Lipid metabolism
You give me fever
Peggy lee
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FATTY ACID OXIDATION
β-oxidation is the pathway by which the fatty acids from both
dietary fat and adipose tissue TAG are oxidized to acetyl-CoA.
The reducing equivalents released during fatty acid oxidation
are captured in the form of FADH2 and NADH, which are used to
support oxidative phosphorylation
In most circumstances, the acetyl-CoA units generated by β oxidation will subsequently be oxidized through the TCA cycle,
generating additional FADH2 and NADH and ultimately,
additional ATP
All cells and tissues except red blood cells and the brain oxidize
fatty acids to generate ATP
The β-oxidation pathway is absent in red blood cells because
they lack mitochondria
Although neuronal cells in the brain do contain mitochondria,
there is only limited transport of fatty acids across the blood2
brain barrier
• The skeletal and heart muscles in particular have a large
capacity for oxidizing fatty acids
• Normally, 60 to 90% of the energy required for contraction of
the heart is derived from the oxidation of fatty acids
• In the fasted state, many cells and tissues depend on β oxidation of fatty acids to provide the ATP needed to
maintain ion gradients and to support biosynthetic processes
such as gluconeogenesis
• During prolonged fasting or starvation the brain meets its
energy needs by oxidizing ketone bodies as well as glucose
• The ketone bodies of physiological significance are fourcarbon anions produced from acetyl-CoA generated by the
β-oxidation of long-chain fatty acids in the liver
• Oxidation of ketone bodies by the brain reduces the brain’s
dependence on glucose and thus decreases the body’s need
to catabolize muscle proteins for gluconeogenesis
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• The major fatty acids of both dietary TAG and adipose stores
in the body contain 16 to 18 carbons and are oxidized through
mitochondrial β-oxidation
• Essentially the same pathway is utilized for short- (C4-C6) and
medium-chain (C8-C12) fatty acids
• Related peroxisomal pathways oxidize the less common
branched-chain and very long-chain (>C22) fatty acids (VLCFA)
Transport and Activation of Fatty Acids
• Since long-chain fatty acids are poorly soluble in aqueous
media, they must be transported in the plasma complexed
with albumin
• When a fatty acid dissociates from albumin it is transferred
from the capillary lumen through the capillary endothelium
and interstitial space to the cells below
• Long-chain fatty acids enter cells both by simple diffusion and
by carrier-mediated transport
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• Intracellularly, fatty acids are bound to cytosolic fatty acid
binding proteins (FABP), which deliver the fatty acids to the
sites where they are metabolized
• Once inside cells, fatty acids must be activated before they can
be metabolized
• In contrast to glucose, which is activated and trapped within
cells as glucose 6-phosphate,fatty acids are converted not to
acyl phosphates but to thioesters of coenzyme A
• Long-chain fatty acids destined for β-oxidation are activated to
their CoA forms primarily on the surface of the outer
mitochondrial membrane
• The inner mitochondrial membrane is, however, impermeable
to long-chain fatty acyl-CoA molecules
• Transport of LCFA across the inner mitochondrial membrane is
facilitated by a fatty acid transport mechanism called the
carnitine shuttle
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• Carnitine is a quaternary amine that has a hydroxyl group to
which a fatty acid can be attached
• Since carnitine can be synthesized in the liver and kidney, it is
not usually considered an essential dietary nutrient
• The activity of the carnitine translocase system is dependent on
two enzymes, carnitine palmitoyltransferase I (CPT-I) and
carnitine palmitoyltransferase II (CPT-II), both of which
catalyze the reversible transfer of long-chain fatty acids
between CoA and carnitine
• CPT-I is localized to the mitochondrial outer membrane and
acts to generate acylcarnitine
• Carnitine translocase, which is embedded in the inner
mitochondrial membrane transports acylcarnitine into the
mitochondria in exchange for free carnitine, which is
concurrently exported from the mitochondrial matrix into the
cytosol
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• CPT-II, which is localized to the matrix face of the inner
mitochondrial membrane, forms mitochondrial acyl-CoA from
CoASH and acylcarnitine
• The net effect is the transfer of a long-chain fatty acid from an
cytosolic CoASH to a mitochondrial CoASH
Intramitochondrial β-Oxidation
• The term β-oxidation is derived from the fact that the critical
chemistry of the four core reactions that comprise the pathway
takes place on the third carbon from the carboxyl end: that is,
the β-carbon atom
• The first of the four reactions of the β-oxidation pathway is
irreversible and is catalyzed by acyl-CoA dehydrogenase
• Two hydrogen atoms are removed-one each from the α and β
and -generating a carbon-carbon double bond between the α
and β carbons
• These hydrogen atoms are transferred to FAD to form FADH7 2
The Mechanism of The Carnitine
Shuttle
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• Mitochondria contain a family of FAD-linked acyl-CoA
dehydrogenases: a long-chain acyl-CoA dehydrogenase, a
medium-chain acyl-CoA dehydrogenase and a short-chain acylCoA dehydrogenase
• The second step in β-oxidation involves hydration of the
carbon-carbon double bond between by enoyl-CoA hydratase.
The hydroxyl group is introduced onto the β-carbon
• In the third step, a second dehydrogenase, NAD +-linked βhydroxyacyl-CoA dehydrogenase, oxidizes the hydroxyacylCoA molecule to generate a β-ketoacyl-CoA and a molecule of
NADH
• The fourth and final step in β-oxidation involves cleavage of the
fatty acid chain with attachment of a second molecule of
CoASH to the β-carbon and generation of one molecule of
acetyl-CoA
• The enzyme that catalyzes this reaction is called β-ketoacyl9
CoA thiolase
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• The net effect of the four steps in β-oxidation is the production
of one molecule of acetyl-CoA and one fatty acyl-CoA molecule
whose carbon chain is two carbons shorter than the original
substrate
• The four steps are then repeated, with successive chain
shortening by two carbon atoms
• The final thiolytic cleavage reaction converts the 4-carbon β ketobutyryl-CoA (acetoacetyl-CoA) into two molecules of
acetyl-CoA
The Energy Yield from β-oxidation
• A cycle of β-oxidation releases one molecule each of FADH2,
NADH and acetyl-CoA
• The four steps are repeated n/2 - 1 times ( n = number of
carbons) until the final acetyl-CoA is released
• n/2 acetyl-CoA and n/2 - 1 each of FADH2 and NADH are
produced by the complete oxidation of a fatty acid with an even
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number of carbons
• Taking the oxidation of palmitate :
Palmitoyl-CoA+ 7 CoA+ 7 FAD+ 7 NAD++7 H2O→ 8 acetyl-CoA+ 7
FADH2+7 NADH+ 7H+
• 8 acetyl-CoA ≈ 8o ATP, 7 FADH2 ≈ 10.5 ATP and 7 NADH ≈ 17.5
ATP → total = 108 ATP
• The equivalent of two ATP molecules is spent on the activation
of fatty acids; this means a net production of 106 ATP
Ancillary Reactions to the Pathway of β-Oxidation
• β –oxidation of saturated fatty acids generates an unsaturated
intermediate with a Δ2,3 -trans double bond which is then
hydrated by enoyl-CoA hydratase
• The metabolism of unsaturated and polyunsaturated fatty acids
requires additional enzymes to act on preexisting cis double
bonds
• For example, the oxidation of linoleic acid proceeds in
essentially the same manner as the β-oxidation of saturated
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fatty acids
• However, after three cycles of β-oxidation, the chain- shortening
process produces an acyl-CoA with a cis-3,4 double bond
• At this point, an additional enzyme, Δ 3, Δ 2 -enoyl-CoA
isomerase, converts the Δ 3,4-cis double bond to a Δ 2,3-trans
double bond, thus providing a suitable substrate for enoyl-CoA
hydratase
• A slightly different situation arises when the chain-shortening
process produces an acyl-CoA molecule that has a cis-4,5 double
bond
• Under these conditions, the acyl-CoA dehydrogenase step
generates a Δ 2,3-trans /Δ 4,5-cis conjugated di-unsaturated fatty
acyl-CoA intermediate
• At this point another enzyme, NADPH-dependent 2,4-dienoylCoA reductase, transfers hydrogen atoms from NADPH to
carbons 4 and 5, generating a Δ 3,4-enoyl-CoA.
• Δ 3, Δ 2 -enoyl-CoA isomerase then converts Δ3,4-trans-enoylCoA to Δ2, 3 -trans-enoyl-CoA
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Oxidation of Medium-Chain Fatty Acids
• Cow’s milk contains relatively large amounts of medium-chain
fatty acids while long-chain fatty acids (particularly palmitic,
oleic, and linoleic acids) predominate in breast milk
• These shorter fatty acids are more soluble than are their more
common C16-C20 counterparts and can enter the
mitochondrion directly from the cytosol without need for the
carnitine transporter system
• The C8-C 12 fatty acids are activated to their corresponding
acyl-CoA derivatives within the mitochondrion and then
undergo β-oxidation
• The initial oxidation reaction is catalyzed by medium-chain
acyl-CoA dehydrogenase (MCAD)
Oxidation of Fatty Acids With an Odd-Number Of Carbons
• Dietary lipids often contain a small amount of odd-carbon fatty
acids such as 17:0
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• Odd-chain fatty acids also undergo β-oxidation
• However, the last thiolytic cleavage step produces one
molecule of acetyl-CoA and one molecule of propionyl-CoA
• Carboxylation of propionyl CoA yields methylmalonyl CoA,
which is ultimately converted to succinyl CoA in a vitamin
B12–dependent reaction
• Propionyl CoA also arises from the oxidation of branched
chain amino acids
• The propionyl CoA to succinyl CoA pathway is a major
anaplerotic route for the TCA cycle
• Thus, this small proportion of the odd-carbon number fatty
acid chain can be converted to glucose
• In contrast, the acetyl-CoA formed from β-oxidation of evenchain-number (and odd-chain) fatty acids in the liver either
enters the TCA cycle or is converted to ketone bodies
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Ketone Bodies
The association of ketone bodies with
the ketoacidosis of diabetes mellitus
has given these substances a bad
reputation
However, ketone bodies are normal
metabolites that serve as circulating
fuels, especially during periods of
moderate (12 to 24 hours) or severe
(>5 days) fasting
The two physiologically significant
ketones are acetoacetate
(β –ketobutyrate) and
β –hydroxybutyrate
Acetone is the product of the nonenzymatic decarboxylation of
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acetoacetate
• Unlike hydrophobic long-chain fatty acids that require
albumin for their transport in the plasma, ketone bodies are
water-soluble and do not require a carrier protein for
transport
• Ketone bodies can hence be thought of as easily
transportable forms of fatty acids
• Ketone bodies are synthesized mainly in the liver with a
smaller contribution from the renal cortex
• In both tissues, the substrate for ketogenesis is mitochondrial
acetyl-CoA, which is derived from β-oxidation and to a lesser
extent, from the oxidation of ketogenic amino acids (e.g.,
leucine)
• In the fasted state, much of the acetyl-CoA generated by
β-oxidation cannot enter the TCA cycle because of a relative
shortage of oxaloacetate which has been diverted to
gluconeogenesis
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• The pathway of ketone body synthesis converts two acetyl-CoA
molecules into one four-carbon acetoacetate molecule while
releasing two free CoASH molecules, which are required for
continued β-oxidation
• Continued β-oxidation, in turn, provides FADH2 and NADH for
oxidative phosphorylation
• The first step in acetoacetate synthesis is catalyzed by
β -ketothiolase, which also catalyzes the last step in
β-oxidation
2Acetyl-CoA⇌acetoacetyl-CoA + CoA
• This reversible reaction is driven to the right by a high
concentration of acetyl-CoA arising from β-oxidation
• The acetoacetyl-CoA is then combined with a third molecule of
acetyl-CoA to form β-hydroxy- β -methylglutaryl-CoA (HMGCoA) in a reaction catalyzed by HMG-CoA synthase
• Most cells contain a second HMG-CoA synthase that is localized
to the cytosol, where it is involved in cholesterol synthesis 19
• Mitochondrial HMG-CoA is then hydrolyzed by HMG-CoA
lyase to produce acetoacetate plus acetyl-CoA
• While about one-third of the acetoacetate produced by
HMG-CoA lyase is secreted by the liver into the circulation,
the other two-thirds is first reduced by mitochondrial
β-hydroxybutyrate dehydrogenase and then secreted:
Acetoacetate + NADH + H+ → β-hydroxybutyrate + NAD+
• This reaction is driven in the direction of β-hydroxybutyrate
synthesis by the relatively high mitochondrial ratio of
NADH/NAD+ generated by active β-oxidation of fatty acids
• β -Hydroxybutyrate is more reduced and more energy-rich
than acetoacetate
Utilization of Ketone Bodies
• Although the liver does not oxidize ketone bodies, the heart
and skeletal muscle are capable of efficiently oxidizing
ketones bodies
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Ketogenesis
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• Ketone utilization is initiated by mitochondrial β –hydroxybutyrate dehydrogenase, which converts β-hydroxybutyrate
back into acetoacetate
• Acetoacetate is then activated (and trapped within the cell) by
one of two mitochondrial enzymatic reactions
• The first trapping reaction is reversible and catalyzed by
succinyl-CoA: β -ketoacid CoA-transferase:
• The other trapping reaction is catalyzed by acetoacetyl-CoA
synthetase
• β-Ketoacid CoA-transferase and acetoacetyl-CoA synthetase
are both absent from hepatocytes, which accounts for the
inability of liver to oxidize ketone bodies
• Acetoacetyl-CoA is then cleaved by β-ketothiolase into two
molecules of acetyl- CoA. Since tissues such as muscle that
oxidize ketone bodies do not perform gluconeogenesis and thus
do not deplete their supply of oxaloacetate in the fasted state,
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the acetyl-CoA molecules join the TCA cycle
The Utilization of Ketone Bodies
Alternate Routes of Fatty Acid
Oxidation
i. Oxidation of Very-Long-Chain Fatty
Acids (VLCFA)
• The initial oxidation of VLCFA
comprised of 22 carbon atoms or
more is accomplished by a modified
β-oxidation pathway that operates in
peroxisomes
• One major difference between the
mitochondrial and peroxisomal
pathways is that, since peroxisomes
lack an electron transport system,
the reduced cofactors generated
during peroxisomal β-oxidation are
not channeled directly into oxidative
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phosphorylation
• The VLCFA are first activated to acyl-CoAs by a distinct acylCoA synthase
• The first FAD-linked dehydrogenase step in the peroxisomal
β-oxidation pathway is different from the corresponding step in
standard mitochondrial β-oxidation
• The peroxisomal FAD-linked dehydrogenase that removes two
hydrogen atoms from the fatty acid chain transfers those
hydrogens to molecular oxygen, thus producing H2O2
• Catalase then breaks down the hydrogen peroxide
• The subsequent steps of the β-oxidation pathway in
peroxisomes are similar to those that operate in mitochondrial
β-oxidation
• The reducing equivalents from the NADH generated by
hydroxyacyl-CoA dehydrogenase are utilized for reactions
within peroxisomes or shuttled out of the peroxisomes and
eventually into mitochondria
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• Once the peroxisomal β-oxidation pathway has reduced the
VLCFA chain to the level of an 8- or 10-carbon acyl-CoA
molecule, the shortened fatty acid chain is transferred to
mitochondria and further catabolized via the mitochondrial
β-oxidation pathway
• The peroxisomal acetyl-CoA units are probably hydrolyzed to
acetate, which is subsequently oxidized in mitochondria
ii. α-Oxidation of Fatty Acids Containing a Branched Methyl
Group
• The chemistry of the β-oxidation pathway entails removal of
both hydrogen atoms from the β-carbon atom
• Therefore, fatty acids that have a methyl group on C3 (the
β -carbon) cannot be oxidized by regular β-oxidation and
require a specialized pathway, which is called α-oxidation
• One such branched-chain fatty acid is phytanic acid, which is
derived from the phytanol side chain of chlorophyll
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• Phytanic acid has methyl groups on carbon atoms 3,7, 11, and 15
• Humans get most of their phytanic acid from dietary dairy
products, beef, and fatty fish
• Phytanic acid hydroxylase in the peroxisomes introduces a
hydroxyl group on the α-carbon, which is then oxidized to a
carboxyl group with release of the original carboxyl group as CO2
• By shortening the fatty acid by one carbon, the methyl groups
will appear on the α-carbon rather than the β-carbon
• Therefore, the product, pristanoic acid, is a suitable substrate for
β-oxidation
• The remaining CH3 of the fatty acyl-CoA chain are now
positioned on even-numbered carbon atoms and therefore do
not present a problem for the enzymes of the standard βoxidation pathway
• Wherever a methyl group is attached to the α-carbon, cleavage
of the carbon chain by β-ketothiolase will generate propionyl26
CoA rather than acetyl-CoA
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iii. ω-Oxidation of Fatty Acids
Fatty acids also may be oxidized at the ω-carbon of the chain
(the terminal methyl group) by enzymes in the endoplasmic
reticulum; the preferred substrates are C10-C12 fatty acids
The ω-methyl group is first oxidized to an alcohol by an
enzyme that uses cytochrome P450, molecular oxygen, and
NADPH
Dehydrogenases convert the alcohol group to a carboxylic acid.
