A 1,2-diacylglycerol that has a phosphate group esterified at carbon atom 3 of the glycerol backbone is a glycerophospholipid, also known as a phosphoglyceride or a glycerol phosphatide F
Trang 1glycerols are normally soluble in benzene, chloroform, ether, and hot ethanol
Al-though triacylglycerols are insoluble in water, monoacylglycerols and diacylglycerols
readily form organized structures in water (see Chapter 9), owing to the polarity of
their free hydroxyl groups
Triacylglycerols are rich in highly reduced carbons and thus yield large amounts
of energy in the oxidative reactions of metabolism Complete oxidation of 1 g of
tri-acylglycerols yields about 38 kJ of energy, whereas proteins and carbohydrates yield
only about 17 kJ/g Also, their hydrophobic nature allows them to aggregate in
highly anhydrous forms, whereas polysaccharides and proteins are highly hydrated
For these reasons, triacylglycerols are the molecules of choice for energy storage in
animals Body fat (mainly triacylglycerols) also provides good insulation Whales
and Arctic mammals rely on body fat for both insulation and energy reserves
8.3 What Are the Structures and Chemistry
of Glycerophospholipids?
A 1,2-diacylglycerol that has a phosphate group esterified at carbon atom 3 of the
glycerol backbone is a glycerophospholipid, also known as a phosphoglyceride or a
glycerol phosphatide (Figure 8.4) These lipids form one of the largest and most
im-portant classes of natural lipids They are essential components of cell membranes
and are found in small concentrations in other parts of the cell It should be noted
A DEEPER LOOK
Polar Bears Prefer Nonpolar Food
The polar bear is magnificently adapted to thrive in its harsh
Arc-tic environment Research by Malcolm Ramsay (at the University
of Saskatchewan in Canada) and others has shown that polar
bears eat only during a few weeks out of the year and then fast
for periods of 8 months or more, consuming no food or water
during that time Eating mainly in the winter, the adult polar
bear feeds almost exclusively on seal blubber (largely composed
of triacylglycerols), thus building up its own triacylglycerol
re-serves Through the Arctic summer, the polar bear maintains
normal physical activity, roaming over long distances, but relies
entirely on its body fat for sustenance, burning as much as 1 to
1.5 kg of fat per day It neither urinates nor defecates for
ex-tended periods All the water needed to sustain life is provided
from the metabolism of triacylglycerols (because oxidation of
fatty acids yields carbon dioxide and water)
Ironically, the word Arctic comes from the ancient Greeks, who
understood that the northernmost part of the earth lay under the
stars of the constellation Ursa Major, the Great Bear Although
un-aware of the polar bear, they called this region Arktikós, which
means “the country of the great bear.”
C
O O
CH2
C
O O
CH2 O P O–
O
O–
FIGURE 8.4 Phosphatidic acid, the parent compound for glycerophospholipids.
Trang 2224 Chapter 8 Lipids
that all glycerophospholipids are members of the broader class of lipids known as
phospholipids.
The numbering and nomenclature of glycerophospholipids present a dilemma in that the number 2 carbon of the glycerol backbone of a phospholipid is asymmetric
It is possible to name these molecules either as D- or L-isomers Thus, glycerol phos-phate itself can be referred to either as D-glycerol-1-phosphate or as L -glycerol-3-phosphate (Figure 8.5) Instead of naming the glycerol phosphatides in this way,
biochemists have adopted the stereospecific numbering or sn- system The
stereospe-cific numbering system is based on the concept of prochirality If a tetrahedral
cen-ter in a molecule has two identical substituents, it is referred to as prochiral because
if either of the like substituents is converted to a different group, the tetrahedral center then becomes chiral Consider glycerol (Figure 8.5): The central carbon of glycerol is prochiral because replacing either of the CH2OH groups would make
the central carbon chiral Nomenclature for prochiral centers is based on the (R,S )
system (see Chapter 4) To name the otherwise identical substituents of a prochiral center, imagine increasing slightly the priority of one of them (by substituting a deu-terium for a hydrogen, for example) as shown in Figure 8.5 The resulting molecule
has an (S ) configuration about the (now chiral) central carbon atom The group that contains the deuterium is thus referred to as the pro-S group As a useful
exer-cise, you should confirm that labeling the other CH2OH group with a deuterium
produces the (R) configuration at the central carbon so that this latter CH2OH
group is the pro-R substituent.
