5CHUCH(CH2)7COOH cis-9-Hexadecenoic acid Palmitoleic acid 1�0.518:1(D9) CH3(CH2)7CHUCH(CH2)7COOH cis-9-Octadecenoic acid Oleic acid 13.4

(Latin oleum,“oil”)

18:2(D9,12) CH3(CH2)4CHUCHCH2CHU cis-,cis-9,12-Octadecadienoic Linoleic acid 1�5CH(CH2)7COOH acid (Greek linon, “flax”)

18:3(D9,12,15) CH3CH2CHUCHCH2CHU cis-,cis-,cis-9,12,15- a-Linolenic acid �11CHCH2CHUCH(CH2)7COOH Octadecatrienoic acid

20:4(D5,8,11,14) CH3(CH2)4CHUCHCH2CHU cis-,cis-,cis-,cis-5,8,11,14- Arachidonic acid �49.5CHCH2CHUCHCH2CHU Icosatetraenoic acidCH(CH2)3COOH

*All acids are shown in their nonionized form. At pH 7, all free fatty acids have an ionized carboxylate. Note that numbering of carbon atoms begins at the carboxylcarbon.†The prefix n- indicates the “normal” unbranched structure. For instance, “dodecanoic” simply indicates 12 carbon atoms, which could be arranged in a variety ofbranched forms; “n-dodecanoic” specifies the linear, unbranched form. For unsaturated fatty acids, the configuration of each double bond is indicated; in biologicalfatty acids the configuration is almost always cis.

table 11–1

(Mr 180), which is 1,100 mg/g. The longer the fatty acyl chain and the fewerthe double bonds, the lower is the solubility in water. The carboxylic acidgroup is polar (and ionized at neutral pH) and accounts for the slight solu-bility of short-chain fatty acids in water.

Melting points are also strongly influenced by the length and degree ofunsaturation of the hydrocarbon chain. At room temperature (25 �C), thesaturated fatty acids from 12:0 to 24:0 have a waxy consistency, whereasunsaturated fatty acids of these lengths are oily liquids. This difference inmelting points is due to different degrees of packing of the fatty acid mole-cules (Fig. 11–1). In the fully saturated compounds, free rotation aroundeach carbon–carbon bond gives the hydrocarbon chain great flexibility; themost stable conformation is the fully extended form, in which the steric hin-drance of neighboring atoms is minimized. These molecules can pack to-gether tightly in nearly crystalline arrays, with atoms all along their lengthsin van der Waals contact with the atoms of neighboring molecules. In un-saturated fatty acids, a cis double bond forces a kink in the hydrocar-bon chain. Fatty acids with one or several such kinks cannot pack togetheras tightly as fully saturated fatty acids and their interactions with eachother are therefore weaker. Because it takes less thermal energy to disor-der these poorly ordered arrays of unsaturated fatty acids, they havemarkedly lower melting points than saturated fatty acids of the same chainlength (Table 11–1).

In vertebrates, free fatty acids (unesterified fatty acids having a freecarboxylate group) circulate in the blood bound noncovalently to a proteincarrier, serum albumin. However, fatty acids are present in blood plasmamostly as carboxylic acid derivatives such as esters or amides. Lacking thecharged carboxylate group, these fatty acid derivatives are generally evenless soluble in water than are the free fatty acids.

Chapter 11 Lipids 365

figure 11–1The packing of fatty acids into stable aggregates. Theextent of packing depends on the degree of saturation.(a) Two representations of the fully saturated acid stearicacid (stearate at pH 7) in its usual extended conforma-tion. Each line segment of the zig-zag represents a singlebond between adjacent carbons. (b) The cis double bond(shaded) in oleic acid (oleate) does not permit rotationand introduces a rigid bend in the hydrocarbon tail. Allother bonds in the chain are free to rotate. (c) Fully satu-rated fatty acids in the extended form pack into nearlycrystalline arrays, stabilized by many hydrophobic interac-tions. (d) The presence of one or more cis double bondsinterferes with this tight packing and results in less stableaggregates.(a)

Carboxyl

Hydrocarbon

group

chain

C

�O O

(b)

C

�O O

Saturatedfatty acids

(c) (d)

Mixture of saturated andunsaturated fatty acids

Triacylglycerols Are Fatty Acid Esters of GlycerolThe simplest lipids constructed from fatty acids are the triacylglycerols,

also referred to as triglycerides, fats, or neutral fats. Triacylglycerols arecomposed of three fatty acids each in ester linkage with a single glycerol(Fig. 11–2). Those containing the same kind of fatty acid in all three posi-tions are called simple triacylglycerols and are named after the fatty acidthey contain. Simple triacylglycerols of 16:0, 18:0, and 18:1, for example,are tristearin, tripalmitin, and triolein, respectively. Most naturally occur-ring triacylglycerols are mixed; they contain two or more different fattyacids. To name these compounds unambiguously, the name and position ofeach fatty acid must be specified.

Because the polar hydroxyls of glycerol and the polar carboxylates ofthe fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hy-drophobic molecules, essentially insoluble in water. Lipids have lower spe-cific gravities than water, which explains why mixtures of oil and water (oil-and-vinegar salad dressing, for example) have two phases: oil, with thelower specific gravity, floats on the aqueous phase.

Triacylglycerols Provide Stored Energy and InsulationIn most eukaryotic cells, triacylglycerols form a separate phase of micro-scopic, oily droplets in the aqueous cytosol, serving as depots of metabolicfuel (Fig. 11–3). In vertebrates, specialized cells called adipocytes, or fatcells, store large amounts of triacylglycerols as fat droplets that nearly fillthe cell. Triacylglycerols are also stored as oils in the seeds of many typesof plants, providing energy and biosynthetic precursors during seed germi-nation (Fig. 11–3b). Adipocytes and germinating seeds contain lipases, en-zymes that catalyze the hydrolysis of stored triacylglycerols, releasing fattyacids for export to sites where they are required as fuel.

There are two significant advantages to using triacylglycerols as storedfuels rather than polysaccharides such as glycogen and starch. First, be-cause the carbon atoms of fatty acids are more reduced than those of sug-ars, oxidation of triacylglycerols yields more than twice as much energy,gram for gram, as the oxidation of carbohydrates. Second, because triacyl-glycerols are hydrophobic and therefore unhydrated, the organism that car-ries fat as fuel does not have to carry the extra weight of water of hydration


Glycerol

HO

CH2

OH

H

CH2

OHC

figure 11–2 Glycerol and a triacylglycerol. The mixed triacylglycerolshown here has three different fatty acids attached to theglycerol backbone. When glycerol has two different fattyacids at C-1 and C-3, the C-2 is a chiral center (p. 59).

8 m(a) �

3 �m(b) �

figure 11–3 Fat stores in cells. (a) Cross section of four guinea pigadipocytes, showing huge fat droplets that virtually fill thecells. Also visible are several capillaries in cross section.(b) Cross section of a cotyledon cell from a seed of theplant Arabidopsis. The large dark structures are proteinbodies, which are surrounded by stored oils in the light-colored oil bodies.

CO

OCH2

1 3

2

O

CO

HCH2

O CO

1-Stearoyl, 2-linoleoyl, 3-palmitoyl glycerol,a mixed triacylglycerol

C

that is associated with stored polysaccharides (2 g per gram of polysaccha-ride). Humans have fat tissue, which is composed primarily of adipocytes,under the skin, in the abdominal cavity, and in the mammary glands. Mod-erately obese people with 15 to 20 kg of triacylglycerols deposited in theiradipocytes could meet their energy needs for months by drawing on theirfat stores. In contrast, the human body can store less than a day’s energysupply in the form of glycogen. Carbohydrates such as glucose and glyco-gen do offer certain advantages as quick sources of metabolic energy, oneof which is their ready solubility in water.

In some animals, triacylglycerols stored under the skin serve not onlyas energy stores but as insulation against low temperatures. Seals, walruses,penguins, and other warm-blooded polar animals are amply padded with tri-acylglycerols. In hibernating animals (bears, for example), the huge fat re-serves accumulated before hibernation serve the dual purposes of insula-tion and energy storage (see Box 17–1, “Fat Bears Carry on b-Oxidation inTheir Sleep”). The low density of triacylglycerols is the basis for another remarkable function of these compounds. In sperm whales, a store of tri-acylglycerols and waxes allows the animals to match the buoyancy of theirbodies to that of their surroundings during deep dives in cold water (Box11–1).

367

box 11–1 Sperm Whales: Fatheads of the Deep

Studies of sperm whales have uncovered anotherway in which triacylglycerols are biologically use-ful. The sperm whale’s head is very large, ac-counting for over one-third of its total bodyweight. About 90% of the weight of the head ismade up of the spermaceti organ, a blubberymass that contains up to 18,000 kg (about 4tons) of spermaceti oil, a mixture of triacylglyc-erols and waxes containing an abundance of un-saturated fatty acids. This mixture is liquid at thenormal resting body temperature of the whale,about 37 �C, but it begins to crystallize at about31 �C and becomes solid when the temperaturedrops several more degrees.

The probable biological function of sperma-ceti oil has been deduced from research on theanatomy and feeding behavior of the spermwhale. These mammals feed almost exclusivelyon squid in very deep water. In their feedingdives they descend 1,000 m or more; the deepestrecorded dive is 3,000 m (almost 2 miles). Atthese depths, there are no competitors for thevery plentiful squid; the sperm whale rests qui-etly, waiting for schools of squid to pass.

