Transcript Utilization and Transport of Fat and Cholesterol

Chapter 17
Lipid Metabolism I: Fatty Acids,
Triacylglycerols, and Lipoproteins
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Biochemistry, 4th Edition
Chapter 17 Outline:
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Utilization and Transport of Fat and Cholesterol
Fatty Acid Oxidation
Fatty Acid Biosynthesis
Biosynthesis of Triacylglycerols
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Utilization and Transport of Fat and Cholesterol
Overview of
intermediary
metabolism with fatty
acid and
triacylglycerol
pathways
highlighted:
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Utilization and Transport of Fat and Cholesterol
Fat storage in a plant seedling:
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The electron micrograph shows a cell
from a cucumber cotyledon (seed leaf)
a few days after germination.
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Fat stored in lipid bodies is degraded,
oxidized, and converted to
carbohydrate in neighboring
glyoxysomes (or microbodies) to
support the growth of the plant.
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Utilization and Transport of Fat and Cholesterol
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Fat has six times more caloric content
by weight than carbohydrate because
fat is more highly reduced and is
anhydrous.
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Utilization and Transport of Fat and Cholesterol
Overview of fat
digestion,
absorption,
storage, and
mobilization in
the human:
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Utilization and Transport of Fat and Cholesterol
Action of bile salts in emulsifying fats in the intestine:
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Cholic acid, a typical bile acid, ionizes to give its cognate bile salt.
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The hydrophobic surface of the bile salt molecule associates with
triacylglycerol, and several such complexes aggregate to form a micelle.
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The polar surface of the bile salts faces outward, allowing the micelle to
associate with pancreatic lipase/colipase.
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Hydrolytic action of this enzyme frees the fatty acids to associate in a much
smaller micelle that can be absorbed through the intestinal mucosa.
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Utilization and Transport of Fat and Cholesterol
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Lipoproteins are lipid–protein
complexes that allow movement of
apolar lipids through aqueous
environments.
Generalized structure of a plasma
lipoprotein:
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The spherical particle, part of which is
shown, contains neutral lipids in the
interior and phospholipids, cholesterol,
and protein at the surface.
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Utilization and Transport of Fat and Cholesterol
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Utilization and Transport of Fat and Cholesterol
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Utilization and Transport of Fat and Cholesterol
Binding of a chylomicron to lipoprotein lipase on the inner
surface of a capillary:
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The chylomicron is anchored by lipoprotein lipase, which is
linked by a polysaccharide chain to the lumenal surface of the
endothelial cell.
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When activated by apoprotein C-II, the lipase hydrolyzes the
triacylglycerols in the chylomicron, allowing uptake into the cell
of the glycerol and the free fatty acids.
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Utilization and Transport of Fat and Cholesterol
Overview of lipoprotein transport pathways and fates:
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Utilization and Transport of Fat and Cholesterol
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A major consequence of liver dysfunction is an
inability to synthesize apolipoproteins and, hence, to
transport fat out of the liver.
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Cholesterol accumulation in the blood is correlated
with development of atherosclerotic plaque.
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Utilization and Transport of Fat and Cholesterol
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Cholesterol esters are synthesized in
plasma from cholesterol and an acyl chain on
phosphatidylcholine (lecithin), through the
action of lecithin:cholesterol
acyltransferase (LCAT), an enzyme that is
secreted from liver into the bloodstream,
bound to HDL and LDL.
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Cholesterol esters are considerably more
hydrophobic than cholesterol itself.
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Utilization and Transport of Fat and Cholesterol
Feedback regulation of HMG-CoA reductase
activity:
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Fibroblasts obtained from a normal subject or
from a patient homozygous for familial
hypercholesterolemia (FH Homozygote) were
grown in monolayer cultures.
a) At time zero, the medium was replaced with fresh
medium depleted of lipoproteins, and HMG-CoA
reductase activity was measured in extracts
prepared at the indicated times.
b) Twenty-four hours after addition of the
lipoprotein-deficient medium, human LDL was
added to the cells at the indicated levels, and
HMG-CoA reductase activity was measured at
the indicated time.
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Utilization and Transport of Fat and Cholesterol
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Uptake of cholesterol from the blood occurs at the LDL receptor via
receptor-mediated endocytosis.
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Intracellular cholesterol regulates its own level by controlling:
1. de novo cholesterol biosynthesis,
2. formation and storage of cholesterol esters
3. LDL receptor density
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Uptake of oxidized LDL by a scavenger receptor is a key event in
atherogenesis.
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Fat mobilization in adipose cells is hormonally controlled, via the
cAMP–dependent phosphorylation of lipolytic enzymes and lipid
droplet-associated proteins.