Now consider the two presentations of glycerol phosphate in Figure 8.5 In the
stereospecific numbering system, the pro-S position of a prochiral atom is denoted
as the 1- position, the prochiral atom as the 2- position, and so on.When this scheme
is used, the prefix sn- precedes the molecule name (glycerol phosphate in this case)
and distinguishes this nomenclature from other approaches In this way, the
glyc-erol phosphate in natural phosphoglycerides is named sn-glycglyc-erol-3-phosphate.
Glycerophospholipids Are the Most Common Phospholipids
Phosphatidic acid, the parent compound for the glycerol-based phospholipids
(Figure 8.4), consists of sn-glycerol-3-phosphate, with fatty acids esterified at the
1- and 2-positions Phosphatidic acid is found in small amounts in most natural systems and is an important intermediate in the biosynthesis of the more common glycerophospholipids (Figure 8.6) In these compounds, a variety of polar groups are esterified to the phosphoric acid moiety of the molecule The phosphate,
HOH2C
CH2OH C
Glycerol
1CHOH D
2C
1-d, 2(S)-Glycerol (S-configuration at C-2)
CH2OH
CH2OPO3–
L -Glycerol-3-phosphate
pro-S position
pro-R position
CH2OH
CH2OPO3–
D -Glycerol-1-phosphate
sn-Glycerol-3-phosphate
(a)
(b) ACTIVE FIGURE 8.5 (a) The two
identi-cal OCH 2 OH groups on the central carbon of glycerol
may be distinguished by imagining a slight increase in
pri-ority for one of them (by replacement of an H by a D) as
shown (b) The absolute configuration of
sn-glycerol-3-phosphate is shown The pro-R and pro-S positions of the
parent glycerol are also indicated Test yourself on the
concepts in this figure at www.cengage.com/login
Go to CengageNOW at www
.cengage.com/login and click BiochemistryInteractive
to learn the structures and names of the
glycerophos-pholipids.
Trang 3together with such esterified entities, is referred to as a “head” group
Phos-phatides with choline or ethanolamine are referred to as phosphatidylcholine
(known commonly as lecithin) or phosphatidylethanolamine, respectively These
phosphatides are two of the most common constituents of biological membranes
Other common head groups found in phosphatides include glycerol, serine,
and inositol (Figure 8.6) Another kind of glycerol phosphatide found in many
tis-sues is diphosphatidylglycerol First observed in heart tissue, it is also called
cardiolipin.In cardiolipin, a phosphatidylglycerol is esterified through the C-1
hy-droxyl group of the glycerol moiety of the head group to the phosphoryl group of
another phosphatidic acid molecule
Phosphatides exist in many different varieties, depending on the fatty acids
ester-ified to the glycerol group As we shall see, the nature of the fatty acids can greatly
Phosphatidylcholine
N+ CH3
CH3
CH3
CH2CH2 O
P O–
O
CH2 C
CH2
O
H O
C O
O C O
GLYCEROLIPIDS WITH OTHER HEAD GROUPS:
NH3
CH2CH2 O
P O–
O
Phosphatidylethanolamine
CH
NH3
COO–
CH2 O P O–
O O
+
Phosphatidylserine
CH OH
CH2 O P O–
O O
Phosphatidylglycerol
CH2 OH
C
CH2 O P O–
O O
Diphosphatidylglycerol (Cardiolipin)
CH2
OH H
O P O–
O O
OH OH HO H
H H
H
H H
O P O–
O O
Phosphatidylinositol
ANIMATED FIGURE 8.6
Structures of several glycerophospholipids and space-filling models of phosphatidyl-choline, phosphatidylglycerol, and
phos-phatidylinositol See this figure animated
at www.cengage.com/login.
Trang 4226 Chapter 8 Lipids
affect the chemical and physical properties of the phosphatides and the membranes that contain them In most cases, glycerol phosphatides have a saturated fatty acid at
position 1 and an unsaturated fatty acid at position 2 of the glycerol Thus, 1-stearoyl-2-oleoyl-phosphatidylcholine(Figure 8.7) is a common constituent in natural
mem-branes, but 1-linoleoyl-2-palmitoylphosphatidylcholine is not.