For a marine animal to remain at a givendepth without a constant swimming effort, itmust have the same density as the surroundingwater. The sperm whale undergoes changes inbuoyancy to match the density of its surround-ings—from the tropical ocean surface to greatdepths where the water is much colder and thusdenser. The key is the freezing point of sperma-ceti oil. When the temperature of the oil is low-ered several degrees during a deep dive, it con-geals or crystallizes and becomes denser. Thusthe buoyancy of the whale changes to match thedensity of seawater. Various physiological mech-anisms promote rapid cooling of the oil during adive. During the return to the surface, the con-gealed spermaceti oil warms and melts, decreas-ing its density to match that of the surface water.Thus we see in the sperm whale a remarkableanatomical and biochemical adaptation. The tria-cylglycerols and waxes synthesized by the spermwhale contain fatty acids of the necessary chainlength and degree of unsaturation to give thespermaceti oil the proper melting point for theanimal’s diving habits.

Unfortunately for the sperm whale popula-tion, spermaceti oil was at one time consideredthe finest lamp oil and continues to be commer-cially valuable as a lubricant. Several centuries ofintensive hunting of these mammals have drivensperm whales onto the endangered species list,with a total worldwide population of about500,000 remaining.

Spermacetiorgan

Many Foods Contain TriacylglycerolsMost natural fats, such as those in vegetable oils, dairy products, and ani-mal fat, are complex mixtures of simple and mixed triacylglycerols. Thesecontain a variety of fatty acids differing in chain length and degree of satu-ration (Fig. 11–4). Vegetable oils such as corn (maize) and olive oil arecomposed largely of triacylglycerols with unsaturated fatty acids and thusare liquids at room temperature. They are converted industrially into solidfats by catalytic hydrogenation, which reduces some of their double bondsto single bonds. Triacylglycerols containing only saturated fatty acids, suchas tristearin, the major component of beef fat, are white, greasy solids atroom temperature.

When lipid-rich foods are exposed too long to the oxygen in air, theymay spoil and become rancid. The unpleasant taste and smell associatedwith rancidity result from the oxidative cleavage of the double bonds in un-saturated fatty acids, which produces aldehydes and carboxylic acids ofshorter chain length and therefore higher volatility.

Waxes Serve as Energy Stores and Water RepellentsBiological waxes are esters of long-chain (C14 to C36) saturated and unsatu-rated fatty acids with long-chain (C16 to C30) alcohols (Fig. 11–5). Theirmelting points (60 to 100 �C) are generally higher than those of triacylglyc-erols. In plankton, the free-floating marine microorganisms at the bottom ofthe food chain, waxes are the chief storage form of metabolic fuel.

Waxes also serve a diversity of other functions in nature related to theirwater-repellent properties and their firm consistency. Certain skin glands ofvertebrates secrete waxes to protect hair and skin and keep it pliable, lu-bricated, and waterproof. Birds, particularly waterfowl, secrete waxes fromtheir preen glands to keep their feathers water-repellent. The shiny leavesof holly, rhododendrons, poison ivy, and many tropical plants are coatedwith a thick layer of waxes, which prevents excessive evaporation of waterand protects against parasites.

Biological waxes find a variety of applications in the pharmaceutical,cosmetic, and other industries. Lanolin (from lamb’s wool), beeswax (Fig.11–5), carnauba wax (from a Brazilian palm tree), and wax extracted fromspermaceti oil (from whales; see Box 11–1) are widely used in the manu-facture of lotions, ointments, and polishes.

368 Part II Structure and CatalysisF

atty

aci

ds (

% o

f to

tal)

Natural fats at 25 °C

Olive oil,liquid

Butter,soft solid

Beef fat,hard solid

C16 and C18saturated

C16 and C18unsaturated

C4 to C14saturated

20

40

60

80

100

figure 11–4 Fatty acid composition of three food fats. Olive oil,butter, and beef fat consist of mixtures of triacylglycerols,differing in their fatty acid composition. The meltingpoints of these fats—and hence their physical state atroom temperature (25 �C)—are a direct function of theirfatty acid composition. Olive oil has a high proportion oflong-chain (C16 and C18) unsaturated fatty acids, whichaccounts for its liquid state at 25 �C. The higher propor-tion of long-chain (C16 and C18) saturated fatty acids inbutter increases its melting point, so butter is a soft solidat room temperature. Beef fat, with an even higher pro-portion of long-chain saturated fatty acids, is a hard solid.

figure 11–5Biological wax. (a) Triacontanoylpalmitate, the majorcomponent of beeswax, is an ester of palmitic acid withthe alcohol triacontanol. (b) A honeycomb, constructed ofbeeswax, is firm at 25 �C and completely impervious towater. The term “wax” originates in the Old English wordweax, meaning “the material of the honeycomb.”

CH3(CH2)14 C

O

O CH2 (CH2)28 CH3

Palmitic acid

(a)

1-Triacontanol

(b)


Structural Lipids in MembranesThe central architectural feature of biological membranes is a double layerof 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 andtheir hydrophilic interactions with water direct their packing into sheetscalled membrane bilayers. In the next sections we describe three generaltypes of membrane lipids: glycerophospholipids, in which the hydrophobicregions are composed of two fatty acids joined to glycerol; sphingolipids, inwhich a single fatty acid is joined to a fatty amine, sphingosine; and sterols,compounds characterized by a rigid system of four fused hydrocarbon rings.The hydrophilic moieties in these amphipathic compounds may be as sim-ple as a single XOH group at one end of the sterol ring system, or they maybe much more complex. In glycerophospholipids and some sphingolipids, apolar head group is joined to the hydrophobic moiety by a phosphodiesterlinkage; these are the phospholipids. Other sphingolipids lack phosphatebut have a simple sugar or complex oligosaccharide at their polar ends;these are the glycolipids (Fig. 11–6). Within these groups of membranelipids, enormous diversity results from various combinations of fatty acid“tails” and polar “heads.” The arrangement of these lipids in membranes,and their structural and functional roles therein, are considered in the nextchapter.

Glycerophospholipids Are Derivatives of Phosphatidic AcidGlycerophospholipids, also called phosphoglycerides, are membranelipids in which two fatty acids are attached in ester linkage to the first andsecond carbons of glycerol, and a highly polar or charged group is attachedthrough a phosphodiester linkage to the third carbon. Glycerol is prochiral;it has no asymmetric carbons, but attachment of phosphate at either endconverts it into a chiral compound, which can be correctly named either L-glycerol 3-phosphate or D-glycerol 1-phosphate (Fig. 11–7). Glycerophos-pholipids are named as derivatives of the parent compound phospha-tidic acid (Fig. 11–8), according to the polar alcohol in the head group.

Storage Membrane lipids (polar)lipids

(neutral)

Phospholipids Glycolipids

Triacylglycerols Glycerophospholipids Sphingolipids Sphingolipids

Gly

cero

lFatty acid Alcohol

Sph

ingo

sin

e

Mono- oroligosaccharide

Fatty acid

Fatty acid

Gly

cero

l

Fatty acid

Fatty acid

PO4

Fatty acid

PO4 Choline

Sph

ingo

sin

e

Fatty acid

figure 11–6 The principal classes of storage and membrane lipids.All the lipids shown here have either glycerol or sphingo-sine as the backbone. A third class of membrane lipids,the sterols, is described later (see Fig. 11–14).

1CH2OH

2C OHH

3CH2 O O�

O

O�

P

figure 11–7L-Glycerol 3-phosphate, the backbone of phospholipids.Note that this compound can also be called D-glycerol 1-phosphate.


Phosphatidylcholine and phosphatidylethanolamine have choline andethanolamine in their polar head groups, for example. In all of these com-pounds, the head group is joined to glycerol through a phosphodiesterbond, in which the phosphate group bears a negative charge at neutral pH.The polar alcohol may be negatively charged (as in phosphatidylinositol

figure 11–8Glycerophospholipids. The common glycerophospho-lipids are diacylglycerols linked to head-group alcoholsthrough a phosphodiester bond. Phosphatidic acid, aphosphomonoester, is the parent compound. Each deriva-

Glycerophospholipid

Head-group

(general structure)

substituent

2C

1CH2 O C

O

3CH2 O P

O

O�

O X

H O

O

Name of X Formula of XNet charge(at pH 7)

H �1

Ethanolamine 0

Choline 0

Serine �1

Glycerol �1

myo-Inositol 4,5- bisphosphate

�4

Phosphatidyl- �2glycerol

CH2 CH2 N�

H3

CH2 CH2 N�

(CH3)3

CH2 C

COO�

H N�

H3

H

OH

HO

C

C

CH2

HOH

H2 O P

O

O�

O CH2

C

CH2 O C

O

R2

H O C

O

R1

C

OH

H

H

H

O P

O P

H H

HO2HCHC2

1

2 3

4

56

HC

OH

Name ofglycerophospholipid

Phosphatidic acid

Phosphatidylethanolamine

Phosphatidylcholine

Phosphatidylserine

Phosphatidylglycerol

Phosphatidylinositol 4,5-bisphosphate

Cardiolipin

Saturated fatty acid(e.g., palmitic acid)

Unsaturated fatty acid(e.g., oleic acid)

tive is named for the head-group alcohol (X), with theprefix “phosphatidyl-.” In cardiolipin, two phosphatidicacids share a single glycerol.