The dicarboxylic acids produced by ω-oxidation can undergo
β–oxidation, forming compounds with 6 to 10 carbons that are
water-soluble
Such compounds may be oxidized further, or be excreted in
urine as dicarboxylic acids
Normally, ω -oxidation is a minor process
However, in some conditions that interfere with β-oxidation ωoxidation produces dicarboxylic acids in increased amounts28
• These dicarboxylic acids are excreted in the urine
• The pathways of peroxisomal β and α-oxidation, and
microsomal ω-oxidation are not feedback regulated
• These pathways function to decrease levels of water-insoluble
fatty acids or of xenobiotic compounds with a fatty acid–like
structure that would become toxic to cells at high
concentrations
• Thus, their rate is regulated by the availability of substrate
ω-Oxidation and Products
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Regulation of Mitochondrial Fatty Acid Oxidation
Regulation by Energy Charge
• The major site of regulation of the mitochondrial β-oxidation
CPT-I, which controls the entry of long-chain fatty acids into
the mitochondrion
• The activity of CPT-I is inhibited by malonyl-CoA, the product
of the key regulatory enzyme of fatty acid synthesis: acetylCoA carboxylase (ACC)
• In the fed state, inhibition of CPT-I by malonyl-CoA prevents
fatty acid oxidation when glucose is plentiful and when acetylCoA is being directed toward fatty acid synthesis
• When a cell is actively synthesizing fatty acids de novo, the
malonyl-CoA concentration in the cytosol increases
• Subsequent inhibition of CPT-I by malonyl-CoA decreases
import of long-chain fatty acids into mitochondria, thereby
preventing a futile cycle of simultaneous fatty acid synthesis
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and β-oxidation
• Conversely, when the energy charge of the cell is low, the
increased concentration of AMP activates AMP-activated
protein kinase (AMPK), which phosphorylates ACC thereby
inhibiting the enzyme –it no longer produces malonyl-CoA
• Thus, the effect of AMP activation of AMPK is to permit
transport of fatty acids into the mitochondrion and ultimately
increase the rate of β-oxidation.
• β-Oxidation of fatty acids within the mitochondrion is also
regulated by the energy charge of the cell
• A high ATP/ADP ratio inhibits entry of reducing equivalents
from NADH and FADH2 into the electron-transport chain
• The resulting increased concentrations of these reduced
cofactors in turn prevent the two dehydrogenases of
β-oxidation from acting when further generation of ATP is not
required
• In liver, in addition to the regulation by the AMPK, acetyl CoA
carboxylase is activated by insulin-dependent mechanisms 31
Regulation by
Energy Charge
Regulation by Gene Transcription
• Peroxisome proliferation-activator receptor-α (PPAR- α) is a
ligand-activated transcription factor that stimulates fatty acid
oxidation (among other things) in liver and muscle
• Certain metabolites bind ligand-activated PPAR-α which is
located in the nucleus
• This binding induces the synthesis of many different genes,
including members of the family of enzymes and proteins
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involved in β-oxidation
Diseases Related to Fatty Acid Oxidation
Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)
• The most common genetic defect in fatty acid oxidation is the
one that affects the medium-chain acyl-CoA dehydrogenase
• A deficiency in MCAD activity is associated with high
concentrations of both medium chain fatty acid and middle
chain acylcarnitines in the plasma and urine of affected
persons.
• Partial oxidation of these intermediate-chain-length fatty acids
also generates dicarboxylic fatty acids whose presence in body
fluids is diagnostic of MCADD
• MCADD causes fasting hypoglycemia and muscle weakness
• Limited utilization of fatty acids as fuels results in an increased
dependence on glucose for muscle work
• At the same time, gluconeogenesis is impaired because of the
limited production of both ATP and NADH substrates needed to
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drive hepatic gluconeogenesis
• Treatment of persons with MCADD involves avoiding periods of
fasting that would tend to produce hypoglycemia
• Patients with MCADD are advised to take frequent small meals
that are relatively high in carbohydrates
• Genetic defects in many of the other proteins required for
mitochondrial β-oxidation have also been documented
• They include deficiencies in the genes encoding CPT-I, CPT-II,
carnitine translocase, acyl-CoA dehydrogenase, and
β-hydroxyacyl- CoA dehydrogenase
• In all of these cases, the clinical manifestations include muscle
weakness and fasting hypoglycemia, similar to those observed
in patients with MCADD
• Unlike the situation with MCADD, people with deficiencies in
enzymes that metabolize long-chain fatty acids do benefit from
diets that contain TAG composed primarily of medium-chain
fatty acids. The utilization of these medium-chain fatty acids is
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not dependent on the carnitine shuttle and LCAD
Impaired Peroxisomal Oxidation
• X-linked adrenoleukodystrophy (ALD) is a relatively common
genetic disease characterized by elevated levels of VLCFA in
plasma
• The accumulation of cholesteryl esters of VLCFA, particularly in
the central nervous system, the adrenal glands, and the testes,
with adverse effects on membrane structure and
steroidogenesis
• The genetic defect lies in defective transport of VLCFA into the
peroxisomes
• Impaired β-oxidation of VLCFA is also observed in patients with
peroxisomal biogenesis disorders such as Zellweger syndrome
• These persons have a defect in one or more of the proteins that
are required to import enzymes into the peroxisome
• Cells of people with peroxisomal biogenesis disorders are
essentially devoid of peroxisomes and exhibit defects in multiple
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peroxisomal metabolic pathways
• These include synthesis of ether lipids, α-oxidation of phytanic
acid, bile acid synthesis as well as β-oxidation of VLCFA
• Refsum Disease is caused by a lack of the α-hydroxylase
required for α-oxidation of fatty acids, such as phytanic acid
• Accumulation of large quantities of phytanic acid in the nervous
tissue and liver results in chronic polyneuropathy and cerebellar
dysfunction
• The reason peroxisomal defects affect the brain is because
VLCFA synthesis occurs in cells of the nervous system, which
incorporate them into the lipids of myelin
• When membranes are remodeled, the VLCFA should be
degraded and resynthesized
FATTY ACID SYNTHESIS
• Fatty acid synthesis serves two main functions
• One is to convert dietary carbohydrates and the carbon
skeletons of excess amino acids into triacylglycerols (TAG) that
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can be stored until needed during periods of fasting
• The other function is to produce a variety of fatty acids, which
are components of the complex lipids of biological membranes
and the precursors of the eicosanoid lipid hormones
• The major pathway of fatty acid synthesis converts acetyl-CoA
molecules derived from dietary carbohydrates and amino acids
into the long-chain fatty acid palmitic acid (16:0)
• Additional enzymes elongate and desaturate both endogenous
palmitate and dietary fatty acids to produce a number of other
fatty acids, of which the most common are stearic and oleic acid
• Two fatty acids, linoleic acid and α-linolenic acid are essential
fatty acids in the sense that they cannot be synthesized by
humans, and as such must be obtained from the diet
• Although neither linoleic acid nor α-linolenic acid can be
synthesized by humans, these dietary fatty acids can be
elongated and further desaturated to produce 20- and 22carbon polyunsaturated fatty acids
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Localization of Fatty Acid Synthesis
• Fatty acid synthesis takes place in the cytosol of most cells and
tissues; however, hepatocytes and adipocytes are endowed
with an especially high capacity for de novo fatty acid
synthesis.
• In the case of fat cells, the fatty acids are esterified to glycerol
and stored in the form of TAG
• In the fasted state, the TAG in adipocytes are hydrolyzed and
the free fatty acids are released from adipocytes and
transported through the blood bound to albumin
• Although the liver is the primary site of fatty acid synthesis in
humans, hepatocytes do not normally accumulate TAG
• Instead, the TAG are packaged into very low density
lipoproteins (VLDL) and secreted into the circulation
• In fact, the accumulation of extensive amounts of TAG in the
liver is pathologic and can ultimately result in cirrhosis
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• Fatty acid synthesis is most active following a meal
• In the first few hours after foods containing carbohydrates such
as starch and sucrose have been digested and absorbed, the
body experiences a period of transient hyperglycemia
• This hyperglycemia triggers insulin secretion
• The resulting high insulin/glucagon ratio signals hepatocytes
and adipocytes to take up glucose from the circulation and
convert it into fatty acids, and ultimately into TAG
• Fatty acid synthesis is thus greater when a person is consuming
a high-carbohydrate diet than a diet that is relatively low in
carbohydrates
The Reactions of Fatty Acid Synthesis
• Even though the main source of of acetyl-CoA for fatty acid
synthesis is glucose, acetyl-CoA is also generated from
oxidation of the carbon skeletons of excess dietary amino acids
and from ethanol
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• In all cases, acetyl-CoA that is not needed for the immediate
generation of ATP is routed to the synthesis of fatty acids
• Once mitochondrial acetyl-CoA has been transported to the
cytosol , it serves as the immediate donor of the two carbons at
the methyl end of a newly synthesized fatty acid
• Malonyl-CoA serves as the high-energy, highly reactive donor of
the additional acetyl units used during the process of fatty acid
synthesis
• Acetyl-CoA carboxylase (ACC), the cytosolic, biotin-containing
enzyme that catalyzes the synthesis of malonyl-CoA, is the ratelimiting step of fatty acid synthesis
• Like in the case of pyruvate carboxylase , biotin is covalently
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attached to a lysine residue of ACC
• During the process of fatty acid synthesis, release of the ionized
carboxyl group of malonyl-CoA as CO2 drives the formation of
carbon-carbon bonds
Fatty Acid Synthase Complex (FAS)
• Fatty acid synthase sequentially adds 2-carbon units from
malonyl-CoA to the growing fatty acyl chain to form palmitate
• After the addition of each 2-carbon unit, the growing chain
undergoes two reduction reactions that require NADPH
• FAS is a large enzyme composed of two identical dimers, which
each have seven catalytic activities and an acyl carrier protein
(ACP) segment in a continuous polypeptide chain
• The ACP segment contains a phosphopantetheine residue that
is derived from the cleavage of coenzyme A
• The two dimers associate in a head-to-tail arrangement, so that
the phosphopantetheinyl sulfhydryl group on one subunit and a
cysteinyl SH group on another subunit are closely aligned 41
A Comparison of
ACP and CoA
• In the initial step of fatty acid synthesis, an acetyl moiety is
transferred from acetyl CoA to the ACP phosphopantetheinyl
sulfhydryl group of one subunit and then to the cysteinyl
sulfhydryl group of the other subunit
• The malonyl moiety from malonyl CoA then attaches to the
ACP phosphopantetheinyl sulfhydryl group of the first
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subunit
• The acetyl and malonyl moieties condense, with the release of
the malonyl carboxyl group as CO2
• Decarboxylation allows the reaction to go to completion, pulling
the whole sequence of reactions in the forward direction
• A 4-carbon -keto acyl chain is now attached to the ACP
phosphopantetheinyl sulfhydryl group
• A series of three reactions reduces the 4-carbon keto group to
an alcohol, removes water to form a double bond, and reduces
the double bond
• NADPH provides the reducing equivalents for these reactions
• The net result is that the original acetyl group is elongated by
two carbons
• The 4-carbon fatty acyl chain is then transferred to the cysteinyl
sulfhydryl group and subsequently condenses with a malonyl
group
• This sequence of reactions is repeated until the chain is 16
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carbons in length (palmitate)
• Palmitate is liberated from the enzyme complex by the
activity of a seventh enzyme in the complex, thioesterase
• The free palmitate must be activated to acyl-CoA before it can
proceed via any other metabolic pathway
• Its usual fate is esterification into acylglycerols, chain
elongation or desaturation, or esterification to cholesteryl
ester
• The oxidative reactions of the pentose phosphate pathway are
the chief source of the hydrogen required for the reductive
synthesis of fatty acids
• Tissues specializing in active lipogenesis possess an active
pentose phosphate pathway. Moreover, both metabolic
pathways are found in the cytosol of the cell; so, there are no
membranes or permeability barriers against the transfer of
NADPH
• The other main source of NADPH is the reaction that converts
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malate to pyruvate catalyzed by the malic enzyme
The FAS Complex
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Reduction
Loading and Condensation
Modification Reactions
• Most cells have the ability to increase the chain length and
degree of unsaturation of long-chain fatty acids
• Modification of both dietary-derived fatty acids and the
palmitate synthesized de novo in the body accounts for the
great diversity of structural fatty acids in membrane lipids and
those involved in signaling
Fatty Acid Chain Elongation
• Elongation of fatty acids occurs primarily in the smooth
endoplasmic reticulum and utilizes malonyl-CoA to add twocarbon units to long-chain fatty acyl-CoAs.
• There is a minor, secondary chain elongation system (elongase)
in mitochondria that utilizes acetyl-CoA as the two-carbon donor
and it appears to be involved primarily in the synthesis of lipoic
acid, a cofactor for PDC and α-ketoglutarate dehydrogenase.