Both structural and functional strategies govern the natural design of the many different kinds of glycerophospholipid head groups and fatty acids The structural roles of these different glycerophospholipid classes are described in Chapter 9 Cer-tain phospholipids, including phosphatidylinositol and phosphatidylcholine, par-ticipate in complex cellular signaling events These roles are described in Section 8.8 and Chapter 32
Ether Glycerophospholipids Include PAF and Plasmalogens
Ether glycerophospholipidspossess an ether linkage instead of an acyl group at the C-1 position of glycerol (Figure 8.8a) One of the most versatile biochemical signal
molecules found in mammals is platelet-activating factor, or PAF, a unique ether
glycerophospholipid (Figure 8.8b) The alkyl group at C-1 of PAF is typically a 16-carbon chain, but the acyl group at C-2 is a 2-carbon acetate unit By virtue of this acetate group, PAF is much more water soluble than other lipids, allowing PAF to function as a soluble messenger in signal transduction
FIGURE 8.7 A space-filling model of
1-stearoyl-2-oleoyl-phosphatidylcholine.
O
O
CH2 CH2 NH3
CH2 CH
H2C O O C
R2
Ester linkage
Ether linkage
+
(a)
O
O
CH2 CH2 N+
CH2 CH
H2C O O
CH3
CH3
CH3
CH3
Platelet-activating factor (b)
FIGURE 8.8 (a) A 1-alkyl 2-acyl-phosphatidylethanolamine (an ether glycerophospholipid) (b) The structure of
1-alkyl 2-acetyl-phosphatidylcholine, also known as platelet-activating factor or PAF.
Trang 5Plasmalogensare ether glycerophospholipids in which the alkyl moiety is cis-
,-unsaturated (Figure 8.9) Common plasmalogen head groups include choline,
ethanolamine, and serine These lipids are referred to as phosphatidal choline,
phosphatidal ethanolamine, and phosphatidal serine.
8.4 What Are Sphingolipids, and How Are They Important
for Higher Animals?
Sphingolipidsrepresent another class of lipids frequently found in biological
mem-branes An 18-carbon amino alcohol, sphingosine (Figure 8.10a), forms the
back-bone of these lipids rather than glycerol Typically, a fatty acid is joined to a
sphin-gosine via an amide linkage to form a ceramide (Figure 8.10b) Sphingomyelins
represent a phosphorus-containing subclass of sphingolipids and are especially
important in the nervous tissue of higher animals A sphingomyelin is formed
by the esterification of a phosphorylcholine or a phosphorylethanolamine to the
1-hydroxy group of a ceramide (Figure 8.10c)
There is another class of ceramide-based lipids that, like the sphingomyelins,
are important components of muscle and nerve membranes in animals These are
the glycosphingolipids, and they consist of a ceramide with one or more sugar
HUMAN BIOCHEMISTRY
Platelet-Activating Factor: A Potent Glyceroether Mediator
Platelet-activating factor (PAF) was first identified by its ability (at
low levels) to cause platelet aggregation and dilation of blood
ves-sels, but it is now known to be a potent mediator in inflammation,
allergic responses, and shock PAF effects are observed at tissue
concentrations as low as 1012M PAF causes a dramatic
inflam-mation of air passages and induces asthmalike symptoms in
labo-ratory animals Toxic shock syndrome occurs when fragments of
destroyed bacteria act as toxins and induce the synthesis of PAF
PAF causes a drop in blood pressure and a reduced volume of
blood pumped by the heart, which leads to shock and, in severe cases, death
Beneficial effects have also been attributed to PAF In repro-duction, PAF secreted by the fertilized egg is instrumental in the implantation of the egg in the uterine wall PAF is produced in significant quantities in the lungs of the fetus late in pregnancy and may stimulate the production of fetal lung surfactant, a protein–lipid complex that prevents collapse of the lungs in a newborn infant
Choline plasmalogen
N+ CH3
CH3
CH3
CH2CH2 O
P O
O –O
CH2 CH
CH2 O
O
C C H
H
The ethanolamine plasmalogens have ethanolamine in place of choline.
FIGURE 8.9 The structure and a space-filling model of a choline plasmalogen.