4,5-bisphosphate), neutral (phosphatidylserine), or positively charged(phosphatidylcholine, phosphatidylethanolamine). As we shall see in Chap-ter 12, these charges contribute significantly to the surface properties ofmembranes.

The fatty acids in glycerophospholipids can be any of a wide variety, soa given phospholipid (phosphatidylcholine, for example) may consist of anumber of molecular species, each with its unique complement of fattyacids. The distribution of molecular species is specific for different organ-isms, different tissues of the same organism, and different glycerophospho-lipids in the same cell or tissue. In general, glycerophospholipids contain aC16 or C18 saturated fatty acid at C-1 and a C18 to C20 unsaturated fatty acidat C-2. With few exceptions, the biological significance of the variation infatty acids and head groups is not yet understood.

Some Phospholipids Have Ether-Linked Fatty AcidsSome animal tissues and some unicellular organisms are rich in ether lipids,in which one of the two acyl chains is attached to glycerol in ether, ratherthan ester, linkage. The ether-linked chain may be saturated, as in the alkylether lipids, or may contain a double bond between C-1 and C-2, as in plas-

malogens (Fig. 11–9). Vertebrate heart tissue is uniquely enriched in etherlipids; about half of the heart phospholipids are plasmalogens. The mem-branes of halophilic bacteria, ciliated protists, and certain invertebrates alsocontain high proportions of ether lipids. The functional significance of etherlipids in these membranes is unknown; perhaps their resistance to the phos-pholipases that cleave ester-linked fatty acids from membrane lipids is im-portant in some roles. At least one ether lipid, platelet-activating factor,

is a potent molecular signal. It is released from leukocytes called basophilsand stimulates platelet aggregation and the release of serotonin (a vaso-constrictor) from platelets. It also exerts a variety of effects on liver, smoothmuscle, heart, uterine, and lung tissues, and plays an important role in in-flammation and the allergic response.

ether-linked alkene

2C

1CH2 O CH

CH

H O C

O3CH2

O

O P�O

O CH2 CH2 N�

(CH3)3

Plasmalogen

O P

O

3C

2C

1CH2 O CH2 CH2

H O C

O

CH3

H2

�O

O CH2 CH2 N�

(CH3)3

acetyl ester

Platelet-activating factor

ether-linked alkane

figure 11–9 Ether lipids. Plasmalogens have an ether-linked alkenylchain where most glycerophospholipids have an ester-linked fatty acid (compare Fig. 11–8). Platelet-activatingfactor has a long ether-linked alkyl chain at C-1 of glyc-erol, but C-2 is ester-linked to acetic acid, which makesthe compound much more water-soluble than most glyc-erophospholipids and plasmalogens. The head-groupalcohol is choline in plasmalogens and in platelet-activating factor.


Sphingolipids Are Derivatives of SphingosineSphingolipids, the second large class of membrane lipids, also have a po-lar head group and two nonpolar tails, but unlike glycerophospholipids theycontain no glycerol. Sphingolipids are composed of one molecule of thelong-chain amino alcohol sphingosine (4-sphingenine) or one of its deriva-tives, one molecule of a long-chain fatty acid, and a polar head group that isjoined by a glycosidic linkage in some cases and by a phosphodiester in oth-ers (Fig. 11–10).

Carbons C-1, C-2, and C-3 of the sphingosine molecule are structurallyanalogous to the three carbons of glycerol in glycerophospholipids. When afatty acid is attached in amide linkage to the XNH2 on C-2, the resultingcompound is a ceramide, which is structurally similar to a diacylglycerol.Ceramide is the structural parent of all sphingolipids.

There are three subclasses of sphingolipids, all derivatives of ceramidebut differing in their head groups: sphingomyelins, neutral (uncharged) gly-colipids, and gangliosides. Sphingomyelins contain phosphocholine orphosphoethanolamine as their polar head group and are therefore classifiedalong with glycerophospholipids as phospholipids (Fig. 11–6). Indeed,sphingomyelins resemble phosphatidylcholines in their general propertiesand three-dimensional structure, and in having no net charge on their head


Sphingolipid

Sphingosine

Fatty acid

(generalstructure)

2C

HO 3CH CH CH (CH2)12 CH3

1CH2 O X

H N

H

C

O

Name of X Formula of XName of sphingolipid

Ceramide

PhosphocholineSphingomyelin

Neutral glycolipids Glucosylcerebroside Glucose

Di-, tri-, or tetrasaccharide

Lactosylceramide (a globoside)

Complex oligosaccharideGanglioside GM2

P

O

O�

O CH2 CH2 N�

(CH3)3

CH2OH

HO

Glc

GalGlc

Gal

GalNAc

Neu5Ac

H

H

OH

HO

OH

HH

H

figure 11–10 Sphingolipids. The first three carbons at the polar end ofsphingosine are analogous to the three carbons of glyc-erol in glycerophospholipids. The amino group at C-2bears a fatty acid in amide linkage. The fatty acid isusually saturated or monounsaturated, with 16, 18, 22, or24 carbon atoms. Ceramide is the parent compound forthis group. Other sphingolipids differ in the polar headgroup (X) attached at C-1. Gangliosides have verycomplex oligosaccharide head groups. Standard abbrevia-tions for sugars are used in this figure: Glc, D-glucose;Gal, D-galactose; GalNAc, N-acetyl-D-galactosamine;Neu5Ac, N-acetylneuraminic acid (sialic acid).


groups (Fig. 11–11). Sphingomyelins are present in the plasma membraneof animal cells and are especially prominent in myelin, a membranoussheath that surrounds and insulates the axons of some neurons—thus thename “sphingomyelins.”

Glycosphingolipids, which occur largely in the outer face of plasmamembranes, have head groups with one or more sugars connected directlyto the XOH at C-1 of the ceramide moiety; they do not contain phosphate.Cerebrosides have a single sugar linked to ceramide; those with galactoseare characteristically found in the plasma membranes of cells in neural tis-sue, and those with glucose in the plasma membranes of cells in nonneuraltissues. Globosides are neutral (uncharged) glycosphingolipids with twoor more sugars, usually D-glucose, D-galactose, or N-acetyl-D-galactosamine.Cerebrosides and globosides are sometimes called neutral glycolipids, asthey have no charge at pH 7.

Gangliosides, the most complex sphingolipids, have oligosaccharidesas their polar head groups and one or more residues of N-acetylneuraminicacid (Neu5Ac), also called sialic acid, at the termini. Sialic acid gives gan-gliosides the negative charge at pH 7 that distinguishes them from globo-sides. Gangliosides with one sialic acid residue are in the GM (M for mono-)series, those with two sialic acids are in the GD (D for di-) series, and so on.

Sphingolipids at Cell Surfaces Are Sites of Biological RecognitionWhen sphingolipids were discovered a century ago by the physician-chemist Johann Thudichum, their biological role seemed as enigmatic asthe Sphinx, for which he therefore named them. In humans, at least 60 dif-ferent sphingolipids have been identified in cellular membranes. Many ofthese are especially prominent in the plasma membranes of neurons, andsome are clearly recognition sites on the cell surface, but for only a fewsphingolipids has a specific function been discovered. The carbohydratemoieties of certain sphingolipids define the human blood groups and there-fore determine the type of blood that individuals can safely receive in bloodtransfusions (Fig. 11–12, p. 374). The kinds and amounts of gangliosides inthe plasma membrane change dramatically with embryonic development,and tumor formation induces the synthesis of a new complement of gan-gliosides. Very low concentrations of a specific ganglioside induce differen-tiation of cultured neuronal tumor cells. Investigation of the biological rolesof diverse gangliosides remains fertile ground for future research.

NCH3

CH3

�C

CH2 O

O

O�

H2

O

H2

Phosphatidylcholine

CH

O C

OC C

O

PH2

OC

CH3

figure 11–11 The similarities in shape and in molecular structure ofphosphatidylcholine (a glycerophospholipid) and sphin-gomyelin (a sphingolipid) are clear when their space-filling and structural formulas are drawn as here.

CH3 N

CH3

CH3

�C

CH2 O

O

O�

H2 O C

PH2

CH

NH

OC

OH

HC

Sphingomyelin

H

OH

COO�

OC

OH

H C

OH

H CH2OH

HN C

O

CH3

N-Acetylneuraminic acid (sialic acid)

H

H

H

HOH

(Neu5Ac)

Johann Thudichum1829–1901


Phospholipids and Sphingolipids Are Degraded in LysosomesMost cells continually degrade and replace their membrane lipids. For eachhydrolyzable bond in a glycerophospholipid, there is a specific hydrolyticenzyme in the lysosome (Fig. 11–13). Phospholipases of the A type removeone of the two fatty acids, producing a lysophospholipid. (These esterasesdo not attack the ether link of plasmalogens.) Lysophospholipases removethe remaining fatty acid.

Gangliosides are degraded by a set of lysosomal enzymes that catalyzethe stepwise removal of sugar units, finally yielding a ceramide. A geneticdefect in any of these hydrolytic enzymes leads to the accumulation of gan-gliosides in the cell, with severe medical consequences (Box 11–2).