• The elongation system is comprised of a condensing enzyme
47
that adds two carbons to a molecule of fatty acyl-CoA
• It also has three additional enzyme activities:β-ketoacyl-CoA
reductase, β-hydroxyacyl-CoA dehydratase, and enoyl-CoA
reductase, whose activities are similar to the enzymes of FAS
that catalyze the reduction sequence
• The major elongation reaction that occurs in the body involves
the conversion of palmitoyl CoA (C16) to stearyl CoA (C18)
• Very-long-chain fatty acids (C22 to C24) are also produced,
particularly in the brain
Desaturation of Fatty Acids
• Desaturation of fatty acids involves a process that requires O2,
NADH, and cytochrome b5
• The reaction, which occurs in the endoplasmic reticulum, results
in the oxidation of both the fatty acid and NADH
• The desaturation complex includes the actual desaturase
enzyme, cytochrome b5 which serves as an electron acceptor,
and NADH-cytochrome b5 reductase, which contains FAD as a
48
prosthetic group
• The most common desaturation reactions involve the
placement of a double bond between carbons 9 and 10 in the
conversion of palmitic acid to palmitoleic acid (16:1,Δ9) and the
conversion of stearic acid to oleic acid (18:1, Δ9)
• Other positions that can be desaturated in humans include
carbons 4, 5, and 6
49
• Polyunsaturated fatty acids with double bonds three carbons
from the methyl end (ω-3 fatty acids) and six carbons from the
methyl end (ω-6 fatty acids) are required for the synthesis of
eicosanoids
• Since human cells cannot introduce double bonds beyond
carbon 9 from the carboxyl end of long-chain fatty acids, ω-3
and ω-6 fatty acids must be present in the diet or the diet must
contain other fatty acids that can be converted to these fatty
acids
• We obtain ω-3 and ω-6 polyunsaturated fatty acids mainly from
dietary plant oils that contain the ω-6 fatty acid linoleic acid
(18:2, Δ9,12) and the ω-3 fatty acid α-linolenic acid (18:3,
Δ9,12,15)
• In the body, linoleic acid can be converted by elongation and
desaturation reactions to arachidonic acid (20:4, Δ5,8,11,14),
which is used for the synthesis of the major class of human
50
eicosanoids
• Elongation and desaturation of α-linolenic acid produces
eicosapentaenoic acid (EPA; 20:5, Δ5,8,11,14,17), which is the
precursor of a different class of eicosanoids
• Another important product of the elongation and desaturation
of fatty acids in the body is docosahexaenoic acid (DHA,22:6
4,7,10,13,16,19
Δ
)
Regulation of Fatty Acid Synthesis
Regulation of the Activity of Acetyl-CoA Carboxylase
• ACC is regulated both by allosteric modulators and by
phosphorylation/dephosphorylation
• The main allosteric activator is citrate
• By contrast, palmitoyl-CoA and other long-chain fatty acylCoAs inhibit ACC; this phenomenon is an example of feedback
inhibition
• Citrate activates ACC by inducing formation of an active
filamentous polymer from relatively inactive enzyme dimers
• Activation of ACC by citrate reflects the energy status of the51cell
52
Elongation and Desaturation of
Polyunsaturated Fatty Acids
• In the fed state, when tissues are energy-replete, the high
mitochondrial concentration of ATP inhibits isocitrate
dehydrogenase and citrate is transported to the cytosol where
it can serve as a signal for the cell to synthesize fatty acids from
excess acetyl-CoA
• In the fasted state when the insulin/glucagon ratio is low, the
concentration of cAMP increases, and cAMP-activated protein
kinase A (PKA) phosphorylates and inhibits ACC
• ACC is also inhibited by the action of a second protein kinase,
AMP-activated protein kinase (AMPK), whose activity reflects
depletion of intracellular ATP
• In the fed state, insulin activates the protein phosphatase,
which dephosphorylates ACC, thus increasing its activity
Regulation of Enzyme Synthesis
• Insulin is a powerful anabolic signal, particularly in hepatocytes
and adipocytes, where it induces synthesis of the lipogenic
53
family of enzymes
• These enzymes include ACC, citrate
The Regulation of ACC
lyase, the malic enzyme, G6PD,
pyruvate kinase, and the FAS
complex
• The mechanism underlying this
action by insulin involves activation
of the sterol regulatory elementbinding protein- I (SREBP- I), a
membrane-bound transcription
factor that enhances transcription
of the genes encoding proteins
required for fatty acid synthesis
• Glucagon, on the other hand,
represses de novo synthesis of these
enzymes in adipocytes and liver,
and stimulates degradation of the
lipogenic family of enzyme proteins
54
•
•
•
•
•
THE EICOSANOIDS
The eicosanoids are a complex family of bioactive lipid
messengers generated by oxygenation of 20-carbon
polyunsaturated fatty acids, primarily arachidonic acid
Eicosanoids are local-acting autocrine and paracrine
hormones that stimulate cells adjacent to their site of
synthesis
In general, eicosanoids have a short half-life, usually on the
order of minutes
They are not stored in cells but instead are released as soon
as they are synthesized
Eicosanoids fall into two main classes:
(1) prostanoids that have a ring structure, including
prostaglandins, thromboxanes, and prostacyclins, and
(2) linear eicosanoids consisting of leukotrienes, lipoxins, and
hydroxyeicosatetraenoic acids (HETEs)
55
The Prostanoids
• The term prostaglandin reflects the original isolation of these
molecules from seminal fluid, into which they are secreted by
the seminal glands (rather than the prostate).
• Prostaglandins act to modulate many physiological
functions, including blood pressure, uterine contraction, and
the induction of pain and fever
• Prostaglandins are designated PGA, PGD, PGE, or PGF, based
on the functional groups on the cyclopentane ring that is
comprised of carbons 8 through 12
• For example, PGE2 contains a 9-keto and an 1 1-hydroxy
group, while PGF2α, contains two hydroxy groups; α
designates the stereochemistry of the C9 hydroxyl group
• All of the prostanoids that are derived from arachidonic acid
have numeral 2 as a subscript, referring to the number of
carbon-carbon double bonds in the two side chains
56
• Prostacyclin (PGI2) and thromboxane (TX2) are two prostanoids
that have somewhat unusual ring structures
• The term thromboxane refers to the platelet-aggregating
activity, which has thrombus-forming potential
Linear Eicosanoids
• Unlike the prostanoids, which contain a ring element in their
structure, the leukotrienes are linear molecules
• The term leukotriene derives from their cell of origin
(leukocytes) and the fact that their structures contain three
carbon-carbon double bonds in conjugation
• The most important leukotrienes in humans are LTA4 and LTB4
and their cysteinyl derivatives, LTC4, LTD4, and LTE4
• All are derived from arachidonic acid and contain four double
bonds
• The cysteinyl leukotrienes constitute the slow-reacting
substance of anaphylaxis (SRS-A)
57
• SRS-A promotes smooth muscle contraction, constriction of
pulmonary airways, trachea and intestine, and increases in
capillary permeability (edema)
• HETEs are closely related in structure to cysteinyl leukotrienes;
they are involved in chemotaxis, degranulation of
polymorphonuclear cells, etc
• Lipoxins have got structures that are distinct from leukotrienes
or HETEs; have got anti angiogenic properties, …
The Synthesis of Eicosanoids
• Synthesis of eicosanoids is initiated by release of arachidonic
acid
• The arachidonic acid present in membrane phospholipids is
released from the lipid bilayer as a consequence of the
activation of membrane-bound phospholipase A2 or C
• This activation occurs when a variety of stimuli (agonists), such
as histamine and the cytokines, interact with a specific plasma
58
membrane receptor on the target cell surface
• Phospholipase A2 is specific for the sn-2 position of
phosphoacylglycerols, the site of attachment of arachidonic acid
to the glycerol moiety
• Phospholipase C hydrolyzes phosphorylated inositol from the
inositol glycerophospholipids, generating a diacylglycerol
containing arachidonic acid
• This arachidonic acid is subsequently released by the action of
other lipases
The Release of Arachidonate
59
The Synthesis of Prostanoids
• The key enzyme of prostanoid biosynthesis is the bifunctional
enzyme prostaglandin G/H synthase (PGS)
• PGS is a single-polypeptide-chain enzyme that has two
catalytic sites
• One catalytic site has cyclooxygenase activity which catalyzes
the addition of two molecules of oxygen to arachidonate to
form the initial prostaglandin, which is PGG2
• The second catalytic site is a glutathione-dependent, hemecontaining peroxidase that converts the hydroperoxide group
(-OOH) on carbon 15 of PGG2 to a hydroxyl group; gives PGH2
• There are two major isozymes of prostaglandin G/H synthase or
cyclooxygenase (COX), commonly called COX-1 and COX-2
• COX-1 is primarily a constitutive enzyme of gastric mucosa,
platelets, vascular endothelium, and kidney
• COX-2 is inducible and expressed in activated macrophages and
monocytes as well as smooth muscle and epithelial cells 60
• Studies have identified a third PGS isozyme, COX-3, which is
made from the COX-1 gene but retains intron 1 in its mRNA
• COX-3 is expressed in the cerebral cortex and is inhibited by
analgesic and antipyretic drugs, such as acetaminophen
(Tylenol), that do not inhibit COX-1 and COX-2
• A family of prostaglandin synthases (i.e., PGD synthase, PGE
synthase, PGI synthase) convert PGH2 to the various
prostaglandins
• PGH2 is also the precursor of thromboxane A2
• The one prostaglandin that is not synthesized directly from
PGH2 is PGF2α, which is synthesized from PGE2
The Synthesis of Linear Eicosanoids
• Lipoxygenases are dioxygenases that attach both atoms of
molecular oxygen to a particular carbon atom of arachidonic
acid (e.g., position 5 , 12, or 15)
61
The Synthesis of Prostanoids
from Arachidonate
62
• In humans, the most important leukotrienes are the 5lipoxygenase (5-LOX) products, which mediate inflammation
• Their synthesis is initiated by the formation of
5-hydroperoxyeicosatetraenoic acid (5-HPETE)
• The reduction of HPETEs to HETEs occurs spontaneously or is
catalyzed by peroxidases
• 5-LOX contains two enzymatic activities: the dioxygenase
activity that converts arachidonic acid to 5-HPETE and a
dehydrase activity that transforms 5-HPETE to LTA4
• LTA4 is an important branch point in the pathway of leukotriene
synthesis. It can be converted to LTB4 or to LTC4
• Successive degradation of LTC4 gives LTD4 and LTE4,
respectively
Eicosanoids from Other Precursors
• The major precursor for the synthesis of human eicosanoids is
arachidonic acid, which gives rise to the 2-series of
63
prostaglandins and the 4-series of leukotrienes
The Synthesis of Leukotrienes
from Arachidonate
64
• Although eicosanoids can also be synthesized from dihomo-y linoleic acid (DGLA, 20:3; ω-6;), the immediate precursor of
arachidonic acid, there is little synthesis of 1-series
prostaglandins and 3-series leukotrienes in humans
• PGE1 has, however, been utilized extensively as a
pharmacological agent; one application of its vasodilatory
effects has been in the treatment of erectile dysfunction
• Eicosapentaenoic acid (EPA, 20:5; ω-3) is the other
physiologically significant precursor of human eicosanoids in
humans
• The main dietary source of EPA is fish oil, particularly oil from
cold-water marine fish; some EPA is also synthesized from
dietary α-linolenic acid
• EPA is the precursor to a family of eicosanoids, each of which
has one more double bond than the corresponding eicosanoid
derived from arachidonate
65
• An increased dietary intake
of fish oil can raise the
ratio of membrane
phospholipid EPA to
arachidonate
• Although dietary fish oils
have been shown to be
cardioprotective, antiinflammatory, and
anticarcinogenic, it is still
unclear how much these
benefits are due to the
partial replacement of
arachidonate-derived
eicosanoids with those
synthesized from EPA
66
Sites of Synthesis of Eicosanoids
• With the exception of red blood cells, eicosanoids are
produced and released by nearly all human cells
Gastrointestinal Tract
• Prostaglandins serve a cytoprotective role in the stomach
• PGE2 is synthesized by epithelial and smooth muscle cells in
the stomach, where it reduces gastric acid secretion while
stimulating the production of protective mucus
• For this reason, synthetic prostaglandins are helpful in
promoting the healing of gastric ulcers
Cardiovascular System
• In blood vessels, different prostaglandins have opposing
effects
• For example, platelets produce TXA2, which promotes
platelet aggregation, whereas vascular endothelial cells
produce prostacyclin (PGI2), which inhibits platelet
67
aggregation
• Both PGE2 and PGI2 are vasodilators that lower systemic
arterial pressure, thereby increasing local blood flow and
decreasing peripheral resistance
• By contrast, both TXA2 and PGF2α, (produced by vascular
smooth muscle) are vasoconstrictors
The Kidneys
• PGE2 is the major prostaglandin in the kidney, and the
collecting ducts are the main site of its production
• PGE2 dilates renal blood vessels and increases blood flow
through the kidney
• It is also an important stimulator of renin release, thus
contributing to the regulation of sodium excretion
The Lungs
• Monocytes and neutrophils in the lungs produce LTB4,
5-HETE, and SRS-A which are bronchoconstrictors
• LTC4 is more potent than histamine in contracting the
smooth muscles of bronchi
68
Female Reproductive Tract
• PGE2 within the ovarian follicle is essential for ovulation
• During parturition, prostaglandins soften tissues in the cervix
and stimulate uterine contractions to expel the fetus
Eicosanoids and Inflammation
• Prostaglandins, PGE2 in particular, are mediators of the
edema, erythema (redness of the skin), and the fever and
pain associated with inflammation
• Inflammatory reactions are most often treated with
corticosteroids that inhibit prostaglandin synthesis
• PGE2, generated in immune cells (e.g., macrophages, mast
cells, B cells), evokes chemotaxis of T cells
• It is thought that pyrogens (fever-inducing agents) activate
the prostaglandin synthesis pathway with release of PGE2 in
the hypothalamus, where body temperature is regulated
• Synthesis of leukotrienes and HETEs is also up-regulated
69
under conditions of allergy and inflammation
Regulation of Eicosanoid Synthesis and Activity
Regulation of Arachidonate Mobilization
• Activation of phospholipase A2 is crucial for the release of
arachidonic acid, which serves as a substrate for eicosanoid
synthesis.
