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Biochemistry
Sixth Edition
Berg • Tymoczko • Stryer
Chapter 22:
Fatty Acid Metabolism
Copyright © 2007 by W. H. Freeman and Company
An
Adipocyte
A fat cell, for
storage of
triacylglycerol.
A Triacylglycerol (TAG)
TAG is primarily storage fat to be used for energy.
Other important functions are thermal insulation
and protection from mechanical shock.
Outline of
Fatty Acid
Metabolism
Synthesis which
occurs in the
cytosol adds 2 C
at a time to the
carboxylate end.
Degradation is
mitochondial and
removes 2 C at a
time from the
carboxylate end.
R = CoA
(catabolic)
and ACP
(anabolic)
Metabolic
reaction
steps
Degradation is
oxidative.
Synthesis is
reductive.
Catabolic
Anabolic
Triacylglycerol (TAG) Hydrolysis
Lipases catalyze enzymatic hydrolysis
of TAG to yield fatty acids and glycerol.
Glycocholic Acid
Bile acids are emulsifiers that aid in
intestinal digestion of triacylglycerols.
(cholic acid + glycine)
Transport intestine to lymph
TAG is hydrolyzed for membrane transport
and reformed for chylomicron transport.
TAG
Hydrolysis
for energy
Lipases in
adipose
tissue are
hormone
stimulated.
Hormonal effects
TAGs are stored in adipocytes, and fatty acids
are released to supply energy demands.
Hormone-sensitive lipases convert TAGs to
free fatty acids and glycerol.
At low carbohydrate and low insulin
concentrations, TAG hydrolysis is stimulated
by increased epinephrine and glucagon
which activate Protein kinase A through Gprotein transduction.
Mobilization of TAG
Stimulated by epinephrine and glucagon.
Chylomicrons
• TAGs, cholesterol and cholesterol esters are
insoluble in water and cannot be transported in
blood or lymph as free molecules so are
transported as lipoproteins.
• Chylomicrons are the largest and most
nonpolar lipoproteins.
• They deliver TAGs from the intestine (via lymph
and blood) to tissues (muscle for energy,
adipose for storage) and are present in blood
only after feeding.
• Cholesterol-rich chylomicron remnants deliver
cholesterol to the liver
Glycerol and Fatty Acid
Metabolism of Glycerol
Fatty Acid Oxidation
• Knoop proposed b-oxidation in 1904 based on
studies with phenyl substituted fatty acids of
odd and even chain lengths.
Even chains -- > phenylacetic acid
Odd chains -- > benzoic acid
• Later discoveries:
(1) reactions occur in the mitochondrial matrix
(2) reactions are enzyme catalyzed
(3) Coenzyme A is required
(4) ATP is required
Fatty Acid Oxidation
• FA Catabolism can be viewed in three stages:
(1) Activation of fatty acids in the cytosol.
A coenzyme A derivative is formed.
(2) Transport into the mitochondria.
Carnitine is used for FA transport.
(3) b-oxidation degrades fatty acids to twocarbon fragments (as acetyl CoA) in the matrix
where it goes to the Krebs cycle.
Fatty Acid (FA) Activation
Occurs in the cytosol. Hydrolysis of
pyrophosphate (PPi  2 Pi) enhances
formation of the acylSCoA product.
This is a two step process catalyzed by
acylCoA synthetase.
AcylCoA synthetase
Activation Steps
There are several chain length specific
acylCoA synthetases (short, medium, long).
Acyl
Adenylate
Precursor to
Acyl
Coenzyme A.
Formation of Acyl Carnitine
Catalyzed by Carnitine acyltransferase.
Carnitine acyltransferase I is in the intermembrane
space and Carnitine acyltransferase II is in the
mitochondrial matrix.