Trang 6228 Chapter 8 Lipids
residues in a -glycosidic linkage at the 1-hydroxyl moiety The neutral
glyco-sphingolipids contain only neutral (uncharged) sugar residues When a single
glu-cose or galactose is bound in this manner, the molecule is a cerebroside (Figure
8.10d) Another class of lipids is formed when a sulfate is esterified at the 3-position
of the galactose to make a sulfatide Gangliosides (Figure 8.10e) are more complex
glycosphingolipids that consist of a ceramide backbone with three or more sugars
esterified, one of these being a sialic acid such as N-acetylneuraminic acid These
OH
C H
H
C +NH3
OH
CH2
C
C
H
H
Sphingosine
(a)
R COOH Fatty acid
OH
C H
H
H
C
NH
OH
CH2
Ceramide
C
R O
H 2 O
C C H
O
CH2OH
H
OH
H OH
H
HO
OH C H
H
H C
NH
O
CH2
C
R O
A cerebroside
C C H
H
N+ CH3
CH 3
CH 3
CH2CH2 O
P O –O
CH 2
C O
O
OH
C H
H
C
NH
Choline sphingomyelin with stearic acid
C C H
H
O
CH2OH H OH
H OH H
HO
H H
O
CH2OH H
OH
H NH H
HO
H H O
CH3 O
O
CH2OH H
H OH
CH2OH H
H OH
H
H H O
O O
CHOH H
OH
H
H H
H N
H
CHOH
CH2OH
COO–
C
O
CH3
OH
C H
H
H
C
NH
O
CH2
C
R O
N-Acetylneuraminidate
(a sialic acid)
GM3
GM2
G M1
C C H C
FIGURE 8.10 Sphingolipids are based on the structure of sphingosine A ceramide with a phosphocholine head group is a choline sphingomyelin A ceramide with a single sugar is a cerebroside Gangliosides are cera-mides with three or more sugars esterified, one of which is a sialic acid.
Trang 7latter compounds are referred to as acidic glycosphingolipids, and they have a net
negative charge at neutral pH
The glycosphingolipids have a number of important cellular functions, despite
the fact that they are present only in small amounts in most membranes
Glyco-sphingolipids at cell surfaces appear to determine, at least in part, certain elements
of tissue and organ specificity Cell–cell recognition and tissue immunity depend on
specific glycosphingolipids Gangliosides are present in nerve endings and are
im-portant in nerve impulse transmission A number of genetically transmitted diseases
involve the accumulation of specific glycosphingolipids due to an absence of the
en-zymes needed for their degradation Such is the case for ganglioside GM2 in the
brains of Tay-Sachs disease victims, a rare but fatal childhood disease characterized
by a red spot on the retina, gradual blindness, and self-mutilation
8.5 What Are Waxes, and How Are They Used?
Waxes are esters of long-chain alcohols with long-chain fatty acids The resulting
molecule can be viewed (in analogy to the glycerolipids) as having a weakly polar
head group (the ester moiety itself) and a long, nonpolar tail (the hydrocarbon
chains) (Figure 8.11) Fatty acids found in waxes are usually saturated The alcohols
found in waxes may be saturated or unsaturated and may include sterols, such as
cholesterol (see later section) Waxes are water insoluble due to their mostly
hy-drocarbon composition As a result, this class of molecules confers water-repellant
character to animal skin, to the leaves of certain plants, and to bird feathers The
glossy surface of a polished apple results from a wax coating Carnauba wax,
ob-tained from the fronds of a species of palm tree in Brazil, is a particularly hard wax
used for high-gloss finishes, such as in automobile wax, boat wax, floor wax, and
shoe polish Lanolin,1a wool wax, is used as a base for pharmaceutical and cosmetic
products because it is rapidly assimilated by human skin The brand name Oil of
Olay®was coined by Graham Wulff, a South African chemist who developed it The
name refers to lanolin, a key ingredient
8.6 What Are Terpenes, and What Is Their Relevance
to Biological Systems?