• Phospholipase A2 is activated in a cell-specific manner by a
variety of agonists (e.g., by thrombin in platelets)
• Steroidal anti-inflammatory drugs such as prednisone and
betamethasone block prostaglandin release in part by
inducing the synthesis of inhibitors of PLA2
Regulation of Prostaglandin G/H Synthase
• Prostaglandin G/H synthase represents the committed step in
prostaglandin and thromboxane synthesis
• Whereas COX-1 is constitutive in many cell types, synthesis of
COX-2 in various cells is induced by a variety of cytokines and
lipid mediators
70
• Although glucocorticoids had long been assumed to block
prostaglandin synthesis at the level of phospholipase A2,
there is increasing evidence that they also suppress the
synthesis of COX-2 in many cell types
• Prostaglandin synthase, a major target for pharmacological
intervention, is inhibited by non-steroidal anti-inflammatory
drugs (NSAIDs) such as aspirin (acetylsalicylic acid) and
ibuprofen
• Most NSAIDs act as reversible inhibitors of the cyclooxygenase component of PGS
• Aspirin acts differently in that it irreversibly inhibits PGS by
acetylating the hydroxyl group of a particular serine hydroxyl
at the active site of cyclooxygenase
• Low-dose aspirin regimens are often used to decrease the
risk of thrombosis and coronary heart disease in older
persons
71
• This therapy is effective because circulating platelets are unable
to synthesize more prostaglandin synthase to replace that
which has been inactivated
• Recent pharmaceutical efforts have focused on the
development of selective COX-2 inhibitors such as Celebrex
(celecoxib), with the goal of developing anti-inflammatory drugs
less likely to cause the gastric toxicity associated with chronic
use of NSAIDs that block COX- 1 as well
• Both Celebrex and Vioxx (now withdrawn from the market)
have been associated with increased adverse cardiovascular
events
• Possible reasons for the adverse effects include:
Decreased production of antithrombotic PGI2 while not
inhibiting COX- 1 -mediated synthesis of thromboxane A2 in
platelets
 COX 2 can be constitutive in the kidneys where PGE2 promotes
72
dilation of arterioles and the excretion of Na+ and water
The Action of NSAIDs
73
Regulation of Leukotriene Metabolism
• Current therapies for asthma include use of 5-lipoxygenase
inhibitors such as zileuton, and cysteinyl-leukotriene (cysLT)
receptor antagonists such as montelukast
Therapeutic Uses of Eicosanoids
• Exogenous prostaglandins have a number of therapeutic uses.
• For example, in the fetus, PGE2 keeps ductus arteriosus open
prior to birth
• In infants born with congenital abnormalities that can be
corrected surgically, infusion of PGE2 will maintain blood flow
through the ductus until surgery is performed
• Conversely, if the ductus remains open after birth in an
otherwise normal infant, closure can be hastened by the COX
inhibitor indomethacin
• PGE2 has also been used to induce cervical ripening and
uterine contractions, leading to parturition
74
• Misoprostol is a synthetic PGE1 analog used to prevent NSAIDinduced ulcers
• In many countries, misoprostol is also used in combination with
the synthetic steroid RU486 to block the action of
progesterone and induce medical (as opposed to surgical)
abortions
Outroduction
Trans Fatty Acids and Omega-3 vs 6 Fatty Acids
• Trans fatty acids are produced during the commercial
hydrogenation of plant oils
• Some margarines contain these fatty acids, as do some
commercially prepared snack foods (e.g. biscuits, cookies,
cakes, chips)
• In addition, bacteria in the rumen of ruminants produce trans
fatty acids, which are therefore present in dairy produce and
meat
75
• Trans fatty acids can be incorporated into the phospholipids of the
plasma membrane of endothelial and other cells, resulting in
damage to the membranes
• Furthermore, these abnormal fatty acids can interfere in the
production of thromboxanes, prostacyclins or leukotrienes
• Series-2 thromboxanes (from the ω-6 arachidonate) have strong
positive effects on aggregation of platelets whereas those of the
3-series (from the ω-6 EPA) have much less effect
• Hence, a low chronic intake of omega-3 fatty acid (linolenic) and a
high intake of omega-6 fatty acid (linoleic) increases the
formation of thromboxanes of the series 2 and increases the
formation of prostaglandin of series 2 which has a lower
vasodilatory effect, which increases the risk of thrombosis
• ω -3 fatty acids are quantitatively important components of the
membranes of neurones in the brain; important in development
of the brain and the repair of damaged membranes
• Chronic deficiency could increase the risk of development of 76
some disorders, including depression,schizophrenia and ADD
77
•
•
•
•
•
TRIACYLGLYCEROL TRANSPORT AND METABOLISM
The main function of TAG in the body is to provide a compact
and relatively unlimited means for storing energy
At 9 kcal/g, the energy content of TAG is more than twice that
of the other major form of energy that humans store, namely
glycogen (4.1 kcal/g)
The fatty acids that comprise TAG are highly reduced. Except
for the carboxyl group, most of the carbon atoms of a fatty acid
have two hydrogen atoms attached to them and are bonded to
another carbon atom
It is the energy that is released during the oxidation of these CH and C-C bonds that ultimately supports the synthesis of ATP
by oxidative phosphorylation
Furthermore, whereas glycogen binds more than twice its
weight in water, TAG are hydrophobic, such that only a small
part of the mass of adipose tissue is water
78
• This contrast in water-binding capacity between fat and
glycogen means that on a weight basis, it is more economical
to store energy in the form of fat than in the form of glycogen
• In the fed state, the TAG in circulating lipoproteins (both dietderived (chylomicrons) and endogenously synthesized (VLDL))
are hydrolyzed in the capillaries of skeletal muscle, adipose,
and other tissues by lipoprotein lipase, and the resulting free
fatty acids are taken up by adipocytes and muscle cells
• Between meals or during a fast when glycogen stores have
been depleted, TAG in adipocytes are hydrolyzed to free fatty
acids and glycerol
• These fatty acids circulate bound to albumin rather than as
components of triacylglycerol-containing lipoproteins
• Formation of non-covalent complexes between fatty acids and
albumin solubilizes the long-chain fatty acids
• Transport on albumin also minimizes the potential damage to
79
membranes by the detergent activity of free fatty acids
• The fatty acids in the sn-1 and -3 positions of TAG tend to be
long-chain saturated fatty acids (e.g., palmitic acid, stearic
acid) or monounsaturated fatty acids (e.g., oleic acid), whereas
those in the sn-2 position tend to be polyunsaturated fatty
acids (e.g., linoleic acid, α-linolenic acid, arachidonic acid)
• The three most abundant fatty acids in the TAG of adipose
tissue and plasma lipoproteins are palmitic acid, oleic acid, and
linoleic acid
Synthesis of Triacylglycerols
• The pathway for triacylglycerol synthesis in most tissues,
including liver and adipocytes, utilizes glycerol 3-phosphate
and fatty acyl-CoA
80
• The activated fatty acids (i.e., fatty acids attached to CoA) are
derived either from endogenous de novo fatty acid synthesis or
from dietary fats
• By contrast, triacylglycerol synthesis in the small intestine
begins with 2-monoacylglycerol
• The sources of glycerol 3-phosphate, which provides the
glycerol moiety for triacylglycerol synthesis, differ in liver and
adipose tissue
• In liver, glycerol 3-phosphate is produced from the
phosphorylation of glycerol by glycerol kinase or from the
glycerol 3-phosphate dehydrogenase reduction of DHAP
derived from glycolysis
• Adipose tissue lacks glycerol kinase and can produce glycerol 3phosphate only from glucose via dihydroxyacetone phosphate
• In the fasting state, adipose and the liver also undergo
glyceroneogenesis –synthesize glycerol 3-phosphate from
81
gluconeogenic precursors, such as alanine, aspartate, malate
• In both adipose tissue
and liver, TAG are
produced by a
pathway in which
glycerol 3-phosphate
reacts with two fatty
acyl-CoA molecules
to form phosphatidic
acid
• Dephosphorylation of
phosphatidic acid
produces
diacylglycerol
• Another fatty acylCoA reacts with the
diacylglycerol to form
TAG
82
Transport of Triacylglycerols in the Blood
• Plasma contains a class of macromolecular aggregates called
lipoproteins that transport otherwise highly water-insoluble
lipids-cholesteryl esters and TAG in particular-in the
circulation
• Lipoproteins also play a key role in the metabolism of these
lipids and facilitate the two-way exchange of TAG between
tissues and the blood
• In addition, cholesteryl esters, TAG and proteins undergo
exchange between the various lipoproteins
• The surface of a lipoprotein particle is coated with proteins,
phospholipids, and free (non-esterified) cholesterol
• The polar ends of the amphipathic lipids face the surface of
the lipoprotein, whereas the hydrophobic portions are
oriented toward the center of the particle
83
• The core of the lipoproteins is composed of highly non-polar
lipids such as TAG and cholesteryl esters
• The proteins (or apoproteins as they are called when
separated from the lipids) serve a number of functions,
including stabilization of the lipoprotein’s structure and as
recognition sites for lipoprotein receptors on cell membranes
• The lipoproteins are differentiated on the basis of density
• The most dense lipoprotein class is called high-density
lipoprotein (HDL) followed by low-density lipoprotein (LDL),
very low-density lipoprotein (VLDL) and chylomicrons
• The major triacylglycerol-transporting lipoproteins are VLDL
and chylomicrons
• LDL and HDL transport primarily cholesteryl esters
• HDL also contains some triacylglycerol; it is also the
lipoprotein with the highest amount of protein
84
85
The Fate of Chylomicrons
• The triacylglycerols of the chylomicrons are digested by
lipoprotein lipase (LPL) attached to the proteoglycans in the
basement membranes of endothelial cells that line the capillary
walls. LPL is activated by apo C II
• LPL is produced mainly by adipose, muscle cells (particularly
cardiac muscle), and cells of the lactating mammary gland
• The isozyme synthesized in adipose cells has a higher Km than
the isozyme synthesized in muscle cells
• Therefore, adipose LPL is more active after a meal, when
chylomicrons levels are elevated in the blood
• Insulin stimulates the synthesis and secretion of adipose LPL,
such that after a meal, when triglyceride levels increase in
circulation, LPL has been upregulated (through insulin release)
to facilitate the hydrolysis of fatty acids from the TAG
• The fatty acids released from triacylglycerols by LPL are not
86
very soluble in water
• They become soluble in blood by forming complexes with
albumin
• The major fate of the fatty acids is storage as triacylglycerol in
adipose tissue.
• However, these fatty acids also may be oxidized for energy in
muscle and other tissues
• The LPL in the capillaries of muscle cells has a lower Km than
adipose LPL
• Thus, muscle cells can obtain fatty acids from blood
lipoproteins whenever they are needed for energy, even if the
concentration of the lipoproteins is low
• The glycerol released from chylomicron TAG by LPL may be
used for triacylglycerol synthesis in the liver in the fed state
• The portion of a chylomicron that remains in the blood after
LPL action is known as a chylomicron remnant
• This remnant binds to receptors on hepatocytes which
87
recognize apo E, and is endocytosed
• Lysosomes fuse with the endocytic vesicles, and the
chylomicron remnants are degraded by lysosomal enzymes
• The products of lysosomal digestion (e.g., fatty acids, amino
acids, glycerol, cholesterol, phosphate) can be reused
o As LPL is bound to the capillary endothelium through binding to
proteoglycans, heparin also can bind to LPL and dislodge it from
the capillary wall. This leads to loss of LPL activity and an
increase of triglyceride content in the blood
Transfer of Proteins Between
HDL and Chylomicrons
88
The Metabolism of
Chylomicrons
89
Fate of VLDL
• The TAG, which is produced in the smooth endoplasmic
reticulum of the liver, is packaged with cholesterol,
phospholipids, and proteins (synthesized in the rough
endoplasmic reticulum) to form VLDL
• The microsomal triglyceride transfer protein (MTP), which is
required for chylomicron assembly, is also required for VLDL
assembly
• The major protein of VLDL is apoB-100. There is one long apoB100 molecule wound through the surface of each VLDL particle
• ApoB-100 is encoded by the same gene as the apoB-48 of
chylomicrons, but is a longer protein
• In intestinal cells, RNA editing produces a smaller mRNA and,
thus, a shorter protein, apoB-48
• VLDL is processed in the Golgi complex and secreted into the
blood by the liver. The fatty acid residues of the triacylglycerols
ultimately are stored in the triacylglycerols of adipose cells 90
• In comparison to chylomicrons , VLDL particles are more dense,
as they contain a lower percentage of triglyceride than do the
chylomicrons
• Similar to chylomicrons, VLDL particles are first synthesized in
a nascent form, and on entering the circulation they acquire
apoproteins CII and E from HDL particles to become mature
VLDL particles
• LPL cleaves the TAG in both VLDL and chylomicrons, forming
fatty acids and glycerol
• The C-II apoprotein, which these lipoproteins obtain from HDL,
activates LPL
• The low Km of the muscle LPL isozyme permits muscle to use
the fatty acids of chylomicrons and VLDL as a source of fuel
even when the blood concentration of these lipoproteins is very
low. The isozyme in adipose tissue has a high Km and is most
active after a meal, when blood levels of chylomicrons and
91
VLDL are elevated
• The fate of the VLDL particle after TAG has been removed by
LPL is the generation of an IDL particle (intermediate-density
lipoprotein), which can further lose triglyceride to become an
LDL particle (low-density lipoprotein)
• The fate of the IDL and LDL particles is closely related with the
metabolism of cholesterol
A VLDL Particle
92
The Synthesis of VLDL
93
The Metabolism of VLDL
(and Chylomicrons)
94
The Mobilization of Adipose Tracylglycerols
• Hormone-sensitive lipase (HSL) is a key enzyme involved in the
hydrolysis of adipocyte triacylglycerol
• Its name reflects the fact that the enzyme is activated by the
signal transduction cascade involving cAMP and protein kinase
A (PKA)
• In adipocytes, activation of PKA is initiated primarily by
epinephrine and glucagon
• For decades, hormone-sensitive lipase was considered to be
the key regulatory enzyme in the lipolysis pathway of adipose
tissue
• However, it is now believed that a recently discovered lipase
called desnutrin or adipose triglyceride lipase (ATGL)
catalyzes the first step in triacylglycerol hydrolysis
• Desnutrin catalyzes the hydrolysis of TAG to diacylglycerols
(DAG) and is the rate-limiting step in triacylglycerol hydrolysis
95
• Hormone-sensitive lipase is
more active against DAG than
against TAG
• Thus, the current hypothesis
regarding the lipolytic cascade
in adipocytes involves three
esterases acting sequentially:
desnutrin hydrolyzes the first
ester bond in TAG generating
DAG; then DAG are
hydrolyzed by hormonesensitive lipase to produce 2monoacylglycerols; and
finally, monoacylglycerol
lipase removes the third fatty
acid to produce glycerol
96
The Regulation of Triacylglycerol Metabolism
• Insulin is the most important regulator of triacylglycerol
metabolism
• Insulin enhances the rate of hydrolysis of lipoproteinassociated TAG by stimulating the synthesis and secretion of
lipoprotein lipase by adipocytes
• Insulin also promotes TAG storage in adipocytes and TAG
synthesis and VLDL export from hepatocytes
• Simultaneously, insulin inhibits the breakdown of TAG, in fat
cells
• By contrast, hydrocortisone, epinephrine, and growth
hormone oppose the action of insulin, inhibiting the
synthesis of fatty acids in both hepatocytes and adipocytes,
and promoting lipolysis in adipocytes in times of energy
need, such as fasting and exercise
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i. Regulation of TAG Synthesis
• Insulin stimulates dephosphorylation and activation of
acetyl-CoA carboxylase
• Insulin also promotes fatty acid synthesis by inducing
enzymes of the fatty acid synthesis family
• In addition, insulin stimulates the catabolism of excess