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Utilization and Transport of Fat and Cholesterol
Receptor-mediated endocytosis of LDL:
•LDL was conjugated with ferritin to permit electron
microscopic visualization.
a)The LDL–ferritin (dark dots) binds to a coated pit on the
surface of a cultured human fibroblast.
b)The plasma membrane closes over the coated pit, forming
an endocytotic vesicle.
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Utilization and Transport of Fat and Cholesterol
Structure of a clathrin-coated pit:
a) Clathrin, the major protein in coated pits, forms
triskelions (named after the symbol of three legs
radiating from the center), which assemble into
polyhedral lattices composed of hexagons and
pentagons, such as the barrel shown in the next panel.
b) Image reconstruction from electron cryomicroscopy of a
clathrin barrel formed from 36 triskelions. A single
clathrin triskelion is highlighted in light blue.
c) A coated pit on the inner surface of the plasma
membrane of a cultured mammalian cell is visualized by
freeze-fracture electron microscopy.
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The cage-like structure of the pit is due to the
clathrin lattice.
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Utilization and Transport of Fat and Cholesterol
Involvement of LDL receptors in cholesterol uptake and
metabolism:
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Utilization and Transport of Fat and Cholesterol
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Polyunsaturated fat (PUFA) ingestion is correlated
with low plasma cholesterol levels.
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The mechanisms involved are not completely
understood.
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Utilization and Transport of Fat and Cholesterol
Mobilization of adipose cell triacylglycerols by lipolysis:
•Three lipases act sequentially to hydrolyze TG to glycerol and FFA.
•These enzymes act at the oil–water interface of the lipid droplet.
•FFA are exported to the blood plasma, where they are bound to
albumin for transport to liver and other tissues for subsequent
oxidation.
•Glycerol is released to the blood to be taken up by liver cells, where it
serves as a gluconeogenic substrate.
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Utilization and Transport of Fat and Cholesterol
Control of lipolysis in adipose cells by a cAMPmediated cascade system:
•Hormonal activation of a b–adrenergic G-protein coupled
receptor on the plasma membrane leads to elevation of
cAMP levels, which in turn, activates protein kinase A
(PKA). PKA phosphorylates perilipin (PL) and HSL.
1.CGI-58 dissociates from phosphorylated-PL, and binds
ATGL .
2.Phosphorylated HSL is recruited to the lipid droplet and
activated by phosphorylated-PL.
3.Phosphorylated-PL also recruits the ATGL/CGI-58
complex to the lipid droplet, activating this lipase.
4.Activated ATGL hydrolyzes TG to activated HSL
hydrolyzes DG to cytoplasmic MGL hydrolyzes MG to free
glycerol + FFA.
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Biochemistry, 4th Edition
Fatty Acid Oxidation
Knoop’s experiments to determine the boxidation of fatty acids:
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When dogs fed fatty acids that had an evennumbered carbon chain, the final breakdown
product, recovered from urine, was phenylacetic
acid.
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When the fed fatty acid had an odd-numbered
chain, the product was benzoic acid.
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These results led Knoop to propose that fatty acids
are oxidized in a stepwise fashion, with initial
attack on carbon 3 (the b-carbon with respect to
the carboxyl group).
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This attack would release the terminal two
carbons, and the remainder of the fatty acid
molecule could undergo another oxidation.
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Fatty Acid Oxidation
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Fatty acids are activated for oxidation by ATP-dependent
acylation of coenzyme A.
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The loss of pyrophosphate is equivalent to 2 ATP’s used
for activation.
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The further hydrolysis of pyrophosphate makes the
activation step irreversible.
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Fatty Acid Oxidation
Overview of the fatty acid oxidation pathway:
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Fatty Acid Oxidation
Mechanism of acyl-CoA synthetase reactions:
•The figure shows reversible formation of the activated
fatty acyl adenylate, nucleophilic attack by the thiol
sulfur of CoA-SH on the activated carboxyl group, and
the quasi-irreversible pyrophosphatase reaction, which
draws the overall reaction toward fatty acyl-CoA.
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Fatty Acid Oxidation
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Carnitine transports acyl-CoAs into mitochondria
for oxidation.
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Fatty Acid Oxidation
The carnitine acyltransferase system, for transport of fatty
acyl-CoAs into mitochondria:
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Fatty Acid Oxidation
Outline of the b-oxidation of fatty acids:
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In the diagram a 16-carbon saturated
fatty acyl-CoA (palmitoyl-CoA)
undergoes seven cycles of oxidation to
yield eight molecules of acetyl-CoA.