The terpenes are a class of lipids formed from combinations of two or more
mole-cules of 2-methyl-1,3-butadiene, better known as isoprene (a five-carbon unit that is
abbreviated C5) A monoterpene (C10) consists of two isoprene units, a sesquiterpene
A DEEPER LOOK
Moby Dick and Spermaceti: A Valuable Wax from Whale Oil
When oil from the head of the sperm whale is cooled, spermaceti,
a translucent wax with a white, pearly luster, crystallizes from the
mixture Spermaceti, which makes up 11% of whale oil, is
com-posed mainly of the wax cetyl palmitate:
CH3(CH2)14OCOOO(CH2)15CH3
as well as smaller amounts of cetyl alcohol:
HOO(CH2)15CH3
Spermaceti and cetyl palmitate have been widely used in the
mak-ing of cosmetics, fragrant soaps, and candles
In the literary classic Moby Dick, Herman Melville describes
Ishmael’s impressions of spermaceti, when he muses that the waxes “discharged all their opulence, like fully ripe grapes their wine; as I snuffed that uncontaminated aroma—literally and truly, like the smell of spring violets.”*
*Melville, H., 1984 Moby Dick London: Octopus Books, p 205 (Adapted from Waddell, T G., and Sanderlin, R R., 1986 Chemistry in Moby Dick.
Journal of Chemical Education 63:1019–1020.)
1 Lanolin is a complex mixture of waxes with 33 different alcohols esterified to 36 different fatty acids.
Trang 8230 Chapter 8 Lipids
O
CH2
Stearyl palmitate
O
CH2
Triacontanol palmitate
O
R2
R1
General forumula of a wax
O
Stearic acid Oleoyl alcohol
the principal component of beeswax Waxes are components of the waxy coating on the leaves of plants, such
as jade plants (shown here) Such species typically contain dozens of different waxy esters.
C C
CH2
CH3
H2C H
Head-to-tail linkage
OH
Tail-to-tail linkage R
R
FIGURE 8.12 The structure of isoprene
(2-methyl-1,3-butadiene) and the structure of head-to-tail and
tail-to-tail linkages Isoprene itself can be formed by distillation
of natural rubber, a linear head-to-tail polymer of
iso-prene units.
O C
OH
Gibberellic acid
HO
COOH
H
H
CH3
DITERPENES
CH2OH
Phytol
O
HO
Eudesmol
CHO
HO
H
Lanosterol
TRITERPENES
Lycopene
MONOTERPENES
Limonene Citronellal Menthol
OH
CHO
Squalene
SESQUITERPENES
Bisabolene
TETRATERPENES
All-trans-retinal
ACTIVE FIGURE 8.13 Many monoterpenes are readily recognized by their characteristic flavors
or odors (limonene in lemons; citronellal in roses, geraniums, and some perfumes; and menthol from peppermint, used in cough drops and nasal inhalers) The diterpenes, which are C 20 terpenes, include retinal (the essential light-absorbing pigment in rhodopsin, the photoreceptor protein of the eye), and phytol (a constituent of chlorophyll) The triterpene lanosterol is a constituent of wool fat Lycopene is a carotenoid found in ripe fruit,
especially tomatoes Test yourself on the concepts in this figure at www.cengage.com/login
Trang 9(C15) consists of three isoprene units, a diterpene (C20) has four isoprene units, and
so on Isoprene units can be linked in terpenes to form straight-chain or cyclic
mol-ecules, and the usual method of linking isoprene units is head to tail (Figure 8.12)
Monoterpenes occur in all higher plants, whereas sesquiterpenes and diterpenes are
less widely known Several examples of these classes of terpenes are shown in Figure
8.13 The triterpenes are C30terpenes and include squalene and lanosterol, two of
the precursors of cholesterol and other steroids (discussed later) Tetraterpenes
(C40) are less common but include the carotenoids, a class of colorful photosynthetic
pigments -Carotene is the precursor of vitamin A, whereas lycopene, similar to
-carotene, is a pigment found in tomatoes.
Long-chain polyisoprenoid molecules with a terminal alcohol moiety are called
polyprenols The dolichols, one class of polyprenols (Figure 8.14), consist of 16 to
22 isoprene units and, in the form of dolichyl phosphates, function to carry
carbo-hydrate units in the biosynthesis of glycoproteins in animals Polyprenyl groups
serve to anchor certain proteins to biological membranes (discussed in Chapter 9)
The Membranes of Archaea Are Rich in Isoprene-Based Lipids Archaea are
found primarily in harsh environments Some thrive in the high temperatures of
geysers and ocean steam vents, whereas others are found in extremely acidic, cold,
or salty environments Archaea also live in extremes of pH in the digestive tracts
of cows, termites, and humans Archaea are ideally adapted to their harsh
CH3
O–
Dolichol phosphate
H3C C HC
CH3
CH2
9
CH2
Undecaprenyl alcohol (bactoprenol)
CH3
CH3
CH2OH
13 – 23
FIGURE 8.14 Dolichol phosphate is an initiation point for the synthesis of carbohydrate polymers in animals.