dietary carbohydrates, thereby increasing the supply of
acetyl-CoA substrate for fatty acid and thus triacylglycerol
synthesis
ii. Regulation of Adipocyte TAG Mobilization
• Lipolysis in adipocytes is under tight hormonal control
• The synthesis of desnutrin is induced by hydrocortisone and
inhibited by insulin
• HSL is activated by epinephrine via a mechanism involving
cAMP-dependent phosphorylation
• By contrast, insulin acts to dephosphorylate HSL, thereby
98
inhibiting lipolysis
• Adipocytes store TAG in the form of lipid droplets surrounded
by a protein called perilipin
• Like HSL, perilipin is phosphorylated by PKA
• In its unphosphorylated state, perilipin acts as a barrier that
limits access of lipases to their substrates, thus maintaining a
low rate of basal triacylglycerol hydrolysis
• Phosphorylation of perilipin causes the lipid droplets to
fragment and disperse, permitting efficient hydrolysis of
adipocyte TAG
iii. Regulation of Lipoprotein Lipase Activity
• LPL is active in both the fasted and fed states
• In the fasted state, it plays an important role in making fatty
acids from VLDL TAG available to cardiac and skeletal muscles,
• In contrast, in the fed state, LPL directs fatty acids from both
chylomicrons and VLDL to adipocytes for storage
• Adipocyte LPL expression is reduced during fasting while its
99
expression in muscle is up-regulated
• Conversely, adipocyte LPL expression is up-regulated in the fed
state
• In addition, muscle- and adipocyte-specific forms of LPL have
different kinetic properties, with the muscle enzyme having a
lower Km , for triacylglycerol than the adipocyte enzyme
• Thus, the active site of the LPL enzyme, which is localized to
the surface of muscle capillaries, is saturated even during the
fasted state, when circulating triacylglycerol-containing
lipoprotein levels are low
• By contrast, the activity of LPL associated with adipose tissue
capillaries increases in the fed state, when the levels of
triacylglycerol-rich lipoproteins are relatively high
The Triacylglycerol Cycle
• An additional factor in the balance between biosynthesis and
degradation of triacylglycerols is that approximately 75% of all
fatty acids released by lipolysis are reesterified to form TAG
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rather than used for fuel
• This ratio persists even under starvation conditions, when
energy metabolism is shunted from the use of carbohydrate
to the oxidation of fatty acids
• Some of this fatty acid recycling takes place in adipose
tissue, with the reesterification occurring before release into
the bloodstream
• Some takes place via a systemic cycle in which free fatty
acids are transported to liver, recycled to TAG, exported
again to the adipose in the form of VLDL
• Flux through this triacylglycerol cycle between adipose tissue
and liver may be quite low when other fuels are available and
the release of fatty acids from adipose tissue is limited
• But the proportion of released fatty acids that are
reesterified remains roughly constant at 75% under all
metabolic conditions
• The function of the apparently futile triacylglycerol cycle101is
• One possibility is that, the excess capacity in the triacylglycerol
cycle (the fatty acid that is eventually reconverted to TAG
rather than oxidized as fuel) could represent an energy reserve
in the bloodstream during fasting, one that would be more
rapidly mobilized in a “fight or flight response” emergency than
would stored TAG
• The glycerol 3-phosphate needed for the recycling of TAG in
the starved state is provided by glyceroneogenesis
• Glyceroneogenesis is a shortened version of gluconeogenesis,
from pyruvate to DHAP followed by conversion of the DHAP to
glycerol 3- phosphate by cytosolic NAD+-linked glycerol 3phosphate dehydrogenase
• In the adipose tissue, glyceroneogenesis coupled with
reesterification of free fatty acids controls the rate of fatty acid
release to the blood
• In addition, glyceroneogenesis in the liver alone can support
the synthesis of enough G 3-P for 65% of reesterification 102
• Flux through the triacylglycerol cycle between liver and adipose
tissue is controlled to a large degree by the activity of PEP
carboxykinase, which limits the rate of both gluconeogenesis
and glyceroneogenesis
• Glucocorticoid hormones such as cortisol and dexamethasone
(a synthetic glucocorticoid) regulate the levels of PEP
carboxykinase reciprocally in the liver and adipose tissue
• These steroid hormones increase the expression of the gene
encoding PEP carboxykinase in the liver, thus increasing
gluconeogenesis and glyceroneogenesis
• Stimulation of glyceroneogenesis leads to an increase in the
synthesis of triacylglycerol molecules in the liver and their
release into the blood
• At the same time, glucocorticoids suppress the expression of
the gene encoding PEP carboxykinase in adipose tissue
• This results in a decrease in glyceroneogenesis in adipose tissue
103
• Recycling of fatty acids declines as a result and more free fatty
acids are released into the blood
• Thus glyceroneogenesis is regulated reciprocally in the liver
and adipose tissue, affecting lipid metabolism in opposite ways
• A lower rate of glyceroneogenesis in adipose tissue leads to
more fatty acid release (rather than recycling), whereas a
higher rate in the liver leads to more synthesis and export of
triacylglycerols
• High levels of free fatty acids in the blood interfere with
glucose utilization in muscle and promote the insulin resistance
that leads to type 2 diabetes
• A class of drugs called thiazolidinediones have been shown to
reduce the levels of fatty acids circulating in the blood and
increase sensitivity to insulin
• Thiazolidinediones bind to and activate a nuclear hormone
receptor called peroxisome proliferator activated receptor γ
(PPAR γ), leading to the induction in adipose tissue PEPCK104
The TAG Cycle
• A higher activity of PEPCK then leads to increased synthesis of
the precursors of glyceroneogenesis
• The therapeutic effect of thiazolidinediones is thus due, in part, to
the increase in glyceroneogenesis, which in turn increases the
resynthesis of triacylglycerol in adipose tissue and reduces the
release of free fatty acid from adipose tissue into the blood
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Diseases Involving Abnormalities in Triacylglycerol Metabolism
Obesity
• The amount of fat an individual can store depends on the
number of fat cells in the body and the amount of
triacylglycerol each cell can accommodate
• In obese individuals, both the number of fat cells and the size of
the cells (i.e., the total storage capacity) is greater than in
individuals with no history of obesity
• Fat cells begin to proliferate early in life, starting in the third
trimester of gestation
• Proliferation essentially ceases before puberty, and thereafter
fat cells change mainly in size
• However, some increase in the number of fat cells can occur in
adulthood if preadipocytes are induced to proliferate by growth
factors and changes in the nutritional state
• Weight reduction results in a decrease in the size of fat cells
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rather than a decrease in number
• There are two major types of fat. One is subcutaneous fat that
is located just below the skin surface, most noticeable in the
thighs, buttocks, arms and face
• The other is visceral fat which lies deep within the abdominal
cavity and is responsible for the size of the waistline
• One way to predict if someone has too much body fat is to
determine their body mass index (BMI) using a ratio of their
weight and height
• It is generally accepted that a BMI value of less than 18.5 is
considered underweight, 18.5-25 is within the normal weight
range, 25-30 is overweight, and greater than 30 is obese
• BMI values do not provide information about the relative
amounts of visceral fat and subcutaneous fat stores
• One of the best ways to predict an individual's disease risk is to
use both their BMI value and the circumference of their waist
in relationship to the size of their hips
107
• By determining a person's waist to hip ratio (WHR), it is
possible to obtain an approximate measurement of the relative
amounts of visceral and subcutaneous fat stored on their body
• A high WHR value corresponds to an "apple-shaped" body
(more visceral fat in the waist than subcutaneous fat on the
hips), whereas, a low WHR value leads to a "pear-shaped" body
• Body weight is determined by a balance between energy intake
and energy expenditure
• The energy expenditure is required to maintain basal
metabolic functions, absorption and digestion of foods ,
physical activity and, in children, linear growth and
development
• If there is a net excess energy intake, BMI and WHR increase,
eventually leading to overweight and ultimately to obesity and
morbid obesity. In a population with stable genetic factors,
increase in obesity is primarily attributable to consumption of
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excess food and decreased physical activity
• Obesity is a worldwide health problem. It is a risk factor for
development of diabetes mellitus, hypertension, and heart
disease, all of which cause decreased quality of life and life
expectancy
• Signals that initiate or inhibit feeding are extremely complex
and include psychological and hormonal factors as well as
neurotransmitter activity
• These signals are integrated and relayed through the
hypothalamus
• Destruction of specific regions of the hypothalamus can lead
to overeating and obesity or to anorexia and weight loss
• The adipocyte, in addition to storing triacylglycerol, secretes
peptide hormones that regulate both glucose and fat
metabolism
• The hormones leptin (Greek leptos, thin), resistin (resists
insulin action), and adiponectin are all secreted from
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adipocytes under different conditions
• Leptin produced in adipocytes moves through the blood to the
brain, where it acts on neuroendocrine system of the
hypothalamus to curtail appetite (induce satiety)
• Leptin carries the message that fat reserves are sufficient, and
it promotes a reduction in fuel intake and increased
expenditure of energy
• Leptin-receptor interaction in the hypothalamus alters the
release of neuronal signals to the region of the brain that
affects appetite
• Leptin also stimulates the sympathetic nervous system,
increasing blood pressure, heart rate, and thermogenesis by
uncoupling electron transfer from ATP synthesis in the
mitochondria of adipocytes
• Two types of neurons control fuel intake and metabolism.
When leptin levels are low, the orexigenic (appetitestimulating) neurons stimulate eating by producing and
110
releasing neuropeptideY (NPY)
• NPY causes the next neuron in the circuit to send the signal to
the brain, Eat!
• When leptin levels are high, the anorexigenic (appetitesuppressing) neurons produce α-melanocyte-stimulating
hormone (α-MSH)
• Release of α-MSH causes the next neuron in the circuit to send
the signal to the brain, Stop eating!
• The amount of leptin released by adipose tissue depends on
both the number and the size of adipocytes
• When weight loss decreases the mass of lipid tissue, leptin
levels in the blood decrease, the production of NPY is increased
• Uncoupling is diminished, slowing thermogenesis and saving
fuel, and fat mobilization slows
• Consumption of more food combined with more efficient
utilization of fuel results in replenishment of the fat reserve in
adipose, bringing the system back into balance
111
112
The Mechanism of Action of Leptin
• Human obesity may be the result of insufficient leptin
production/action
• Blood levels of leptin are usually much higher in obese animals
(including humans) than in animals of normal body mass
• Some downstream element in the leptin response system may
be defective in obese individuals, and the elevation in leptin
could be the result of an (unsuccessful) attempt to overcome
the leptin resistance
• In humans, adiponectin is secreted from adipocytes in inverse
proportion to their adipose mass, lean individuals secreting
more adiponectin than obese individuals
• This is the exact opposite of leptin secretion.
• Adiponectin circulates in the blood and powerfully affects the
metabolism of fatty acids and carbohydrates in liver and
muscle
• It increases the uptake of fatty acids from the blood by
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myocytes and the rate at which fatty acids undergo oxidation
• It also blocks fatty acid synthesis and gluconeogenesis in
hepatocytes, and it stimulates glucose uptake and catabolism
in muscle and liver
• These effects of adiponectin occur indirectly, through
activation of the key regulatory enzyme AMPK
• Mice with defective adiponectin genes are less sensitive to
insulin than those with normal adiponectin, and they show
poor glucose tolerance; ingestion of dietary carbohydrate
causes a long-lasting rise in their blood glucose
• These metabolic defects resemble those of humans with type II
diabetes, who also are insulin-insensitive and clear glucose
from the blood only slowly
• Individuals with obesity or type II diabetes have lower blood
adiponectin levels than non-diabetic controls
• Moreover, thiazolidinediones increase the expression of
adiponectin mRNA in adipose tissue and increase blood
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adiponectin levels in experimental animals
• They also activate AMPK
• It appears that adiponectin, acting through AMPK, modulates
the sensitivity of cells and tissues to insulin
• This hormone may prove to be one of the links between type II
diabetes and its most important predisposing factor, obesity
The Mechanism of
Action
of Adiponectin 115
• The adipocytes have been shown to release a hormone
known as resistin
• This hormone may contribute to insulin resistance
• The mechanism by which resistin causes an insensitivity of
cells to the actions of insulin is unknown
• Thiazolidinediones, suppress resistin transcription, reduce
resistin levels, and increase sensitivity to insulin in these
patients
• Proteins in a family of ligand-activated transcription factors,
the peroxisome proliferator-activated receptors (PPARs),
respond to changes in dietary lipid by altering the expression
of genes involved in fat and carbohydrate metabolism
• These transcription factors were first recognized for their
roles in peroxisome synthesis—thus their name
• Their normal ligands are fatty acids or fatty acid derivatives,
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but they can also bind synthetic agonists
• PPARα, PPARδ, and PPARγ are members of the nuclear
receptor superfamily
• They act in the nucleus by forming heterodimers with another
nuclear receptor, RXR (retinoid X receptor), binding to
regulatory regions of DNA near the genes under their control
and changing the rate of transcription of those genes
• PPAR γ , expressed primarily in liver and adipose tissue, is
involved in turning on genes necessary to the differentiation
of fibroblasts into adipocytes and genes that encode proteins
required for lipid synthesis and storage in adipocytes
• The PPAR-y/RXR heterodimer regulates the transcription of
genes encoding proteins such as acyl CoA-synthetases,
lipoprotein lipase, GLUT4, mitochondrial uncoupling
protein,…
• PPAR α in hepatocytes turns on the genes necessary for
β-oxidation of fatty acids and formation of ketone bodies
117
during fasting
• PPAR δ is a key regulator of
fat oxidation, which acts by
sensing changes in dietary
lipid
• It acts in liver and muscle,
stimulating the
transcription of at least nine
genes encoding proteins for
β-oxidation and for energy
dissipation through
uncoupling of mitochondria
• By stimulating fatty acid
breakdown in uncoupled
mitochondria, PPARδ
causes fat depletion, weight
loss, and thermogenesis
Mode of Action of
PPARs 118
• Short-term eating behavior can be modified by hormones
• Ghrelin is a peptide hormone produced in cells lining the
stomach
• It is a powerful appetite stimulant that works on a shorter time
scale (between meals) than leptin
• Ghrelin receptors are located in the pituitary gland (presumably
mediating growth hormone release) and in the hypothalamus
(affecting appetite), as well as in heart muscle and adipose
tissue
• The concentration of ghrelin in the blood varies strikingly
between meals, peaking just before a meal and dropping
sharply just after the meal
To summarize….