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Fatty Acid Oxidation
Reaction 1: The Initial Dehydrogenation
•The first reaction is catalyzed by an acyl-CoA dehydrogenase, which catalyzes the
removal of two hydrogen atoms from the a- and b-carbons to give a trans a,bunsaturated acyl CoA (trans-D2-enoyl-CoA) as the product.
•The pro-R hydrogen on the b-carbon is then transferred as a hydride equivalent to FAD
to give the trans double bond and enzyme-bound FADH2.
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Fatty Acid Oxidation
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Fatty Acid Oxidation
Reaction 4: Thiolytic Cleavage
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The fourth and last reaction in each cycle of the b-oxidation
pathway involves attack of the nucleophilic thiol sulfur of
coenzyme A on the electron-poor keto carbon of 3-ketoacylCoA, with cleavage of the bond and release of acetyl-CoA.
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The other product is a shortened fatty acyl-CoA, ready to
begin a new cycle of oxidation:
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Fatty Acid Oxidation
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Fatty Acid Oxidation
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Fatty acids are oxidized by repeated
cycles of dehydrogenation, hydration,
dehydrogenation, and thiolytic
cleavage, with each cycle yielding
acetyl-CoA and a fatty acyl-CoA shorter
by two carbons than the input acyl-CoA.
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Fatty Acid Oxidation
Energy Yield from Fatty Acid Oxidation:
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Fatty Acid Oxidation
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Two enzymes, enoyl-CoA isomerase
and 2,4-dienoyl-CoA reductase, play
essential roles in the oxidation of
unsaturated fatty acids.
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Fatty Acid Oxidation
b-Oxidation pathway for polyunsaturated
fatty acids:
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This example, using linoleyl-CoA, shows
sites of action of enoyl-CoA isomerase
and 2,4-dienoyl-CoA reductase, enzymes
specific to unsaturated fatty acid
oxidation (identified with red type).
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Fatty Acid Oxidation
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Odd-numbered fatty acid chains yield upon b-oxidation 1
mole of propionyl-CoA, whose conversion to succinylCoA involves a biotin-dependent carboxylation and a
coenzyme B12–dependent rearrangement.
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Biochemistry, 4th Edition
Fatty Acid Oxidation
Pathway for catabolism of
propionyl-CoA:
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Biochemistry, 4th Edition
Fatty Acid Oxidation
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Biochemistry, 4th Edition
Fatty Acid Oxidation
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During fasting or starvation, when carbohydrate intake is too low, oxaloacetate levels fall
so that flux through citrate synthase is impaired, causing acetyl-CoA levels to rise.
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Under these conditions, 2 moles of acetyl-CoA undergo a reversal of the thiolase reaction
to give acetoacetyl-CoA.
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Acetoacetyl-CoA can react in turn with a 3rd mole of acetyl-CoA to give 3-hydroxy-3methylglutaryl-CoA (HMG-CoA), catalyzed by HMG-CoA synthase.
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In the cytosol, HMG-CoA is an early intermediate in cholesterol biosynthesis. In the
mitochondria, HMG-CoA is acted on by HMG-CoA lyase to yield acetoacetate plus
acetyl-CoA.
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Acetoacetate undergoes NADH-dependent reduction to give D-b-hydroxybutyrate or
spontaneous decarboxylation to acetone.
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Collectively, acetoacetate, acetone, and b-hydroxybutyrate are called ketone bodies.
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Fatty Acid Oxidation
Biosynthesis of ketone
bodies in the liver:
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Fatty Acid Biosynthesis
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When carbohydrate catabolism is limited, acetyl-CoA is
converted to ketone bodies, mainly acetoacetate and bhydroxybutyrate—important metabolic fuels in some
circumstances.
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Animals readily convert carbohydrate to fat, but cannot carry
out net conversion of fat to carbohydrate.
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Fatty acid synthesis occurs through intermediates similar to
those of fatty acid oxidation, but with differences in electron
carriers, carboxyl group activation, stereochemistry, and
cellular location.
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Fatty Acid Biosynthesis
Acetyl-CoA as a key intermediate between fat and carbohydrate
metabolism:
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Citrate serves as a carrier to transport acetyl units from the
mitochondrion to the cytosol for fatty acid synthesis.
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Fatty Acid Biosynthesis
Biosynthesis of Palmitate from Acetyl-CoA:
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Synthesis of malonyl-CoA is the first committed step in fatty acid
biosynthesis from acetyl-CoA and bicarbonate, catalyzed by
acetyl-CoA carboxylase (ACC).
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Fatty Acid Biosynthesis
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Fatty Acid Biosynthesis
Chemical similarities between oxidation and synthesis
of a fatty acid:
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Fatty Acid Biosynthesis
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Fatty Acid Biosynthesis
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Malonyl/acetyl-CoA-ACP transacylase (MAT) loads fatty acid
synthase with substrates.