Ceramide

Gal

O Antigen

A Antigen

B Antigen

Sphingosine

Fatty acid

Glc Gal GalNAc Gal

Fuc

GalNAc

figure 11–12 Glycosphingolipids as determinants of blood groups.The human blood groups (O, A, B) are determined in partby the oligosaccharide head groups (blue) of these gly-cosphingolipids. The same three oligosaccharides arealso found attached to certain blood proteins of individ-uals of blood types O, A, and B, respectively. (Fuc repre-sents the sugar fucose.)

figure 11–13 The specificities of phospholipases. Phospholipases A1

and A2 hydrolyze the ester bonds of intact glycerophos-pholipids at C-1 and C-2 of glycerol, respectively. Phos-pholipases C and D each split one of the phosphodiesterbonds in the head group. Some phospholipases act ononly one type of glycerophospholipid, such as phos-phatidylinositol 4,5-bisphosphate (shown here) or phos-phatidylcholine; others are less specific. When one of thefatty acids has been removed by a type A phospholipase,the second fatty acid is cleaved from the molecule by alysophospholipase (not shown).

Phospholipase A1

Phospholipase C

Phospholipase A2

Phospholipase D

O P

O

3C

2C

1CH2 O C

O

H O C

O

H2

O�

O

H

OHHOOH

H

H

H

O P

O P

H H

375

box 11–2 Inherited Human Diseases Resulting from Abnormal Accumulations of Membrane Lipids

�-galactosidase

�-galacto-sidase A

�-galactosidase

Ceramide

GM1

Generalized gangliosidosis

glucocerebrosidase

Ceramide

Ceramide

Gaucher’s disease

hexosaminidase A

Ceramide

Tay-Sachs disease

GM2

hexosaminidaseA and B

Ceramide

Sandhoff ’s disease

Globoside

Ceramide

Fabry’s disease

gangliosideneuraminidase

Ceramide

GM3

sphingo-myelinase

Ceramide Phosphocholine

Sphingomyelin

Phosphocholine

Niemann-Pick disease

Ceramide

Glc

Gal

GalNAc

Neu5Ac

the brain, spleen, and liver. The disease becomesevident in infants, and causes mental retardationand early death.

More common is Tay-Sachs disease (Fig. 2), inwhich ganglioside GM2 accumulates in the brainand spleen owing to lack of the enzyme hexos-aminidase A. The symptoms of Tay-Sachs dis-ease are progressive retardation in development,paralysis, blindness, and death by the age of 3 or4 years.

Genetic counseling can predict and avertmany inheritable diseases. Tests on prospectiveparents can detect abnormal enzymes, then DNAtesting can determine the exact nature of the de-fect and the risk it poses for offspring. Once apregnancy occurs, fetal cells obtained by sam-pling a part of the placenta (chorionic villusbiopsy) or the fluid surrounding the fetus (am-niocentesis) can be tested in the same way.

figure 1Pathways for the breakdown of GM1, globoside, and sphingomyelin toceramide. A defect in the enzyme hydrolyzing a particular step is indi-cated by �, and the disease that results from accumulation of the partialbreakdown product is noted.

1 m�

figure 2 Electron micrograph of a portion of a braincell from an infant with Tay-Sachs disease,showing abnormal ganglioside deposits inthe lysosomes.

The polar lipids of membranes undergo constantmetabolic turnover, the rate of their synthesis nor-mally counterbalanced by the rate of breakdown.The breakdown of lipids is promoted by hydrolyticenzymes in lysosomes, each enzyme capable of hy-drolyzing a specific bond. When sphingolipiddegradation is impaired by a defect in one of theseenzymes (Fig. 1), partial breakdown products ac-cumulate in the tissues, causing serious disease.

For example, Niemann-Pick disease is causedby a rare genetic defect in the enzyme sphingo-myelinase, which cleaves phosphocholine fromsphingomyelin. Sphingomyelin accumulates in

Sterols Have Four Fused Carbon RingsSterols are structural lipids present in the membranes of most eukaryoticcells. Their characteristic structure is the steroid nucleus consisting of fourfused rings, three with six carbons and one with five (Fig. 11–14). Thesteroid nucleus is almost planar and is relatively rigid; the fused rings do notallow rotation about CXC bonds. Cholesterol, the major sterol in animaltissues, is amphipathic, with a polar head group (the hydroxyl group at C-3) and a nonpolar hydrocarbon body (the steroid nucleus and the hydro-carbon side chain at C-17) about as long as a 16-carbon fatty acid in its ex-tended form. Similar sterols are found in other eukaryotes: stigmasterol inplants and ergosterol in fungi, for example. Bacteria cannot synthesizesterols; a few bacterial species, however, can incorporate exogenous sterolsinto their membranes. The sterols of all eukaryotes are synthesized fromsimple five-carbon isoprene subunits, as are the fat-soluble vitamins,quinones, and dolichols described below.

In addition to their roles as membrane constituents, the sterols serve asprecursors for a variety of products with specific biological activities.Steroid hormones, for example, are potent biological signals that regulategene expression. Bile acids are polar derivatives of cholesterol that act asdetergents in the intestine, emulsifying dietary fats to make them morereadily accessible to digestive lipases. We shall return to cholesterol andother sterols in later chapters, to consider the structural role of cholesterol,in biological membranes (Chapter 12), signaling by steroid hormones(Chapter 13), the remarkable biosynthetic pathway to cholesterol, and thetransport of cholesterol by lipoprotein carriers (Chapter 21).

Lipids as Signals, Cofactors, and PigmentsThe two functional classes of lipids considered thus far (storage lipids andstructural lipids) are major cellular components; membrane lipids make up5% to 10% of the dry mass of most cells, and storage lipids more than 80%of the mass of an adipocyte. With some important exceptions, these lipidsplay a passive role in the cell; lipid fuels are stored until oxidized by en-zymes, and membrane lipids form impermeable barriers that surround cellsand cellular compartments. Another group of lipids, present in muchsmaller amounts, have active roles in the metabolic traffic as metabolitesand messengers. Some serve as potent signals—as hormones carried in theblood from one tissue to another, or as intracellular messengers generatedin response to an extracellular signal (hormone or growth factor). Othersfunction as enzyme cofactors in electron-transfer reactions in chloroplastsand mitochondria, or in the transfer of sugar moieties in a variety of glyco-sylation (addition of sugar) reactions. A third group consists of lipids witha system of conjugated double bonds: pigment molecules that absorb visi-


Alkyl

Polar Steroid

side

head nucleus

chain

20C

22C

23C

24C

25C

19CH3

H 27CH3

H2

H2

H2

H 21CH3

BA

C D

HO

26CH3

18CH3

5

11

4

21

8

3

9

76

10

1312 17

16

1514

figure 11–14 Cholesterol. The stick structure of cholesterol is visiblethrough a transparent surface contour model of the mole-cule. In the chemical structure, the rings are labeled Athrough D to simplify reference to derivatives of thesteroid nucleus and the carbon atoms are numbered inblue. The hydroxyl group on C-3 (red in both representa-tions) represents the polar head group. For storage andtransport of the sterol, this hydroxyl group condenses witha fatty acid to form a sterol ester.

HO

CH3

OH

17C

O

NH CH2 CH2 SO3�

Taurocholic acid(a bile acid)

OH

CH3

CH3


ble light. Some of these act as light-capturing pigments in vision and pho-tosynthesis; others produce natural colorations, such as the orange ofpumpkins and carrots and the yellow of canary feathers. Specialized lipidssuch as these are derived from lipids of the plasma membrane or from thefat-soluble vitamins A, D, E, and K. We will briefly describe a few of thesebiologically active lipids. In later chapters, their synthesis and biologicalroles are considered in more detail.

Phosphatidylinositols Act as Intracellular SignalsPhosphatidylinositol and its phosphorylated derivatives act at several levelsto regulate cell structure and metabolism (Fig. 11–15). Phosphatidylinosi-tol 4,5-bisphosphate (Fig. 11–8) in the cytosolic (inner) face of plasmamembranes serves as a specific binding site for certain cytoskeletal proteinsand for some soluble proteins involved in membrane fusion during exocy-tosis. It also serves as a reservoir of messenger molecules that are releasedinside the cell in response to extracellular signals interacting with specificreceptors on the outer surface of the plasma membrane. The signals actthrough a series of steps (Fig. 11–15) that begins with enzymatic removalof a phospholipid head group and ends with activation of an enzyme (pro-tein kinase C). For example, when the hormone vasopressin binds toplasma membrane receptors on the epithelial cells of the renal collectingduct, a specific phospholipase C is activated. Phospholipase C hydrolyzesthe bond between glycerol and phosphate in phosphatidylinositol 4,5-bisphosphate, releasing two products: inositol 1,4,5-trisphosphate (IP3),which is water-soluble, and diacylglycerol, which remains associated withthe plasma membrane. IP3 triggers release of Ca2� from the endoplasmicreticulum, and the combination of diacylglycerol and elevated cytosolicCa2� activates the enzyme protein kinase C. This enzyme catalyzes thetransfer of a phosphoryl group from ATP to a specific residue in one or moretarget proteins, thereby altering their activity and consequently the cell’smetabolism. Membrane sphingolipids can also serve as sources of intracel-lular messengers; both ceramide and sphingomyelin (Fig. 11–10) are po-tent regulators of protein kinases.

figure 11–15 Phosphatidylinositols in cellular regulation. Phos-phatidylinositol 4,5-bisphosphate in the plasma mem-brane is hydrolyzed by a specific phospholipase C inresponse to hormonal signals. Both products of hydrolysisact as intracellular messengers.