• Although an increase in food intake beyond the daily
requirements results in an increase in body weight and in fat
stores, there is a large variation among individuals in the
amount of weight gained for the excess calories consumed119
• Both genetic and environmental factors influence the
development of obesity
• Still, although there are certainly wide variations in body
structure and metabolism between individuals, the current
epidemic of obesity in the many countries is due more to
environmental than to genetic factors
• The efficient storage of excess calories as triacylglycerol may
have been advantageous for our distant ancestors who were
physically very active and for whom food scarcity was often
the norm
• However, in the current context of sedentary lifestyles and in
environments where there is an overabundance of calorically
dense foods containing large amounts of fats and sugars,
efficient fuel storage in the form of fat can result in obesity
and its undesirable medical and social sequelae
• So, what should be done? MOVE!
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Exercise Generating Maximum Power
• ATP, phosphocreatine and anaerobic metabolism of muscular
glycogen are the primary sources of energy
High Intensity Endurance Exercise
• Phosphocreatine is the initial energy source; glycogenolysis
ensues, anaerobically at first, with lactate production, but with
increasing aerobic oxidation as oxygen availability increases
• Glycogen utilization is the major fuel for the first 15 minutes,
but it is superseded by fatty acid and plasma glucose utilization
• During the exercise, branched-chain amino acid oxidation
contributes some energy
• In endurance exercise, hepatic glycogen serves as a major fuel
for muscle, and muscle glycogen is the initial fuel
Low-Level Non-fatiguing Exercise
• This exercise is characteristic of an individual in normal
occupational tasks that could reasonably be continued for long
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hours
• The principal substrates used are similar to those in long
endurance exercise but without depletion of phosphocreatine
and with minimal muscle glycogen utilization
• The main source of energy is the aerobic oxidation of fatty
acids, glucose, and branched-chain amino acids
• To the extent that one utilizes free fatty acids during exercise, one
decreases fat stores proportionally
Diabetes Mellitus
• Type I diabetes is caused by absent or insufficient insulin
production
• Although the most common cause is autoimmune destruction
of the β-cells of the pancreas, it can also result from chronic
pancreatitis
• The resulting insulin deficiency has been described as
“starvation in the midst of plenty,” with metabolic pathways
active in fasting mode despite high plasma levels of nutrients in
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the blood
• Diabetes affects lipid and protein metabolism as well as that
of glucose
• A lack of insulin results in fasting hyperglycemia with both
overproduction of glucose by the liver and underutilization of
glucose by both muscle and adipocytes
• There is going to be an increased catabolism of muscle
proteins to provide gluconeogenetic precursors;
hyperaminoacidemia will result
• Chronically raised levels of glucose in the blood bring damage
to many organs, including the eyes, kidneys, blood vessels,
and nerves
• The high glucagon/insulin ratio stimulates adipose
triacylglycerol hydrolysis and increases the plasma free fatty
acid concentration
• A great proportion of the fatty acids is going to travel to the
liver and be metabolized in two main ways
123
1. Reesterification and release into the blood in the form of VLDL
• But because insulin is lacking, LPL is not upregulated and
hydrolysis of the triacylglycerols in chylomicrons and in VLDL is
decreased, and hypertriglyceridemia results
2. β-oxidation and formation of ketone bodies (since oxaloacetate
is being diverted to gluconeogenesis)
• Higher than normal quantities of ketone bodies present in the
blood or urine constitute ketonemia or ketonuria, respectively
The overall condition is called ketosis
• The basic form of ketosis occurs in starvation and feeding of
high-fat diets and involves depletion of available carbohydrate
coupled with increased levels of free fatty acids
• This general pattern of metabolism is exaggerated to produce
the pathologic states found in diabetes mellitus
• In uncontrolled diabetes mellitus, the concentration of plasma
ketone bodies can rise to levels of 8 to 15 mmol/L or more
124
(normal is in the range of 0.2–2 mmol/L)
• Since both acetoacetate and β -hydroxybutyrate are organic
acids, ketosis is a form of metabolic acidosis. In order to be
excreted in the urine, the anionic metabolic acids in the urine
must be counterbalanced by equivalent numbers of cations
• Therefore, ketosis may result in depletion of body stores of
sodium and potassium and in some instances even in the loss of
divalent cations such as calcium and magnesium
• The combined effect of the ketone bodies and the associated
protons is known as diabetic ketoacidosis (DKA)
• Along with the effects of the hyperosmolarity of the blood (due
to the excess glucose), DKA can lead to coma and death
• Currently, the only effective treatment for type I diabetes
mellitus is insulin therapy
• Most people with diabetes mellitus have type II diabetes rather
than type I, and it is type II diabetes that is now reaching
epidemic incidence in many countries
125
• Type II diabetes mellitus is characterized by insulin resistance
rather than primary insulin insufficiency
• A person has insulin resistance when larger-than-normal
amounts of insulin are required to support insulin-dependent
metabolic processes
• Insulin resistance is also commonly seen in the obese and in
those with the metabolic syndrome
• In most instances of insulin resistance, insulin secretion is not
impaired and insulin receptors are functional
• Although people in the early stages of insulin resistance can
maintain normal blood glucose concentrations by increasing
their insulin secretion, this compensation often becomes
inadequate and they eventually progress to hyperglycemia and
eventually to type II diabetes
• Like type I diabetes, type II diabetes is characterized by
hyperglycemia, hypertriglyceridemia, hyperaminoacidemia,
126
and elevated levels of free fatty acids
• The high levels of free fatty acids in the blood are the result of
increased TAG lipolysis by adipocytes
• Elevated free fatty acid levels, in turn, result in increased TAG
synthesis by the liver and export of TAG-rich VLDL particles
• Unlike people with type I diabetes those with type II diabetes
usually do not develop ketoacidosis, in part because - at least
in the early stages of the disease - the liver is less insulin
resistant than skeletal muscle or adipocytes
• Obesity often leads to insulin resistance in muscle
• In people with insulin resistance, the muscle cells do not
sufficiently up-regulate the acyltransferases involved in
triacylglycerol synthesis to cope with the increased availability
of free fatty acids
• As a result, a high intracellular concentration of metabolic
intermediates inhibits glucose uptake and glycolysis by muscle
cells, and higher levels of insulin are required for glucose
127
utilization
• Although the mechanisms have not been fully elucidated, the
relatively insulin-resistant state associated with obesity may be
the result of an imbalance in adipokine production as well as
elevated plasma levels of free fatty acids released from the
excess adipocyte stores
• Exercise stimulates both TAG and glycogen synthesis in skeletal
muscle and improves insulin sensitivity in both normal-weight
and obese persons
• Among the mechanisms involved are up-regulation of GLUT4
transporters and induced expression of diacylglycerol
acyltransferase
• Type II diabetes can be treated with a variety of drugs that
stimulate insulin secretion (e.g., sulfonylureas) or increase
insulin sensitivity (e.g, thiazolidinediones), or reduce hepatic
gluconeogenesis (e.g., metformin)
• Many cases of type II diabetes, however, eventually progress to
128
the point of pancreatic β-cell failure and dependence on insulin
•
•
•
•
•
Ethanol Metabolism
Since there is no significant renal or pulmonary excretion of
ethanol and no storage of ethanol in the body, whatever
ethanol is consumed must be disposed of through
metabolism
The major site of alcohol metabolism is the liver, which
contains both alcohol dehydrogenase (ADH) and the
microsomal ethanol-oxidizing system (MEOS), the two
enzymes most responsible for ethanol metabolism
However, alcohol dehydrogenase activity is also present in the
gastric mucosa (more so in men than women), and to a lesser
extent in other organs, including the kidneys, lungs and small
intestine
ADH is a non-specific enzyme, able to use a wide range of
simple alcohols, hydroxysteroids and retinol as substrates
It also acts on ethanol produced by the fermentation of sugars
129
by microorganisms in the colon
• The overall pathway for metabolizing ethanol involves
oxidation of the alcohol to acetaldehyde, which is then
oxidized to acetate
• The acetate derived from ethanol oxidation is activated to
acetyl-CoA by acetate thiokinase
• The resulting acetyl-CoA can be metabolized through the TCA
cycle or utilized for fatty acid synthesis. There are three
enzymes or enzyme systems that convert ethanol to
acetaldehyde: ADH, MEOS, and catalase
ADH
• NAD+-dependent ADH, which is a cytosolic enzyme converts
ethanol to acetaldehyde with the concomitant release of
NADH
• The NADH can be transported to the mitochondria and be
used for ATP synthesis
• The gastric isozyme of ADH has a higher Km for ethanol than
130
do the ADH isozymes in liver
MEOS
• The liver has a second pathway for oxidizing ethanol, which
even though it can oxidize a variety of compounds in addition
to ethanol, is designated the microsomal ethanol-oxidizing
system
• Other substrates for MEOS include fatty acids, steroids, and
certain drugs
• Ethanol and NADPH both donate electrons in the reaction,
which reduces O2 to 2H2O
• The cytochrome P450 enzymes all have two major catalytic
protein components: an electron-donating reductase system
that transfers electrons from NADPH (cytochrome P450
reductase) and a cytochrome P450
• The cytochrome P450 protein contains the binding sites for O2
and the substrate (e.g., ethanol) and carries out the reaction
• MEOS is part of the superfamily of cytochrome P450 enzymes,
131
all of which catalyze similar oxidative reactions
• Within the superfamily, at least 10 distinct gene families are
found in mammals
• More than 100 different cytochrome P450 isozymes exist
within these 10 gene families
• Each isoenzyme has a distinct classification according to its
structural relationship with other isoenzymes
• The isoenzyme that has the highest activity toward ethanol is
called CYP2E1
• A great deal of overlapping specificity exists among the various
P450 isoenzymes, and ethanol is also oxidized by several other
P450 isoenzymes “MEOS” refers to the combined ethanol
oxidizing activity of all the P450 enzymes
• CYP2E1 has a much higher Km for ethanol than ADH. Thus, a
greater proportion of ingested ethanol is metabolized through
CYP2E1 at high levels of ethanol consumption than at low
levels
132
The General Structure of
Cytochrome P450 Enzymes
Catalase
• Catalase is also capable of oxidizing ethanol; however, its
contribution to ethanol metabolism is minimal
• Oxidation of ethanol by catalase utilizes hydrogen peroxide:
ethanol + H2O2 → acetaldehyde + 2H2O
• Acetaldehyde generated by ADH, MEOS, or catalase is oxidized
to acetate by NAD+-dependent aldehyde dehydrogenase
• Although the major isozyme of aldehyde dehydrogenase is
133
located in the mitochondria, there is also a cytosolic isozyme
• The acetate produced by the oxidation of ethanol is activated
by acetyl-CoA synthetase (acetate thiokinase)
acetate + CoASH + ATP → acetyl-CoA + AMP + PPi
• The major liver isozyme of acetate thiokinase is cytosolic, and
the acetyl-CoA it generates is used for fatty acid and
cholesterol synthesis
• However, when these two pathways are inactive (due primarily
to a high ratio of glucagon to insulin), acetate will diffuse out of
the hepatocytes and be taken up and oxidized by heart and
skeletal muscle which have high concentrations of
mitochondrial acetyl-CoA synthetase
• Thus, if ethanol is consumed along with significant amounts of
carbohydrate, the acetate generated from ethanol will be used
mainly as a substrate for hepatic fatty acid synthesis
• If, however, ethanol is consumed in the absence of
carbohydrate, the acetate derived from the oxidation of
134
ethanol will be used mostly as fuel
•
•
•
•
•
Regulation of Ethanol Metabolism
Chronic consumption of ethanol can increase hepatic levels
of CYP2E1 many fold
When induction of MEOS increases the rate of metabolism of
ethanol, the increased production of acetaldehyde may
exceed the ability of the acetaldehyde dehydrogenases to
further oxidize acetaldehyde
Gender differences and genetic variants in the enzymes
responsible for metabolizing ethanol may account for some
of the individual variation in tolerance to ethanol
As noted earlier, women normally have lower levels of gastric
ADH than men
The lower level of gastric ADH activity in women, as well as
gender-based differences in body size and total body-water
space, are believed to account for the lower tolerance to
135
ethanol in women relative to men
• A number of genetic polymorphisms in ethanol-metabolizing
enzymes have been characterized
• The inducibility of CYP2E1 can vary as much as 10-fold
between persons
• Similarly, many persons of East Asian descent have an
inactive or less active form of ALDH2, the hepatic
mitochondrial isozyme of acetaldehyde dehydrogenase
• When people with mutation in ALDH2 consume ethanol,
they are more susceptible to flushing, headache, and nausea,
apparently because of acetaldehyde accumulation
• The drug Antabuse (disulfiram), which is used to discourage
alcoholics from drinking, acts by inhibiting acetaldehyde
dehydrogenase; people who consume ethanol while taking
Antabuse develop symptoms similar to those of people who
have a genetic lack of ALDH2 activity
136
Toxic Effects of Alcohol Metabolism
• Alcohol-induced liver disease, a common and sometimes fatal
consequence of chronic ethanol abuse, may manifest itself in
three forms: fatty liver, alcohol-induced hepatitis, and cirrhosis
• Each may occur alone, or they may be present in any
combination in a given patient
• Acetaldehyde and free radicals generated from ethanol
metabolism can result in alcohol-induced hepatitis, a condition
in which the liver is inflamed and cells become necrotic and die
• However, ethanol ingestion also has acute effects on liver
metabolism, including inhibition of fatty acid oxidation and
stimulation of triacylglycerol synthesis, leading to a fatty liver
• It also can result in ketoacidosis or lactate acidosis and cause
hypoglycemia
Changes in Fatty Acid Metabolism
• The high NADH/NAD+ ratio generated from ethanol oxidation
137
inhibits the oxidation of fatty acids in the liver
• The source of the fatty acids can be dietary fat, fatty acids
synthesized in the liver, or fatty acids released from adipose
tissue stores
• These fatty acids are re-esterified into TAG by combining with
glycerol 3-P
• The increased NADH/NAD+ ratio increases the availability of
glycerol 3-P by promoting its synthesis from intermediates of
glycolysis
• The TAG are incorporated into VLDL which accumulate in the
liver and enter the blood, resulting in an ethanol-induced
hyperlipidemia
• Fatty acids that are oxidized are converted to acetyl-CoA and
subsequently to ketone bodies
• Enough NADH is generated from oxidation of ethanol and fatty
acids that there is no need to oxidize acetyl-CoA in the TCA
cycle. And the very high NADH/NAD+ ratio shifts the
138
oxaloacetate in the TCA cycle to malate
• The acetyl-CoA enters the pathway for ketone body synthesis
instead of the TCA cycle
• Although ketone bodies are being produced at a high rate, their
metabolism in other tissues is restricted by the supply of
acetate, which is the preferred fuel
• Thus, the blood concentration of ketone bodies may be much
higher than found under normal fasting conditions
Acetaldehyde Toxicity
• Accumulation of acetaldehyde, produced both by alcohol
dehydrogenase and MEOS, is believed to be responsible for
most of the alcohol-induced liver damage known as cirrhosis
• By virtue of its aldehyde group, acetaldehyde is a highly
reactive molecule that can form adducts with many different
intracellular proteins
• In particular, reaction of acetaldehyde with tubulin impairs
secretion of serum proteins from hepatocytes, damaging these
139
cells further
• Increased oxidative stress, resulting from production of free
radicals by CYP2E1, also contributes to liver damage in chronic
alcoholics
• Acetaldehyde binds directly to glutathione and diminishes its
ability to protect against H2O2 and prevent lipid peroxidation. It
also binds to free radical defense enzymes
• Liver injury is irreversible at the stage that hepatic cirrhosis
develops
• During the development of cirrhosis, many of the normal
metabolic functions of the liver are lost, including biosynthetic
and detoxification pathways
• Synthesis of blood proteins, including blood coagulation
factors and serum albumin, is decreased. The capacity to
incorporate amino groups into urea is decreased, resulting in
the accumulation of toxic levels of ammonia in the blood.