Transport of
activated FA
1. Acylcarnitine
formation
2. Translocation
3. AcylSCoA
formation
Maintains separate
CoA pools.
b-Oxidation of Fatty Acids
The pathway for degradation of fatty acids
is in the mitochondrial matrix and is
called the b-oxidation spiral.
Oxidation occurs at the b-carbon of the
fatty acid.
b-Oxidation
Sequence
1.
2.
3.
4.
Oxidation (FAD)
Hydration (HOH)
Oxidation (NAD+)
Cleavage (CoASH)
FAD
Oxidation
Double bond is trans.
Compare to succinate
dehydrogenase in the
Kreb’s cycle.
AcylCoA DH
Electron Flow/Proton Pumps
Electrons from FAD Oxidation
Hydration
1.
2.
3.
4.
Oxidation (FAD)
Hydration (HOH)
Oxidation (NAD+)
Cleavage (CoASH)
EnoylCoA hydratase
NAD+
Oxidation
1.
2.
3.
4.
Oxidation (FAD)
Hydration (HOH)
Oxidation (NAD+)
Cleavage (CoASH)
b-hydroxyacylCoA
dehydrogenase
Cleavage
1.
2.
3.
4.
Oxidation (FAD)
Hydration (HOH)
Oxidation (NAD+)
Cleavage (CoASH)
b-ketothiolase
b-Oxidation Summary
The pathway for degradation of fatty acids
is called the b-oxidation spiral.
Successive
2 carbon
cleavages
Each comes off
as acetylSCoA and
the four b-oxidation
steps are required.
Unsaturated
FA
metabolism
Normal b-oxidation
occurs until the cis
double bond is in the
b,g position.
Isomerization with
migration moves the
bond to a trans a,b.
An FADH2 is lost due
to the already present
double bond.
Multiple Unsaturations
For multiple double bonds, D9,12, b-oxidation
proceeds as before until the second db is in
the g,d position.
b-oxidation inserts an a,b unsaturation giving two
conjugated double bonds.
The NADPH enzyme, 2,4-dienoyl reductase, acts
with migration to yield a trans b,g unsaturation.
An isomerase catalyzes migration to a,b and
retains the trans structure & b-ox. continues.
For the second db an FADH2 is not lost but an
NADPH is used negating the NADH produced.
Multiple Unsat.
3 cycles of b-ox
Odd Chain Fatty Acids
b-oxidation of a FA with an odd number of
carbon atoms produces propionic acid.
Odd Chain Fatty Acids
Odd-chain fatty acids occur in bacteria and
microorganisms and is a minor metabolic route in
mammals (occurs in liver).
Three enzymes are needed to convert propionyl
CoA to succinyl CoA (so the last three carbons do
not yield acetylCoA but do give a citric acid cycle
intermediate).
Propionyl CoA carboxylase (requires biotin)
Malonyl CoA racemase
Malonyl CoA mutase (requires B-12)
Odd Chain FA Pathway
Malonyl CoA
racemase
Propionyl CoA
carboxylase
Malonyl CoA
mutase
B-12, a coenzyme for mutase
Methylmalonyl
SCoA mutase
Co+3 -- > Co+2
Deoxyadenosyl Radical
Homolytic bond cleavage between
the deoxyadenosyl 5'CH2 and
Cobalt.
Radical Mechanism
Methyl malonylSCoA mutase catalyzes a
radical migration of the thioester carbonyl.
Peroxisome oxidation
b-oxidation occurs in liver peroxisomes as
well as the mitochondrial matrix. All boxidation in plants occurs in the
peroxisomes(glyoxysomes) and none in the
matrix.
Peroxisomes use oxygen and produce
peroxide. Peroxisomes do well with very
long chain fatty acids and can do branched
chains. The peroxisomal thiolase does not
do well with small chains which are moved
to the matrix for further oxidation.
Peroxisome oxidation
Ketone Body Pathway
Ketone bodies are synthesized in the liver. The
pathway is active under conditions that
cause elevated levels of acetylCoA.