Phosphatidylinositol

phosphorylation 2ATPin plasmamembrane 2ADP

Phosphatidylinositol 4,5-bisphosphate

hormone-sensitive H2Ophospholipase Cin plasmamembrane

DiacylglycerolInositol 1,4,5-trisphosphate

Activation ofprotein kinase C

Release of intracellular Ca2�

Regulation of other enzymes(by protein phosphorylation)

Regulation of other enzymes(by Ca2�)

Eicosanoids Carry Messages to Nearby CellsEicosanoids are paracrine hormones, substances that act only on cells nearthe point of synthesis instead of being transported in the blood to act oncells in other tissues or organs. These fatty acid derivatives have a varietyof dramatic effects on vertebrate tissues. They are known to be involved inreproductive function, in the inflammation, fever, and pain associated withinjury or disease, in the formation of blood clots and the regulation of bloodpressure, in gastric acid secretion, and in a variety of other processes im-portant in human health or disease.

All eicosanoids are derived from the 20-carbon polyunsaturated fattyacid arachidonic acid, 20:4(D5,8,11,14) (Fig. 11–16), from which they taketheir general name (Greek eikosi, “twenty”). There are three classes ofeicosanoids: prostaglandins, thromboxanes, and leukotrienes.

Prostaglandins (PG) contain a five-carbon ring originating from thechain of arachidonic acid. They derive their name from the prostate gland,the tissue from which they were first isolated by Bengt Samuelsson andSune Bergström. Two groups of prostaglandins were originally defined:PGE, for ether-soluble, and PGF, for phosphate buffer–soluble (fosfat inSwedish). Each contains numerous subtypes, named PGE1, PGE2, and soforth.

Prostaglandins act in many tissues by regulating the synthesis of the in-tracellular messenger molecule 3�,5�-cyclic AMP (cAMP). Because cAMPmediates the action of many hormones, the prostaglandins affect a widerange of cellular and tissue functions. Some prostaglandins stimulate con-traction of the smooth muscle of the uterus during menstruation and labor.Others affect blood flow to specific organs, the wake-sleep cycle, and the re-sponsiveness of certain tissues to hormones such as epinephrine andglucagon. Prostaglandins in a third group elevate body temperature (pro-ducing fever) and cause inflammation and pain.


figure 11–16 Arachidonic acid and some eicosanoid derivatives. (a) In response to hormonal signals, phospholipase A2

cleaves arachidonic acid–containing membrane phospho-lipids to release arachidonic acid (arachidonate at pH 7),the precursor to various eicosanoids. (b) These includeprostaglandins such as PGE1, in which C-8 and C-12 ofarachidonate are joined to form the characteristic five-membered ring. In thromboxane A2, the C-8 and C-12are joined and an oxygen atom is added to form the six-membered ring. Leukotriene A4 has a series of three con-jugated double bonds. Nonsteroidal antiinflammatorydrugs (NSAIDs) such as aspirin, acetaminophen, andibuprofen block the formation of prostaglandins andthromboxanes from arachidonate by inhibiting theenzyme cyclooxygenase (prostaglandin H2 synthase).

C

C

C

O

X

H2

H O C

O

H2 O C

O

Membrane

Polar

phospholipid

headgroup

(a)

Phospholipase A2

148

5 11

Arachidonic acid

1C

O

OH

CH3

OC

O

O� O

H

CH3

Prostaglandin E1(PGE1)

C

O

O�

CH3

Leukotriene A4

OCH3

OHO

NSAIDs

C

O

O�8

12

O

HO

8

12

8

11

5

14

(b)

Thromboxane A2

Eicosanoids


The thromboxanes have a six-membered ring containing an ether.They are produced by platelets (thrombocytes) and act in the formation ofblood clots and the reduction of blood flow to the site of a clot. The non-steroidal antiinflammatory drugs (NSAIDs)—aspirin, ibuprofen, aceta-minophen, and meclofenamate, for example—were shown by John Vane toinhibit the enzyme prostaglandin H2 synthase (also called cyclooxygenaseor COX), which catalyzes an early step in the pathway from arachidonate toprostaglandins and thromboxanes (Fig. 11–16; see also Box 21–2).

Leukotrienes, found first in leukocytes, contain three conjugated dou-ble bonds. They are powerful biological signals. For example, leukotrieneD4, derived from leukotriene A4, induces contraction of the muscle liningthe airways to the lung. Overproduction of leukotrienes causes asthmaticattacks, and leukotriene synthesis is one target of antiasthmatic drugs suchas prednisone. The strong contraction of the smooth muscles of the lungthat occurs during anaphylactic shock is part of the potentially fatal allergicreaction in individuals hypersensitive to bee stings, penicillin, or variousother agents.

Steroid Hormones Carry Messages between TissuesSteroids are oxidized derivatives of sterols; they have the sterol nucleus but lack the alkyl chain attached to ring D of cholesterol, and they are more polar than cholesterol. Steroid hormones move through the blood-stream on protein carriers from their site of production to target tissues, where they enter cells, bind to highly specific receptor proteins in the nucleus, and trigger changes in gene expression and metabolism. Be-cause hormones have very high affinity for their receptors, very low con-centrations of hormones (nanomolar or less) are sufficient to produce re-sponses in target tissues. The major groups of steroid hormones are the male and female sex hormones and the hormones produced by the adrenal cortex, cortisol and aldosterone (Fig. 11–17). Prednisone and prednisolone are steroid drugs with potent antiinflammatory activities, mediated in part by the inhibition of arachidonate release by phospholi-pase A2 (Fig. 11–16) and consequent inhibition of the synthesis of leukotrienes, prostaglandins, and thromboxanes. They have a variety of medical applications, including the treatment of asthma and rheumatoid arthritis.

John Vane, Sune Bergström, and Bengt Samuelsson

figure 11–17 Steroids derived from cholesterol. Testosterone, themale sex hormone, is produced in the testes. Estradiol,one of the female sex hormones, is produced in theovaries and placenta. Cortisol and aldosterone are hor-mones synthesized in the cortex of the adrenal gland;they regulate glucose metabolism and salt excretion,respectively. Prednisolone and prednisone are steroiddrugs used as antiinflammatory agents.

C

CH2

OHO

Prednisolone PrednisoneO

H C3

H C3

OH

HO

C

CH2

OHO

O

H C3

H C3

OH

O

C

CH2

OHO

CortisolO

H C3

H C3

O

OH

HO

OH

Testosterone Estradiol

H3

H

OH

H C3

H C3

O O

C

CO

H2OH

O

Aldosterone

HO

H C3

O

C

C H


Vitamins A and D Are Hormone PrecursorsDuring the first third of the twentieth century, a major focus of research inphysiological chemistry was the identification of vitamins—compoundsthat are essential to the health of humans and other vertebrates but cannotbe synthesized by these animals and must therefore be obtained in the diet.Early nutritional studies identified two general classes of such compounds:those soluble in nonpolar organic solvents (fat-soluble vitamins) and thosethat could be extracted from foods with aqueous solvents (water-soluble vi-tamins). Eventually the fat-soluble group was resolved into the four vitamingroups A, D, E, and K, all of which are isoprenoid compounds synthesizedby the condensation of multiple isoprene units. Two of these (D and A)serve as hormone precursors.

Vitamin D3, also called cholecalciferol, is normally formed in the skinfrom 7-dehydrocholesterol in a photochemical reaction driven by the UVcomponent of sunlight (Fig. 11–18). Vitamin D3 is not itself biologically ac-tive, but is converted by enzymes in the liver and kidney to 1,25-dihydroxy-cholecalciferol, a hormone that regulates calcium uptake in the intestineand calcium levels in kidney and bone. Deficiency of vitamin D leads to de-fective bone formation and the disease rickets, for which administration of

CH3

CIsoprene

CH CH2CH2

figure 11–18Vitamin D3 production and metabolism. (a) Cholecalciferol(vitamin D3) is produced in the skin by UV irradiation of 7-dehydrocholesterol. In the liver, a hydroxyl group is added atC-25; in the kidney, a second hydroxylation at C-1 producesthe active hormone, 1,25-dihydroxycholecalciferol. Thishormone regulates the metabolism of Ca2� in kidney, intes-tine, and bone. (b) Dietary vitamin D prevents rickets, adisease once common in cold climates where heavy clothingblocks the UV component of sunlight necessary for the pro-duction of vitamin D3 in skin. On the left is a 2 1/2-year-oldboy with severe rickets; on the right, the same boy at age 5,after 14 months of vitamin D therapy.

CH3

CH3

CH3

CH3

CH3

7-DehydrocholesterolUV light

2 steps (in skin)

OH

1,25-Dihydroxycholecalciferol(1,25-dihydroxyvitamin D3)

HO

HO

CH2

76

76

4

4

1

5

5

3

3

2

2

10

9

8

1

CH3

CH3 CH3

CH325

25

HO

CH2

76

45

3

21

CH3

CH3 CH3

CH3

OH

(a)

Cholecalciferol (vitamin D3)

1 step in the liver

1 step in the kidney

Before vitamin D treatment After 14 months of vitamin D treatment(b)


vitamin D produces a dramatic cure. Vitamin D2 (ergocalciferol) is a com-mercial product formed by UV irradiation of the ergosterol of yeast. VitaminD2 is structurally similar to D3, with slight modification to the side chain at-tached to the sterol D ring. Both have the same biological effects, and D2 iscommonly added to milk and butter as a dietary supplement. Like steroidhormones, the product of vitamin D metabolism, 1,25-dihydroxycholecal-ciferol, regulates gene expression—for example, turning on the synthesis ofan intestinal Ca2�-binding protein.