Conjugation and excretion of bilirubin (from heme degra140
dation) is diminished, and bilirubin accumulates in the blood
141
The Synthesis of Membrane Lipids
Glycerophospholipids
• The pathway for synthesis of glycerophospholipids, like that for
TAG, starts with glycerol 3-phosphate
• Glycerol 3-phosphate reacts with two fatty acyl-CoA molecules
to form phosphatidic acid
• Two different mechanisms are then used to add a head group
to the molecule
• In the first mechanism, phosphatidic acid is cleaved by a
phosphatase to form diacylglycerol (DAG)
• DAG then reacts with an activated head group. Head groups
are activated by reacting them with CTP
• Phosphatidylcholine (PC) and phosphatidylethanolamine (PE)
are synthesized using this mechanism
• In the second mechanism, phosphatidic acid reacts with CTP to
form CDP-diacylglycerol
142
• CDP-DAG can react with phosphatidylglycerol (which itself is
formed from the condensation of CDP-DAG and glycerol 3phosphate) to produce cardiolipin or with inositol to produce
phosphatidylinositol (PI)
• Human cells do not synthesize phosphatidylserine (PS)
through any one of the above listed mechanisms
• Instead, PS is produced by a novel mechanism that involves
polar head group exchange between PE and free serine
• PS can be converted back to PE by a decarboxylation reaction.
• PC can be synthesized from dietary choline when it is available
• If dietary choline is not available, choline can be synthesized
from glucose
• But the rate of synthesis is inadequate to provide for the
necessary amounts of choline. Thus, choline has been classified
as an essential nutrient
• PE can also be methylated to form PC
143
• The donor of the methyl group is S-adenosylmethionine (SAM)
The Synthesis of
PC, PE and PS
144
The Synthesis of PG, Cardiolipin and PI
Ether Lipids
• The pathway for synthesis of ether lipids starts with acylation
of C-1 of the glycolytic intermediate DHAP
• The fatty acid is then replaced by a fatty alcohol
• The carbonyl group on C-2 of DHAP is then reduced by an
NADPH dependent enzyme and acylated
145
• At this point, an alkyl analog of phosphatidate has been made
• Head groups are added in a manner similar to that of PC
• Formation of a double bond between carbons 1 and 2 of the
alkyl group produces a plasmalogen
• Platelet-activating factor (PAF) is similar to choline
plasmalogen except that an acetyl group replaces the fatty
acyl group at carbon 2 of the glycerol moiety, and the alkyl
group on carbon 1 is saturated
• PAF is released from phagocytic blood cells in response to
various stimuli
• It causes platelet aggregation, edema, and hypotension, and
it is involved in the allergic response
• Plasmalogen synthesis occurs within peroxisomes, and, in
individuals with Zellweger’s syndrome , plasmalogen
synthesis is compromised
• If severe enough, this syndrome leads to death at an early
146
age
147
The Synthesis of Ethanolamine
Plasmalogen
Sphingolipid Synthesis
• The synthesis of sphingolipids begins with the formation of
ceramide
• The starting materials for the synthesis of palmitoyl-CoA and
serine
• Ceramide reacts with phosphatidylcholine to form
sphingomyelin
• Ceramide also reacts with UDP-sugars to form cerebrosides
(which contain a single monosaccharide, usually galactose or
glucose)
• Galactocerebroside may react with 3-phosphoadenosine 5phosphosulfate (PAPS, an active sulfate donor) to form
sulfatides, the major sulfolipids of the brain
• Additional sugars may be added to ceramide to form
globosides, and gangliosides are produced by the addition of
N-acetylneuraminic acid (NANA) as branches from the
148
oligosaccharide chains
The Synthesis of
Sphingolipids
from Ceramide
149
• The sphingolipidoses (lipid storage diseases) are a group of
inherited diseases that are caused by a genetic defect in the
catabolism of lipids containing sphingosine
• They are part of a larger group of lysosomal disorders and
exhibit several constant features:
(1) Complex lipids containing ceramide accumulate in cells,
particularly neurons, causing neurodegeneration and
shortening the life span
(2) The rate of synthesis of the stored lipid is normal
(3) The enzymatic defect is in the lysosomal degradation
pathway of sphingolipids
(4) The extent to which the activity of the affected enzyme is
decreased is similar in all tissues
• There is no effective treatment for many of the diseases,
although some success has been achieved with enzyme
replacement therapy and bone marrow transplantation in the
150
treatment of Gaucher’s and Fabry’s diseases
Examples of Sphingolipidoses
151
•
•
•
•
•
•
CHOLESTEROL METABOLISM
Cholesterol is a ubiquitous and essential component of
mammalian cell membranes
It is also present in small amounts in the outer membrane of
mitochondria
Cholesterol is especially abundant in myelinated structures of
the central nervous system, with 25% of the body’s cholesterol
located in the brain
In contrast to plasma, where most of the circulating
cholesterol exists esterified to a fatty acid, most cholesterol in
cellular membranes is present in the free (unesterified) form
The fluidity of membranes is determined by the degree of
unsaturation of the hydrocarbon chains of the phospholipids and
by the amount of cholesterol in the membrane
Hydrocarbon chains with cis-double bonds produce kinks and
allow a greater degree of freedom of movement for the
152
neighboring alkyl side chains
• Hence, these unsaturated chains give rise to more fluidity than do
saturated alkyl chains, which associate in ordered arrays
• Cholesterol is packed between fatty alkyl chains, the ring bearing
the polar hydroxyl group interacting with the polar groups of
phospho- and glycolipids
• The presence of cholesterol disrupts the orderly stacking of alkyl
side chains, restricts their mobility, and causes increased
membrane viscosity
• Cholesterol is abundant in bile. The solubilization of free
cholesterol in bile is achieved in part by the detergent property
of phosphatidylcholine (lecithin), which is produced in liver and
secreted into bile
• Bile acids, which are metabolites of cholesterol, also aid in
solubilizing cholesterol in bile
• Increased biliary secretion of cholesterol or decreased secretion
of phospholipids or bile acids into bile may lead to deposition of
153
cholesterol-rich gallstones
• Indeed, the name cholesterol was derived some 200 years ago
from the Greek words chole (bile) + stereos (solid)
• A little more than half the cholesterol of the body arises by
synthesis (about 700 mg/d), and the remainder is provided by
the average diet
• Virtually all tissues containing nucleated cells are capable of
cholesterol synthesis
• However , the liver and intestine account for approximately
10% each of total synthesis in humans
• In addition, the adrenal cortex, and reproductive tissues,
including ovaries, testes, and placenta have got a high capacity
of cholesterol synthesis
• Cholesterol synthesis occurs in the endoplasmic reticulum and
the cytosol
• Humans cannot metabolize cholesterol to CO2 and water.
Excretion of cholesterol and of bile acids synthesized from
154
cholesterol occurs by way of the liver, gallbladder, and intestine
Cholesterol Synthesis
• Acetyl-CoA is the source of all carbon atoms in cholesterol
• The biosynthesis of cholesterol may be divided into five steps:
Step 1: Synthesis of Mevalonate
• HMG-CoA is formed by the reactions used in mitochondria to
synthesize ketone bodies
• However, since cholesterol synthesis is extramitochondrial, the
two pathways are distinct
• Initially, two molecules of acetyl-CoA condense to form
acetoacetyl-CoA catalyzed by cytosolic thiolase
• Acetoacetyl-CoA condenses with an additional molecule of
acetyl-CoA catalyzed by HMG-CoA synthase to form HMG-CoA,
which is reduced to mevalonate by NADPH catalyzed by HMGCoA reductase
• This is the rate-limiting step in the pathway of cholesterol
synthesis
155
Step 1
Step 2: Formation of Isoprenoid Units
• Mevalonate is phosphorylated
sequentially by ATP by three
kinases, and after decarboxylation
the active isoprenoid unit,
isopentenyl diphosphate, is formed
Step 3: Formation of Squalene
• Isopentenyl diphosphate is
isomerized by a shift of the double
bond to form dimethylallyl
diphosphate, then condensed with
another molecule of isopentenyl
diphosphate to form the tencarbon intermediate geranyl
diphosphate
• A further condensation forms
156
farnesyl diphosphate
• Two molecules of farnesyl diphosphate condense at the
diphosphate end to form squalene
• The polyisoprenoids dolichol and ubiquinone are formed from
farnesyl diphosphate by the further addition of up to 16
(dolichol) or 3–7 (ubiquinone) isopentenyl diphosphate residues
• Some GTP-binding proteins in the cell membrane are prenylated
with farnesyl or geranylgeranyl (20 carbon) residues
• Protein prenylation is believed to facilitate the anchoring of
proteins into lipoid membranes and may also be involved in
protein-protein interactions and membrane-associated protein
trafficking
Step 4: Formation of Lanosterol
• Squalene can fold into a structure that closely resembles the
steroid nucleus
Step 5: Formation of Cholesterol
• The steroid nucleus and the side chain of lanosterol (30 C) are
157
modified by at least eight enzymes to give cholesterol (27 C)
158
Step 3
Step 2
Steps 4 and 5
• Two key enzymes can esterify
cholesterol
• One is an intracellular enzyme,
acyl-CoA: cholesterol
acyltransferase (ACAT), which
transfers a fatty acyl group
from its CoA carrier
• The other is an extracellular
enzyme called
lecithin:cholesterol
acyltransferase (LCAT), which
esterifies cholesterol with a
fatty acyl derived from
phosphatidylcholine
159
The Transport of Cholesterol
• The total fasting cholesterol concentration in plasma of
healthy people is usually 150 to 200 mg per 100 mL, which is
about twice the normal plasma glucose concentration
• Such a high concentration of cholesterol in plasma is possible
due to cholesterol-rich plasma lipoproteins that solubilize and
transport cholesterol
• Only about 30% of the total plasma cholesterol is free
(unesterified); the rest is esterified with a long-chain fatty acid,
usually linoleic acid, which increases the hydrophobicity of
cholesterol
• Chylomicrons are the main vehicle for transporting dietaryderived cholesterol and cholesteryl esters
• As the chylomicrons circulate in the plasma, they lose TAG and
shrink to become chylomicron remnants
• The chylomicron remnants then deliver cholesterol and other
lipophilic molecules (e.g., fat-soluble vitamins) to the liver 160
• Although the main function of VLDL is to export
endogenous TAG that are made in the liver, VLDL is also
involved in the transport of both free cholesterol and
cholesteryl esters between tissues
• VLDL are the main vehicle for exporting both dietaryderived and endogenously synthesized cholesterol from
hepatocytes into the plasma
• Like chylomicrons, VLDL changes its composition and size
as it circulates; it loses TAG through hydrolysis by LPL and
by acquiring additional cholesteryl esters from HDL
• As their TAG component undergoes hydrolysis, VLDL
particles become remnants of various sizes (sometimes
called IDL, or intermediate-density lipoproteins)
• The VLDL remnants contain apo E; approximately twothirds of the VLDL remnants are removed from the
161
circulation by the liver
• The remaining IDL are converted in the circulation to LDL
as a result of the actions of LPL and hepatic triacyglycerol
lipase (HTGL)
• LDL is the major lipoprotein that transports cholesterol in
blood
• Unlike chylomicrons and VLDL that are rich in TAG, the
core of LDL contains primarily cholesteryl esters
• The surface of each LDL particle contains one molecule of
apo B100
• Since its concentration in plasma is positively correlated
with cardiovascular disease (stroke, myocardial infarction,
blood clots), LDL-cholesterol is popularly termed the
“bad” cholesterol
• LDL functions primarily to deliver cholesterol and
cholesteryl esters to peripheral tissues such as the adrenal
162
glands, testes, and ovaries
• Since LDL contains some cholesteryl esters derived from
HDL, it also contributes to reverse cholesterol transport,
whereby cholesteryl esters are transported from peripheral
tissues to the liver for excretion as cholesterol or as bile salts
• Both hepatocytes and peripheral cells express LDL receptors
which recognize apo B 100 and internalize LDL via receptormediated endocytosis
• The receptors are then recycled back to the cell surface, and
the LDL is transported to lysosomes, where hydrolysis of
cholesteryl esters generates free cholesterol
• The main role of HDL is reverse cholesterol transport whereby
HDL extracts cholesterol from peripheral tissues and
transports that cholesterol to the liver for excretion
• Circulating HDL can also donate cholesteryl esters to other
lipoproteins such as VLDL and IDL
• HDL also plays a central role in lipoprotein metabolism by
163
donating proteins such as apo C2 and apo E to chylo. and VLDL
LDL Metabolism
164
• Since the concentration of HDL is inversely correlated with
cardiovascular disease, HDL-cholesterol is described as the
“good” cholesterol
• The primary apoprotein in HDL is apo A
• The major form of apo A is apo A 1 , which is synthesized by
both the liver and the intestine
• There are three major structural forms of apo A1 circulating in
the plasma: (1) amorphous or lipid-free HDL (apo A1), which
does contain some phospholipid; (2) nascent or discoidal, lipidpoor HDL; and ( 3 ) mature, spherical HDL (HDL2, HDL3),
which is rich in cholesteryl esters
• Lipid-free apo A1 is secreted by the liver and intestine and
acquires free cholesterol and phospholipids in the plasma,
thereby becoming nascent HDL
• As nascent HDL acquires additional free cholesterol, the
cholesterol is esterified by the action of LCAT to generate
165
cholesteryl esters –HDL2 is formed
• There are two mechanisms by which circulating HDL gives up
some of its cholesteryl esters
• One is the exchange of cholesteryl esters in HDL for TAG in
VLDL, IDL, or, to a lesser extent, in LDL
• The exchange process is mediated by cholesteryl ester
transfer protein (CETP)
• Cholesteryl esters may also be removed by selective uptake by
the liver, which is mediated by a scavenger receptor class B1
(SR-B1) and occurs without the intracellular uptake of HDL
proteins
• The resulting HDL3 particles are smaller and have a higher
ratio of TAG to cholesteryl esters than HDL2 has, and can be
cleared by the liver or the kidney
• In the process of HDL3 formation from HDL2 some of the
excess phospholipid is released from the surface of the
particles, thus regenerating lipid-poor HDL. The lipid-poor
HDL can then acquire and esterify additional cholesterol 166
• ABCA-1 and ABCG-1 are
members of the ATPbinding cassette
transporter (ABC) family
of transporters
• LCAT acquired from the
circulation and activated
by apo A1
HDL
Metabolism
167
Receptor-Mediated Endocytosis of Lipoproteins
• Receptor-mediated endocytosis of lipoproteins provides a
mechanism both for their clearance from the circulation and
for the delivery of key lipid components to target cells
• Targeting of lipoproteins to sites of metabolism and removal is
mediated primarily by the apoproteins on their surfaces
• The LDL receptor (LDLR) is a transmembrane glycoprotein
with an apo B-100-binding domain
• LDL receptors are expressed on liver cells and extrahepatic
tissues and they recognize apo B 100 but not the smaller apo
B48 molecule present on chylomicrons and chylomicron
remnants
• Once the LDL receptor is occupied by LDL, the LDL : LDLR
complex clusters in coated pits, which are then internalized by
receptor-mediated endocytosis
• Intracellularly, as the clathrin-coated vesicles lose their clathrin
168
the LDL receptors are recycled back to the plasma membrane
• The LDL-containing endosomes then fuse with lysosomes to
form endolysosomes
• Within the endolysosomes, the cholesteryl esters are
hydrolyzed by “acid lipase” to free cholesterol and fatty
acids, while the apo B-100 is hydrolyzed to amino acids
• LDL Receptor-Related Protein (LRP) is expressed on the
surface of hepatocytes but not peripheral cells. Its function is
to bind and clear chylomicron remnants
• Scavenger Receptors A (SR-A’s) are a family of molecules
that are expressed on tissue macrophages, Kupffer cells, and
various extrahepatic endothelial cells
• Scavenger Receptors B 1 (SR-B1’s) are different in that they
are not internalized by receptor-mediated endocytosis
• Instead, SR-B 1 permit hepatocytes (and the adrenal glands,
ovaries, and other steroidogenic tissues) to selectively
remove and internalize HDL-associated cholesteryl esters
169
170
Receptor-Mediated Endocytosis of LDL
The Regulation of Cholesterol Metabolism
Regulation of HMG-CoA Reductase
• The activity of HMG-CoA reductase is under strict metabolic
control
• The simultaneous regulation of HMG-CoA reductase synthesis
and degradation can alter steady-state levels of the enzyme
200-fold
• Sterol regulatory element binding protein (SREBP) plays a
central role in regulating the expression of HMG-CoA
reductase levels
• SREBPs, after synthesis, are integral ER proteins associated
with SCAP (SREBP cleavage-activating protein). The active
component of the protein is released by two proteases, S1P
(site 1 protease) and S2P. Once released, the active amino
terminal component travels to the nucleus to bind to SREs.