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The acyl groups (acetyl or malonyl) are transferred from the SH
group of CoA to the SH group of the phosphopantetheine
moiety of acyl carrier protein (ACP).
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The reactions shown here produce acetyl-KS and malonyl-ACP,
which are used in the remaining reactions of the cycle.
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Fatty Acid Biosynthesis
Synthesis of palmitate, starting with malonylACP and acetyl-KS:
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The first cycle of four reactions generates
butyryl-ACP.
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Following translocation from ACP, butyryl-KS
reacts with a second molecule of malonyl-ACP,
leading to a second cycle of two-carbon
addition.
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A total of seven such cycles generates
palmitoyl-ACP.
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Hydrolysis of this product releases palmitate.
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Fatty Acid Biosynthesis
Structure and swinging arm mechanism in the mammalian
fatty acid synthase complex:
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Fatty Acid Biosynthesis
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Malonyl-CoA represents an activated source of two-carbon
fragments for fatty acid biosynthesis, with the loss of CO2
driving C-C bond formation.
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In eukaryotes, fatty acid synthesis is carried out by a
megasynthase, an organized multienzyme complex that
contains multifunctional proteins.
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Fatty Acid Biosynthesis
Transport of acetyl units and reducing equivalents
used in fatty acid synthesis:
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Citrate serves as a carrier of two-carbon fragments from
mitochondria to cytosol for fatty acid biosynthesis.
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Fatty Acid Biosynthesis
Fatty acid desaturation system:
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The black arrows indicate the path of electron flow as the two
substrates are oxidized.
•
D5 and D6 desaturases use the same mechanism.
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Fatty Acid Biosynthesis
Pathway for synthesis of polyunsaturated
fatty acids (PUFAs) in plants and animals:
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Fatty Acid Biosynthesis
Regulation of fatty acid synthesis in animal
cells:
•The rate-limiting enzyme, acetyl-CoA carboxylase
(ACC), is controlled by both allosteric (citrate and
long-chain fatty acids) and covalent modification
mechanisms.
•Phosphorylation by AMP-activated protein kinase
(AMPK) or cyclic AMP–dependent protein kinase
(PKA) inactivates ACC.
•Insulin stimulates fatty acid synthesis by increasing
glucose uptake and increasing flux through pyruvate
dehydrogenase to produce acetyl-CoA.
•The dephosphorylated form of PDH is the
enzymatically active form.
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Fatty Acid Biosynthesis
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Fatty Acid Biosynthesis
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•
A related series of pathways in bacteria and fungi is
involved in the biosynthesis of a class of antibiotics
called polyketides.
•
Examples include Erythromycin, from
Saccharopolyspora erythraea, and oxytetracycline,
from Streptomyces rimosus.
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These polyketide antibiotics are potent inhibitors of
bacterial protein synthesis.
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Other polyketides, such as lovastatin and
simvastatin have found clinical use as cholesterollowering drugs by inhibiting HMG-CoA reductase.
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Polyketides are synthesized in assembly-line fashion
by giant enzyme megasynthases that consist of
individual modules for rounds of carbon addition, with
each module closely resembling the process whereby
two carbons are added in a cycle of the fatty acid
synthesis pathway.
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Biosynthesis of Triacylglycerols
Glycerolipid/free fatty acid cycle and
glyceroneogenesis:
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Mammals hydrolyze and resynthesize TG in a
glycerolipid/free fatty acid (GL/FFA) cycle.
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FFA are released from TG by lipases acting on lipid
droplets. ATGL, adipose triglyceride lipase; HSL,
hormone-sensitive lipase; MGL, monoacylglycerol
lipase.
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Some of the FFAs are released into the blood for
transport and oxidation, but ~75% are re-esterified
back to TG.
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ATP hydrolysis in this futile cycle is shown in red.
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Biosynthesis of Triacylglycerols
Glycerolipid/free fatty acid cycle and
glyceroneogenesis:
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The glycerol backbone is produced via
glyceroneogenesis, involving reactions catalyzed
by pyruvate carboxylase and phosphoenolpyruvate
carboxykinase (PEPCK), and reversal of glycolytic
steps to give DHAP.
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DHAP is reduced to glycerol-3-phosphate.
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AMP-activated protein kinase (AMPK)-dependent
phosphorylation of GPAT and HSL inhibits these
steps of the cycle.
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The lysophosphatidic acid (LPA), phosphatidic acid
(PA), and sn-1,2-diacylglycerol (DAG)
intermediates in the TG resynthesis pathway are
also important lipid signaling molecules.
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