Vitamin A (retinol) in its various forms functions as a hormone andas the visual pigment of the vertebrate eye (Fig. 11–19). Acting through re-ceptor proteins in the cell nucleus, the vitamin A derivative retinoic acidregulates gene expression in the development of epithelial tissue, includingskin. Retinoic acid is the active ingredient in the drug tretinoin (Retin-A),used in the treatment of severe acne and wrinkled skin. The vitamin A de-rivative retinal is the pigment that initiates the response of rod and conecells of the retina to light, producing a neuronal signal to the brain. This roleof retinal is described in detail in Chapter 13.

�-Carotene

Vitamin A1

point ofcleavage

(retinol)

C

oxidation ofalcohol toaldehyde

11-cis-Retinal

Neuronal

all-trans-Retinal

signal to brain

CH3

CH3

CH3

CH3

CH3CH3

12

CH3

CH3

CH3

CH3

CH2OH

CH3

15

11

CH3

CH3

CH3

CH3

2

7

6

12

C

1111

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

visiblelight

OH OH

(visual pigment)

(a)

(b) (e)

(d)oxidationof aldehydeto acid

Retinoic acid Hormonalsignal toepithelialcells

(c)

figure 11–19 Vitamin A1, its precursor, and derivatives. (a) b-Carotene is the precursor of vitamin A1.Isoprene structural units are set off by dashed red lines. Cleavage of b-carotene yieldstwo molecules of vitamin A1 (retinol) (b). Oxidation at C-15 converts retinol to the alde-hyde, retinal (c), and further oxidation produces retinoic acid (d), a hormone that regu-lates gene expression in skin. Retinal combines with the protein opsin to form rhodopsin(not shown), a visual pigment widely employed in nature. In the dark, retinal of rhodopsinis in the 11-cis form (c). When a rhodopsin molecule is excited by visible light, the 11-cis-retinal undergoes a series of photochemical reactions that convert it to all-trans-retinal(e), forcing a change in the shape of the entire rhodopsin molecule. This transformationin the rod cell of the vertebrate retina sends an electrical signal to the brain that is thebasis of visual transduction, a topic we address in more detail in Chapter 13.

Vitamin A was first isolated from fish liver oils; liver, eggs, whole milk,and butter are good dietary sources. In vertebrates, b-carotene, the pig-ment that gives carrots, sweet potatoes, and other yellow vegetables theircharacteristic color, can be enzymatically converted to vitamin A. Defi-ciency of vitamin A leads to a variety of symptoms in humans, including dry-ness of the skin, eyes, and mucous membranes; retarded development andgrowth; and night blindness, an early symptom commonly used in diagnos-ing vitamin A deficiency.

Vitamins E and K and the Lipid Quinones Are Oxidation-Reduction CofactorsVitamin E is the collective name for a group of closely related lipids calledtocopherols, all of which contain a substituted aromatic ring and a longisoprenoid side chain (Fig. 11–20a). Because they are hydrophobic, toco-pherols associate with cell membranes, lipid deposits, and lipoproteins inthe blood. Tocopherols are biological antioxidants. The aromatic ring reactswith and destroys the most reactive forms of oxygen radicals and other freeradicals, protecting unsaturated fatty acids from oxidation and preventingoxidative damage to membrane lipids, which can cause cell fragility. Toco-pherols are found in eggs and vegetable oils, and are especially abundant inwheat germ. Laboratory animals fed diets depleted of vitamin E developscaly skin, muscular weakness and wasting, and sterility. Vitamin E defi-ciency in humans is very rare; the principal symptom is fragile erythrocytes.

The aromatic ring of vitamin K (Fig. 11–20b) undergoes a cycle of ox-idation and reduction during the formation of active prothrombin, a bloodplasma protein essential in blood clot formation. Prothrombin is a prote-olytic enzyme that splits peptide bonds in the blood protein fibrinogen toconvert it to fibrin, the insoluble fibrous protein that holds blood clots to-gether. Henrik Dam and Edward A. Doisy independently discovered that vi-tamin K deficiency slows blood clotting, which can be fatal. Vitamin K defi-ciency is very uncommon in humans, aside from a small percentage ofinfants who suffer from hemorrhagic disease of the newborn, a potentiallyfatal disorder. In the United States, newborns are routinely given a 1 mg in-jection of vitamin K. Vitamin K1 (phylloquinone) is found in green plantleaves; a related form, vitamin K2 (menaquinone), is formed by bacteria re-siding in the vertebrate intestine.

Warfarin (Fig. 11–20c) is a synthetic compound that inhibits the for-mation of active prothrombin. It is particularly poisonous to rats, causingdeath by internal bleeding. Ironically, this potent rodenticide is also an in-valuable anticoagulant drug for treating humans at risk for excessive bloodclotting, such as surgical patients and victims of coronary thrombosis.

Ubiquinone (also called coenzyme Q) and plastoquinone (Fig. 11–20d, e) are isoprenoids that function as lipophilic electron carriers inthe oxidation-reduction reactions that drive ATP synthesis in mitochondriaand chloroplasts, respectively. Both ubiquinone and plastoquinone can ac-cept either one or two electrons and either one or two protons, as shown inFigure 19–2.

Dolichols Activate Sugar Precursors for BiosynthesisDuring assembly of the complex carbohydrates of bacterial cell walls, andduring the addition of polysaccharide units to certain proteins (glycopro-teins) and lipids (glycolipids) in eukaryotes, the sugar units to be added arechemically activated by attachment to isoprenoid alcohols called dolichols

(Fig. 11–20f). These compounds have strong hydrophobic interactionswith membrane lipids, anchoring the attached sugars to the membranewhere they participate in sugar-transfer reactions.

Edward A. Doisy1893–1986

Henrik Dam1895–1976



HOCH3

O

CH2 CH2 CH2 C

CH3

H CH2 CH2 CH2 C

CH3

H CH2 CH2 CH2 C

CH3

H CH3

OCH3

CH2 CH C

CH3

CH2 CH2 CH2 C

CH3

H CH2 2 CH2 CH2 C

CH3

H CH3

O

OHC

CH

H2 C

O

CH3

CH3O

OCH3

CH2 CH C

CH3

CH2 CH2 CH C

CH3

CH2 n CH2 CH C

CH3

CH3

CH3

OCH2 CH C

CH3

CH2

HO CH2 CH2 C

CH3

H CH2 CH2 CH C

CH3

CH2 n CH2 CH C

CH3

CH3

CH3

CH3

O

CH3

O

O

O

CH3O

CH3

CH2 CH C

CH3

CH2 n CH2 CH C

CH3

CH3

��

�

�

�

�

��

(b)Vitamin K1: a blood-clotting

cofactor (phylloquinone)

(d)Ubiquinone: a mitochondrial electron carrier (coenzyme Q)

(n 4–8)

(e)Plastoquinone: a chloroplast

electron carrier (n 4–8)

(f)Dolichol: a sugar carrier

(n 9–22)

(a)Vitamin E: an antioxidant

(c)Warfarin: a blood

anticoagulant

figure 11–20 Some other biologically active isoprenoid compounds or derivatives. Isoprene structural units are set off bydashed red lines. In most mammalian tissues, ubiquinone(also called coenzyme Q) has ten isoprene units.Dolichols of animals have 17 to 21 isoprene units (85 to105 carbon atoms), bacterial dolichols have 11, andthose of plants and fungi have 14 to 24.

Separation and Analysis of LipidsIn exploring the biological role of lipids in cells and tissues, it is useful toknow which lipids are present and in what proportions. Because lipids areinsoluble in water, their extraction and subsequent fractionation require theuse of organic solvents and some techniques not commonly used in the pu-rification of water-soluble molecules such as proteins and carbohydrates. Ingeneral, complex mixtures of lipids are separated by differences in the po-larity or solubility of the components in nonpolar solvents. Lipids that con-tain ester- or amide-linked fatty acids can be hydrolyzed by treatment withacid or alkali or with highly specific hydrolytic enzymes (phospholipases,


glycosidases) to yield their component parts for analysis. Some methodscommonly used in lipid analysis are shown in Figure 11–21 and discussedbelow.

Lipid Extraction Requires Organic SolventsNeutral lipids (triacylglycerols, waxes, pigments, etc.) are readily extractedfrom tissues with ethyl ether, chloroform, or benzene, solvents in whichlipid clustering driven by hydrophobic interactions does not occur. Mem-brane lipids are more effectively extracted by more polar organic solvents,such as ethanol or methanol, which reduce the hydrophobic interactionsamong lipid molecules while also weakening the hydrogen bonds and elec-trostatic interactions that bind membrane lipids to membrane proteins. Acommonly used extractant is a mixture of chloroform, methanol, and water,initially in volume proportions (1:2:0.8) that are miscible, producing a sin-gle phase. After tissue is homogenized in this solvent to extract all lipids,more water is added to the resulting extract and the mixture separates intotwo phases, methanol/water (top phase) and chloroform (bottom phase).The lipids remain in the chloroform layer, and more polar molecules such asproteins and sugars partition into the methanol/water layer.