• Cholesterol and cholesterol derivatives block the proteolytic
171
activation and transport of SREBP
• Transcription of the LDL receptor is also regulated by the
intracellular cholesterol concentration through SREBP
• There are multiple isoforms of SREBP. One of these, SREBP-2,
selectively activates transcription of cholesterol biosynthetic
genes and the LDL receptor gene
• By contrast, SREBP-1 also activates transcription of acetyl-CoA
carboxylase, fatty acid synthase,…
• SREBP-1 thus controls not only cholesterol synthesis but also
the synthesis of fatty acids, TAG, and phospholipids
• In sterol-depleted cells, HMG-CoA reductase is slowly degraded
with a half-life greater than 12 hours
• In the presence of abundant sterols the degradation of HMGCoA reductase is accelerated
• HMG-CoA reductase is also inhibited when it is phosphorylated
by AMP-activated kinase (AMPK)
• Glucagon and sterols activate AMPK while insulin favors the
172
action of phosphatase
The Regulation
of HMG-CoA
Reductase
173
Abnormalities in Cholesterol Metabolism
Hypercholesterolemia
• Hypercholesterolemia refers to plasma levels of cholesterol
that exceed the normal range
• The risk of coronary heart disease (CHD) is correlated with LDLcholesterol (LDL-C) level, while a high fasting HDL-C level is a
negative risk factor for CHD
• Since the risk of developing CHD is related mainly to the LDLcomponent, treatment is aimed at decreasing the level of LDL
• Hypercholesterolemia could have genetic origins
• A deficiency of LDLR is the most common cause of familial
hypercholesterolemia (FH)
• FH heterozygotes usually exhibit an elevated fasting plasma
LDL-C concentration and a normal triglyceride level
• They have an increased risk of CHD, with onset in the fourth or
fifth decade. Patients with heterozygous FH are generally
174
responsive to treatment
• Persons who are homozygous for FH usually have very high
plasma LDL-C levels, even in early childhood
• These patients invariably have cutaneous deposits of
cholesterol called xanthomas on the hands, wrists, elbows,
and/or knees
• Coronary heart disease usually manifests within the first two
decades of life
• Due to the lack of functional receptors, patients with
homozygous FH are largely unresponsive to drug therapies
• Currently, the preferred treatment is the selective removal of
VLDL, IDL and LDL from the plasma
Atherogenic Dyslipidemia
• The dyslipidemic or atherogenic profile is a combination of
three abnormalities in plasma lipoprotein levels: high VLDL
triacylglycerol , low HDL-C, and the presence of relatively
small, dense LDL particles
175
• Independent of the concentration of either total plasma
cholesterol or LDL-C, the dyslipidemic profile is a major risk
factor for coronary artery disease
• The dyslipidemic profile is commonly associated with insulin
resistance, type II diabetes, central obesity, and hypertension,
which are included in a constellation of findings that have been
termed the metabolic syndrome
Tangier Disease
• The role of ABC 1-transporter in cholesterol efflux is
exemplified by an autosomal recessive disease known as
Tangier disease where mutations in the gene encoding the
ABC-1 transporter lead to accumulation of cholesterol esters in
the tissues with almost complete absence of HDL cholesterol
Alzheimer's Disease
• Abnormal cholesterol metabolism may be a factor in
Alzheimer’s disease (AD)
176
• Of the several genotypes for apo E, the acquisition of two E4
alleles may increase the risk for Alzheimer's disease up to
eightfold and shifts the onset to the lower ages
Treatment of Hypercholesterolemia
• Treatment of persons with moderate hypercholesterolemia
usually begins with dietary and other behavioral changes (e.g.,
exercise)
• Diets low in total fat, saturated fat, and cholesterol and
relatively high in oleic acid (as in olive oil) tend to lower both
the total cholesterol level and the LDL-C level
• If additional reductions in cholesterol are needed, various
medications are available
• Statin drugs decrease cholesterol synthesis by inhibiting HMGCoA activity. This, in turn, results in up-regulation of the LDLR,
particularly in hepatocytes
• On a mass basis, the major metabolic products of cholesterol
177
metabolism are the bile salts
• Most bile salts are normally reabsorbed in the distal ileum
• Soluble dietary fiber, such as that found in fruits and oat bran,
binds bile salts and decrease their absorption
• This, in turn, increases synthesis of bile salts in the liver,
consuming cholesterol in the process and decreasing the
concentration of cholesterol in hepatocytes
• Bile-acid sequestrants are insoluble polymers that bind bile
acids strongly inside the resin matrix; the bile acidsequestrant complex is excreted in the feces
• Depletion of the body's bile acid pool results in the increased
conversion of cholesterol to bile acids
• The depletion of hepatic cholesterol increases the expression
of LDLR and lowers the plasma LDL-cholesterol concentration
• However, since the decreased intrahepatic concentration of
cholesterol also stimulates synthesis of HMG-CoA reductase,
statins may be prescribed in conjunction with resins
178
• Plant sterols interfere with the absorption of dietary
cholesterol. Increased intake of vegetables and therapeutic
doses of plant sterols have been used to lower the plasma
cholesterol level
• Cholesterol absorption can also be reduced by Ezetimibe which
inhibits the transporter that moves cholesterol from the
intestinal lumen into enterocytes
• In addition, niacin and a class of drugs known as fibrates
increase HDL levels and lower plasma TAG levels
The Development of Atherosclerosis
• Atherosclerosis is a degenerative condition of the arteries in
which fat, yellowish plaque, known as atheroma, is present in
medium and large arteries
• The current view is that atheroma starts with injury to
endothelial cells in an artery and that his affects the immune
system in a similar manner to the development of chronic
179
inflammation. This is the injury– inflammation hypothesis
• Many factors are suspected of causing injury to the endothelial
cells, including pollutants, chemicals in tobacco smoke, various
lipids (e.g. cholesterol, long-chain fatty acids), proinflammatory
cytokines and bacteria
• Disturbance in the pattern of blood flow at bends and
branchpoints in the major arteries can also lead to endothelial
injury, particularly if the blood pressure is raised
• Such injuries result in the appearance of adhesion molecules on
the surface of the endothelial cells to which monocytes adhere
before entering the subendothelial space, where they develop
into macrophages
• Factors that are released from these cells attract more
monocytes and other immune cells, and the process of
inflammation begins
• A central event in the generation of plaque is the uptake of LDL
by macrophages in the subendothelial space. LDL enters this
180
space through the damaged endothelial cells
• The uptake occurs mainly by endocytosis mediated by the
non-specific SR-A1 receptors
• The uptake is not feedback regulated; LDL keep on entering
the macrophage
• Damage to LDL prevents its recognition by the normal
receptor and facilitates its uptake by the scavenger receptors
• Damage to LDL is caused by high blood glucose levels, which
cause glycation of apo B 100, and free radicals, which oxidize
unsaturated fatty acids in the phospholipids of the LDL
• Within the macrophage, the LDL is degraded and the resultant
free cholesterol is esterified to form cholesterol ester
• This accumulates and then damages and eventually kills the
macrophages to produce what are known as foam cells
• The dead and dying macrophages secrete cytokines and
chemotactic agents, which encourage the entry of more
monocytes and lymphocytes into the developing plaque 181
• They also secrete growth factors that stimulate proliferation of
smooth muscle cells to further increase the size of the plaque
• The process, therefore, has the characteristics of a vicious circle
Injury to Endothelial Cells
182
Formation of Plaque
Derivatives of Cholesterol
• Oxygenated derivatives of
cholesterol play many roles in the
body
• Cholesterol is the precursor of
two important classes of
molecules, bile acids and steroid
hormones.
• In addition, 7-dehydrocholesterol,
the immediate precursor of
cholesterol, can be converted to
cholecalciferol (vitamin D3), which
ultimately produces 1,25dihydroxycholecalciferol [ 1,25(OH)2D3, calcitriol], the active
hormone that regulates calcium
183
metabolism
• Many of the enzymes that catalyze oxygenation of cholesterol
and cholesterol derivatives are monooxygenases (mixedfunction oxidases)
• The monooxygenases that modify cholesterol are members of
the cytochrome P450 superfamily of enzymes and usually
utilize NADPH as a cofactor
• These monooxygenases are all membrane-bound and localized
to either the endoplasmic reticulum or the inner mitochondrial
membrane
• Cholesterol is a 27-carbon lipid containing a fused four-ring
structure and a hydrocarbon chain
• Except for the one hydroxyl group at C3, cholesterol is
completely non-polar
• By contrast, bile acids contain 24-carbon atoms and are more
polar than cholesterol. The steroid ring of bile acids contains
one or more additional hydroxyl groups and the shorter
184
hydrocarbon side chain terminates in a carboxyl group
• The so-called “bile salts” are actually bile acids that contain an
amino acid which is conjugated in amide linkage to the side
chain of the carboxyl group of the bile acid
• The two amino acids used most commonly by the liver to
conjugate human bile acids are glycine, and the sulfur amino
acid taurine
• Conjugated bile acids are more ionized at the pH of the
intestinal lumen than their non-conjugated counterparts
• They are therefore better emulsifying agents
• Bile salts play a major role in the digestion and absorption of
triacylglycerols and cholesteryl esters
• Bile salts emulsify dietary lipids in the gastrointestinal tract and
stabilize the resulting mixed micelles
• Along with lecithin, bile salts solubilize the cholesterol and bile
pigments present in bile, preventing formation of precipitates
(stones) of cholesterol or bilirubin in the gallbladder and bile
185
ducts
• In addition, formation of bile salts represents the major
metabolic mechanism for eliminating excess cholesterol from
the body
• The synthesis of the primary bile acids, cholic acid and
chenodeoxycholic acid, and their conjugation with taurine or
glycine to form bile salts occurs exclusively in the liver
• The conjugated bile acids are reabsorbed in intestine and
transported through the blood back to the liver
• Intestinal bacteria deconjugate and dehydroxylate some of
the bile salts; the products are called secondary bile acids
• The secondary bile acids are less soluble and, therefore, less
readily reabsorbed from the intestinal lumen than the bile
salts that have not been subjected to bacterial action
• Lithocholic acid, a secondary bile salt that has a hydroxyl
group only at position 3, is the least soluble bile salt. Its major
186
fate is excretion
The Synthesis of Bile Acids
• Greater than 95% of
the bile salts are
resorbed in the ileum
and return to the liver
via the enterohepatic
circulation
• In addition to
secondary bile acids,
the feces contains a
mixture of
cholesterol and
cholesterol
metabolites, such as
cholestanol and
coprostanol,
generated by
intestinal bacteria
187
The Conjugation and Deconjugation of Bile Acids
188
• Five types of hormones are produced by the oxidation and
the removal of the side chain of cholesterol: glucocorticoids,
mineralcorticoids, androgens, estrogens and progestins
• The synthesis of steroid hormones begins with the
formation of pregnenolone through the action of the
mitochondrial enzyme desmolase
• Desmolase removes 6 carbons from cholesterol (27 C)
• Pregnenolone moves from the mitochondria to the SER
where it is changed to all the different types of steroid
hormones
• The cholesterol used for steroid hormone synthesis is
either synthesized in the tissues from acetyl CoA, extracted
from intracellular cholesterol ester pools, or taken up by
the cell in the form of cholesterol-containing lipoproteins
(either internalized by the LDL-receptor, or absorbed by
189
the SR-B1 receptor)
190
• Cholecalciferol (vitamin D3) can either be obtained from the
diet or formed in the skin by the non-enzymatic action of
ultraviolet (UV) light on 7-dehydrocholesterol
• Ergocalciferol (vitamin D2) is formed by the action of UV light
on a structurally similar plant sterol (ergosterol)
• Vitamin D3 is preferred for vitamin D supplementation
• Conversion of cholecalciferol to its active hormonal form is a
multiorgan process
• First, cholecalciferol is hydroxylated in the liver by 25hydroxylase to form 25-hydroxycholecalciferol
• A second hydroxylation step, catalyzed by 1α-hydroxylase in
the kidney, generates the active hormone 1,25dihydroxycholecalciferol
• Kidney, bone, cartilage, and intestine contain a 24-hydroxylase
that converts 25-dihydroxycholecalciferol to the inactive 24,25dihydroxycholecalciferol,thus preventing formation of excess
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active calcitriol
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