The analogous alcohol in bacterial systems,
undeca-prenol, also known as bactoundeca-prenol, consists of 11
iso-prene units Undecaprenyl phosphate delivers sugars from the cytoplasm for the synthesis of cell wall compo-nents such as peptidoglycans, lipopolysaccharides, and glycoproteins.
A DEEPER LOOK
Why Do Plants Emit Isoprene?
The Blue Ridge Mountains of Virginia are so named for the misty
blue vapor or haze that hangs over them through much of the
summer season This haze is composed in part of isoprene that
is produced and emitted by the plants and trees of the
moun-tains Global emission of isoprene from vegetation is estimated at
3 1014g/yr Plants frequently emit as much as 15% of the
car-bon fixed in photosynthesis as isoprene, and Thomas Sharkey, a
botanist at the University of Wisconsin, has shown that the kudzu
plant can emit as much as 67% of its fixed carbon as isoprene as
the result of water stress Why should plants and trees emit large
amounts of isoprene and other hydrocarbons? Sharkey has
shown that an isoprene atmosphere or “blanket” can protect
leaves from irreversible damage induced by high (summerlike)
temperatures He hypothesizes that isoprene in the air around
plants dissolves into leaf-cell membranes, altering the lipid
bi-layer and/or lipid–protein and protein–protein interactions
within the membrane to increase thermal tolerance 䊱 Blue Ridge Mountains
Trang 10232 Chapter 8 Lipids
environments, and one such adaptation is found in their cell membranes, which contain isoprene-based lipids (Figure 8.15) These isoprene chains are linked at both ends by ether bonds to glycerols Ether bonds are more stable to hydrolysis than the ester linkages of glycerophospholipids (Figure 8.6) With a length twice that of typical glycerophospholipids, these molecules can completely span a cell membrane, providing additional stability Interestingly, the glycerols in archaeal
lipids are in the (R ) configuration, whereas glycerolipids of animals, plants, and eu-bacteria are almost always in the (S ) configuration.
HUMAN BIOCHEMISTRY
Coumadin or Warfarin—Agent of Life or Death
The isoprene-derived molecule whose structure is shown here is
known alternately as Coumadin and warfarin By the former
name, it is a widely prescribed anticoagulant By the latter name,
it is a component of rodent poisons How can the same
chemi-cal species be used for such disparate purposes? The key to both
uses lies in its ability to act as an antagonist of vitamin K in the
body
Vitamin K is necessary for the carboxylation of glutamate
residues on certain proteins, including some proteins in the
blood-clotting cascade (including prothrombin, factor VII, factor IX, and
factor X, which undergo a Ca2-dependent conformational change
in the course of their biological activity, as well as protein C and
protein S,two regulatory proteins in coagulation) Carboxylation
of these coagulation factors is catalyzed by a carboxylase that
re-quires the reduced form of vitamin K (vitamin KH2), molecular
oxygen, and carbon dioxide KH2is oxidized to vitamin K epoxide,
which is recycled to KH2by the enzymes vitamin K epoxide
reduc-tase (1) and vitamin K reductase (2, 3) Coumadin/warfarin exerts
its anticoagulant effect by inhibiting vitamin K epoxide reductase
and possibly also vitamin K reductase This inhibition depletes
vit-amin KH2and reduces the activity of the carboxylase
Coumadin/warfarin, given at a typical dosage of 4 to 5 mg/day,
prevents the deleterious formation in the bloodstream of small
blood clots and thus reduces the risk of heart attacks and strokes for
individuals whose arteries contain sclerotic plaques Taken in much
larger doses, as for example in rodent poisons, Coumadin/warfarin
can cause massive hemorrhages and death
K
K
1 2
3
Warfarin resistant
CH HO
O
O
CH3
CH2 C
Warfarin (Coumadin)
O
H
O
-O C
CH2 Glu O
H2C
O
O
CH2
CO2
O C
O
O H
-carboxy-Glu
Warfarin inhibits
CH2OH
O
CH2 O
H2C O
H HOCH2
Isoprene units
Caldarchaeol
Glycerol Glycerol
FIGURE 8.15 The structure of caldarchaeol, an isoprene-based lipid found in archaea.