Adsorption Chromatography Separates Lipids of Different PolarityThe complex mixture of tissue lipids can be fractionated further by chro-matographic procedures based on the different polarities of each class oflipid. In adsorption chromatography, an insoluble, polar material such as sil-ica gel (a form of silicic acid, Si(OH)4) is packed into a glass column, andthe lipid mixture (in chloroform solution) is applied to the top of the col-umn. (In high-performance liquid chromatography, the column is of smallerdiameter, and solvents are forced through the column under high pressure.)The polar lipids bind tightly to the polar silicic acid, but the neutral lipidspass directly through the column and emerge in the first chloroform wash(Fig. 11–21b). The polar lipids are then eluted, in order of increasing po-larity, by washing the column with solvents of progressively higher polarity.Uncharged but polar lipids (cerebrosides, for example) are eluted with ace-tone, and very polar or charged lipids (such as glycerophospholipids) areeluted with methanol.

Con

cen

trat

ion

Elution time

homogenized inchloroform/methanol/water

Water

Methanol/water

Chloroform

Adsorptionchromatography

(d)

(e) (f)

(c)

NaOH/methanol

Fatty acyl methyl esters

Polarlipids

Chargedlipids

Neutrallipids

Gas-liquidchromatography

High-performance

liquidchromatography

Tissue

18:0 16:114:0

16:0

(b)

Thin-layerchromatography

1 2 3 4 5 6 7 8 9

(a)

figure 11–21 Common procedures in the extraction, separation, and identification of cellular lipids.(a) Tissue is homogenized in a chloroform/methanol/water mixture, which on addition ofwater and removal of unextractable sediment by centrifugation yields two phases. Dif-ferent types of extracted lipids in the chloroform phase may be separated by (b) adsorp-tion chromatography on a column of silica gel, through which solvents of increasingpolarity are passed, or (c) thin-layer chromatography (TLC), in which lipids are carried upa silica gel–coated plate by a rising solvent front, less polar lipids traveling farther thanmore polar or charged lipids. TLC with appropriate solvents can also be used to separateclosely related lipid species; for example, the charged lipids phosphatidylserine, phos-phatidylglycerol, and phosphatidylinositol are easily separated by TLC. For the determina-tion of fatty acid composition, a lipid fraction containing ester-linked fatty acids is trans-esterified in a warm aqueous solution of NaOH and methanol (d), producing a mixture offatty acyl methyl esters. These methyl esters are then separated on the basis of chainlength and degree of saturation by (e) gas-liquid chromatography (GLC) or (f) high-per-formance liquid chromatography (HPLC). Precise determination of molecular mass bymass spectrometry (see Box 5–3) allows unambiguous identification of individual lipids.


Thin-layer chromatography on silicic acid employs the same principle. A thin layer of silica gel is spread onto a glass plate, to which it adheres. A small sample of lipids dissolved in chloroform is applied near one edge of the plate, which is dipped in a shallow container of an organic solvent or solvent mixture—all of which is enclosed within a chamber saturated with the solvent vapor. As the solvent rises on the plate by capillary action, it carries lipids with it (Fig. 11–21c). The less polar lipids move farthest, as they have less tendency to bind to the silicic acid. The lipids can be de-tected after separation by spraying the plate with a dye (rhodamine) that fluoresces when associated with lipids, or by exposing the plate to iodine fumes. Iodine reacts reversibly with the double bonds in fatty acids, such that lipids containing unsaturated fatty acids develop a yellow or brown color. A number of other spray reagents are also useful in detecting specific lipids. For subsequent analysis, regions containing separated lipids can be scraped from the plate and the lipids recovered by extraction with an organic solvent.

Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid DerivativesGas-liquid chromatography separates volatile components of a mixture ac-cording to their relative tendencies to dissolve in the inert material packedin the chromatography column and to volatilize and move through the col-umn, carried by a current of an inert gas such as helium. Some lipids arenaturally volatile, but most must first be derivatized to increase their volatil-ity (that is, lower their boiling point). For an analysis of the fatty acids in asample of phospholipids, the lipids are first heated in a methanol/HCl ormethanol/NaOH mixture, which converts fatty acids esterified to glycerolinto their methyl esters (transesterification). These fatty acyl methyl estersare then loaded onto the gas-liquid chromatography column, and the col-umn is heated to volatilize the compounds. Those fatty acyl esters most sol-uble in the column material partition into (dissolve in) that material; theless-soluble lipids are carried by the stream of inert gas and emerge firstfrom the column. The order of elution depends on the nature of the solidadsorbant in the column and on the boiling point of the components of thelipid mixture. Using these techniques, mixtures of fatty acids of variouschain lengths and various degrees of unsaturation can be completely re-solved (Fig. 11–21e).

Specific Hydrolysis Aids in Determination of Lipid StructureCertain classes of lipids are susceptible to degradation under specific con-ditions. For example, all ester-linked fatty acids in triacylglycerols, phos-pholipids, and sterol esters are released by mild acid or alkaline treatment,and somewhat harsher hydrolysis conditions release amide-bound fattyacids from sphingolipids. Enzymes that specifically hydrolyze certain lipidsare also useful in the determination of lipid structure. Phospholipases A, C,and D (Fig. 11–13) each split particular bonds in phospholipids and yieldproducts with characteristic solubilities and chromatographic behaviors.Phospholipase C, for example, releases a water-soluble phosphoryl alcohol(such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of spe-cific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determina-tion of a lipid structure.


Mass Spectrometry Reveals Complete Lipid StructureTo establish unambiguously the length of a hydrocarbon chain or the posi-tion of double bonds, mass spectral analysis of lipids or their volatile deriv-atives is invaluable. The chemical properties of similar lipids (for example,two fatty acids unsaturated at different positions, or two isoprenoids withdifferent numbers of isoprene units) are very much alike, and their posi-tions of elution from the various chromatographic procedures often do notdistinguish between them. When the output from a chromatography col-umn is constantly sampled by mass spectrometry, however, the componentsof a lipid mixture can be simultaneously separated and identified.

Lipids are water-insoluble cellular componentsthat can be extracted by nonpolar solvents.Some lipids serve as structural components ofmembranes and others as storage forms of fuel.Fatty acids, which provide the hydrocarboncomponents of many lipids, usually have an evennumber of carbon atoms (usually 12 to 24) andmay be saturated or unsaturated; unsaturatedfatty acids have double bonds in the cis configu-ration. In most unsaturated fatty acids, one dou-ble bond is at the D9 position (between C-9 and C-10).

Triacylglycerols contain three fatty acid mole-cules esterified to the three hydroxyl groups ofglycerol. Simple triacylglycerols contain only onetype of fatty acid, mixed triacylglycerols two orthree types. Triacylglycerols are primarily stor-age fats; they are present in many types of foods.

The polar lipids, with polar heads and nonpo-lar tails, are major components of membranes.The most abundant are the glycerophospho-lipids, which contain two fatty acid molecules es-terified to two hydroxyl groups of glycerol, and asecond alcohol, the head group, esterified to thethird hydroxyl of glycerol via a phosphodiesterbond. Glycerophospholipids differ in the struc-ture of the head group; common glycerophos-pholipids are phosphatidylethanolamine andphosphatidylcholine. The polar heads of theglycerophospholipids carry electric charges atpH near 7. The sphingolipids, also membranecomponents, contain sphingosine, a long-chainaliphatic amino alcohol, but no glycerol. Sphin-gomyelin has, in addition to phosphoric acid andcholine, two long hydrocarbon chains, one con-tributed by a fatty acid and the other by sphin-gosine. Three other classes of sphingolipids are

cerebrosides, globosides, and gangliosides,which contain sugar components. Cholesterol, asterol, is a precursor of steroids and an importantcomponent of the plasma membranes of animalcells.

Some types of lipids, although present in rela-tively small quantities, play critical roles as co-factors or signals. Phosphatidylinositol is hy-drolyzed to yield two intracellular messengers,diacylglycerol and inositol 1,4,5-trisphosphate.Prostaglandins, thromboxanes, and leukotrienesare extremely potent hormones derived fromarachidonate. Steroid hormones are derivedfrom sterols. Vitamins D, A, E, and K are fat-soluble compounds made up of isoprene units.All play essential roles in the metabolism orphysiology of animals. Vitamin D is precursor toa hormone that regulates calcium metabolism.Vitamin A furnishes the visual pigment of thevertebrate eye and acts as a regulator of gene ex-pression during epithelial cell growth. Vitamin Efunctions in the protection of membrane lipidsfrom oxidative damage, and vitamin K is essen-tial in the blood-clotting process. Ubiquinonesand plastoquinones, also isoprenoid derivatives,function as electron carriers in mitochondria andchloroplasts, respectively. Dolichols activate andanchor sugars on cellular membranes for use inthe synthesis of certain complex carbohydrates,glycolipids, and glycoproteins.

In the determination of lipid composition,lipids are extracted from tissues with organic sol-vents and separated by thin-layer, gas-liquid, orhigh-performance liquid chromatography. Indi-vidual lipids are identified by their chromato-graphic behavior, their susceptibility to hydroly-sis by specific enzymes, or by mass spectrometry.

summary


further readingGeneral

Gurr, M.I. & Harwood, J.L. (1990) Lipid Bio-

chemistry: An Introduction, 4th edn, Chapman &Hall, London.

A good general resource on lipid structure and me-tabolism, at the intermediate level.

Harwood, J.L. & Russell, N.J. (1984) Lipids in

Plants and Microbes, George Allen & Unwin, Ltd.,London.

Short, clear descriptions of lipid types, their distribu-tion, metabolism, and function in plants and mi-crobes; intermediate level.

Mead, J.F., Alfin-Slater, R.B., Howton, D.R., &

Popják, G. (1986) Lipids: Chemistry, Biochem-

istry and Nutrition, Plenum Press, New York.An intermediate-level textbook on chemical, meta-bolic, and nutritional aspects of lipids.

Vance, D.E. & Vance, J.E. (eds) (1996) Biochem-

istry of Lipids, Lipoproteins and Membranes, NewComprehensive Biochemistry, Vol. 31, Elsevier Sci-ence Publishing Co., Inc., New York.

An excellent collection of reviews on various aspectsof lipid structure, biosynthesis, and function.

Structural Lipids in Membranes

Gravel, R.A., Clarke, J.T.R., Kaback, M.M.,

Mahuran, D., Sandhoff, K., & Suzuki, K. (1995)The GM2 gangliosidoses. In The Metabolic and Mole-

cular Bases of Inherited Disease, 7th edn (Scriver,C.R., Beaudet, A.L., Sly, W.S., & Valle, D., eds), pp.2839–2879, McGraw-Hill, Inc., New York.

This article is one of many in a three-volume set thatcontains definitive descriptions of the clinical, bio-chemical, and genetic aspects of hundreds of humanmetabolic diseases—an authoritative source and fas-cinating reading.

Hakamori, S. (1986) Glycosphingolipids. Sci. Am.

254 (May), 44–53.

Hoekstra, D. (ed.) (1994) Cell Lipids, CurrentTopics in Membranes, Vol. 4, Academic Press, Inc.,San Diego.

Ostro, M.J. (1987) Liposomes. Sci. Am. 256 (Janu-ary), 102–111.

Sastry, P.S. (1985) Lipids of nervous tissue: compo-sition and metabolism. Prog. Lipid Res. 24, 69–176.

Spector, A.A. & Yorek, M.A. (1985) Membranelipid composition and cellular function. J. Lipid Res.

26, 1015–1035.

Lipids with Specific Biological Activities

Bell, R.M., Exton, J.H., & Prescott, S.M. (eds)

(1996) Lipid Second Messengers, Handbook ofLipid Research, Vol. 8, Plenum Press, New York.

Chojnacki, T. & Dallner, G. (1988) The biologicalrole of dolichol. Biochem. J. 251, 1–9.

Machlin, L.J. & Bendich, A. (1987) Free radicaltissue damage: protective role of antioxidant nutri-ents. FASEB J. 1, 441–445.

Brief discussion of tocopherols as antioxidants andtheir role in preventing damage by oxygen free radi-cals.

Prescott, S.M., Zimmerman, G.A., & McIntyre,

T.M. (1990) Platelet-activating factor. J. Biol. Chem.

265, 17,381–17,384.

Snyder, F., Lee, T.-C., & Blank, M.L. (1989)Platelet-activating factor and related ether lipid me-diators: biological activities, metabolism, and regula-tion. Ann. N. Y. Acad. Sci. 568, 35–43.

Suttie, J.W. (1993) Synthesis of vitamin K-depen-dent proteins. FASEB J. 7, 445–452.

Vermeer, C. (1990) g-Carboxyglutamate-containingproteins and the vitamin K-dependent carboxylase.Biochem. J. 266, 625–636.

Describes the biochemical basis for the requirementof vitamin K in blood clotting, and the importance ofcarboxylation in the synthesis of the blood-clottingprotein thrombin.

Viitala, J. & Järnefelt, J. (1985) The red cell sur-face revisited. Trends Biochem. Sci. 10, 392–395.

Includes discussion of the human A, B, and O bloodtype determinants.

Separation and Analysis of Lipids

Kates, M. (1986) Techniques of Lipidology: Isola-

tion, Analysis and Identification of Lipids, 2ndedn, Laboratory Techniques in Biochemistry and Mol-ecular Biology, Vol. 3, Part 2 (Burdon, R.H. & vanKnippenberg, P.H., eds), Elsevier Science PublishingCo., Inc., New York.

Matsubara, T. & Hagashi, A. (1991) FAB/Massspectrometry of lipids. Prog. Lipid Res. 30, 301–322.

An advanced discussion of the identification of lipidsby fast atom bombardment (FAB) mass spectrome-try, a powerful technique for structure determina-tion.


problems

1. Operational Definition of Lipids How is thedefinition of “lipid” different from the types of defi-nitions used for other biomolecules that we have considered, such as amino acids, nucleic acids, andproteins?

2. Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are: stearic acid,69.6 �C; oleic acid, 13.4 �C; linoleic acid, �5 �C; andlinolenic acid, �11 �C. (a) What structural aspect ofthese 18-carbon fatty acids can be correlated with themelting point? Provide a molecular explanation for thetrend in melting points. (b) Draw all of the possibletriacylglycerols that can be constructed from glycerol,palmitic acid, and oleic acid. Rank them in order of in-creasing melting point. (c) Branched-chain fatty acidsare found in some bacterial membrane lipids. Wouldtheir presence increase or decrease the fluidity of themembranes (that is, make them have a lower or highermelting point)? Why?

3. Preparation of Béarnaise Sauce During thepreparation of béarnaise sauce, egg yolks are incorpo-rated into melted butter to stabilize the sauce andavoid separation. The stabilizing agent in the egg yolksis lecithin (phosphatidylcholine). Suggest why thisworks.

4. Hydrophobic and Hydrophilic Components of

Membrane Lipids A common structural feature ofmembrane lipid molecules is their amphipathic nature.For example, in phosphatidylcholine, the two fattyacid chains are hydrophobic and the phosphocholinehead group is hydrophilic. For each of the followingmembrane lipids, name the components that serve asthe hydrophobic and hydrophilic units: (a) phosphati-dylethanolamine; (b) sphingomyelin; (c) galactosyl-cerebroside; (d) ganglioside; (e) cholesterol.

5. Alkali Lability of Triacylglycerols A commonprocedure for cleaning the grease trap in a sink is toadd a product that contains sodium hydroxide. Ex-plain why this works.

6. The Action of Phospholipases The venom ofthe Eastern diamondback rattler and the Indian cobracontains phospholipase A2, which catalyzes the hydroly-sis of fatty acids at the C-2 position of glycerophos-pholipids. The phospholipid breakdown product ofthis reaction is lysolecithin (lecithin is phosphatidyl-choline). At high concentrations, this and otherlysophospholipids act as detergents, dissolving themembranes of erythrocytes and lysing the cells. Ex-tensive hemolysis may be life-threatening.

(a) Detergents are amphipathic. What are the hy-drophilic and hydrophobic portions of lysolecithin?

(b) The pain and inflammation caused by a snake bitecan be treated with certain steroids. What is the basisof this treatment?

(c) Though high levels of phospholipase A2 can bedeadly, this enzyme is necessary for a variety of nor-mal metabolic processes. What are these processes?

7. Intracellular Messengers from Phosphati-

dylinositols When the hormone vasopressin stimu-lates cleavage of phosphatidylinositol 4,5-bisphos-phate by hormone-sensitive phospholipase C, twoproducts are formed. Compare their properties andtheir solubilities in water, and predict whether eitherwould diffuse readily through the cytosol.

8. Storage of Fat-Soluble Vitamins In contrast towater-soluble vitamins, which must be a part of ourdaily diet, fat-soluble vitamins can be stored in thebody in amounts sufficient for many months. Suggestan explanation for this difference based on solubilities.

9. Hydrolysis of Lipids Name the products of mildhydrolysis with dilute NaOH of (a) 1-stearoyl-2,3-dipalmitoylglycerol; (b) 1-palmitoyl-2-oleoylphos-phatidylcholine.

10. Effect of Polarity on Solubility Rank the fol-lowing in order of increasing solubility in water: a tri-acylglycerol, a diacylglycerol, and a monoacylglycerol,all containing only palmitic acid.

11. Chromatographic Separation of Lipids Amixture of lipids is applied to a silica gel column, andthe column is then washed with increasingly polar sol-vents. The mixture consists of: phosphatidylserine,cholesteryl palmitate (a sterol ester), phosphati-dylethanolamine, phosphatidylcholine, sphingomyelin,palmitate, n-tetradecanol, triacylglycerol, and choles-terol. In what order do you expect the lipids to elutefrom the column?

12. Identification of Unknown Lipids JohannThudichum, who practiced medicine in London about100 years ago, also dabbled in lipid chemistry in hisspare time. He isolated a variety of lipids from neuraltissue, and characterized and named many of them.His carefully sealed and labeled vials of isolated lipidswere rediscovered many years later. (a) How wouldyou confirm, using techniques not available to Thu-dichum, that the vials labeled “sphingomyelin” and“cerebroside” actually contain these compounds? (b) How would you distinguish sphingomyelin fromphosphatidylcholine by chemical, physical, or enzy-matic tests?

13. Ninhydrin to Detect Lipids on TLC Plates

Ninhydrin reacts specifically with primary amines toform a purplish-blue product. A thin-layer chro-matogram of rat liver phospholipids is sprayed withninhydrin, and the color is allowed to develop. Whichphospholipids can be detected in this way?

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