Ketone bodies are:
acetoacetate
b-hydroxybutyrate and
acetone
The pathway requires the enzymes:
1. b-ketothiolase
2. hydroxymethylglutarylCoA synthase
3. hydroxymethylglutarylCoA lyase
4. b-hydroxybutyrate dehydrogenase
Ketone Body Pathway
1. b-ketothiolase
In presence of
excess acetyl
CoA, the last
reaction of
b-oxidation
reverses to form
acetoacetylCoA
(not a ketone body).
Ketone Body Pathway
2. Hydroxymethyl
glutarylCoA
synthase
AcetoacetylCoA
combines with
another acetylCoA
with loss of
Coenzyme A.
Ketone Body Pathway
3. Hydroxymethyl
glutarylCoA lyase
To get the ketone
body, acetoacetate,
acetylCoA is cleaved
from HMGCoA.
Ketone Body Pathway
4. b-hydroxybutyrate
dehydrogenase
Acetoacetate is
reduced with NADH
to b-hydroxybutyrate
Decarboxylation of
the b-ketoacid to
acetone is slow but
Is spontaneous and
irreversible.
Ketone Body Pathway
1. b-ketothiolase
2. hydroxymethylglutarylCoA synthase
3. hydroxymethylglutarylCoA lyase
4.
b-hydroxybutyrate dehydrogenase
Ketone
Body
Use
Activating Acetoacetate
Acetoacetate is
converted to
acetylCoA for
energy.
First it must be
converted back
to the CoA form.
The acylCoA
synthetase used
to activate fatty
acids is not used
for acetoacetate.
Hydroxybutyrate
Hydroxybutyrate is converted back to acetoacetate
to get acetylCoA and acetone is lost.
b-hydroxybutyrate DH
Fatty Acid Synthesis (FAS)
Synthesis occurs in the cytosol of a liver cell or an
adipocyte in mammals. It starts with acetyl CoA and
requires NADPH. The NADPH required comes from
the HMS or malic enzyme in the citrate shuttle. Note
the cytosolic NADH/NADPH exchange.
The initial product is palmitoyl CoA (no intermediate
size fatty acids are observed).
As one might expect, b-oxidation and fatty acid
synthesis (FAS) occur by different pathways.
When glucose is plentiful, acetylCoA can be used for
FAS.
Oxidation vs Synthesis
Oxidation
Synthesis
Localization
mitochondria/
peroxisomes
cytosol
Transport
Carnitine shuttle
Citrate Shuttle
Acyl carrier
CoenzymeA
AcylCarrierProtein
Carbon units
C2
C2
Acceptor/donor
AcetylSCoA, C2
MalonylSCoA, C3
Redox Cofactors FAD, NAD+
NADPH
Enzymes
Multifunctional
dimer
Separate
enzymes
Acetyl CoA into the Cytosol
b-Oxidation produces acetylCoA in the mitochondria.
Transport to the cytosol is via the Citrate Shuttle.
NADPH and Acetyl CoA
Synthesis is in the cytosol starting with acetyl CoA.
The Citrate Shuttle
Enzymes:
Pyruvate Carboxylase (ATP, Biotin, CO2)
Citrate Synthase
Citrate Lyase (ATP)
Malate Dehydrogenase (NADH)
Malic enzyme (NADP+)
Two ATPs used per transport cycle.
Transport:
Pyruvate into mito. uses a Pyr/H+ symport
Citrate out of mito. one of several antiports
Acetyl CoA Carboxylase
Fatty acid synthesis starts in the cytosol with the
addition of CO2 to acetyl CoA.
A bifunctional enzyme in animals.
A multienzyme complex in bacteria:
Biotin carrier protein
Biotin carboxylase
Carboxyl transferase
biotin
Acetyl CoA Carboxylase (ACC)
This the regulatory enzyme for fatty acid anabolism
in animals. Under influence of epinephrine or
glucagon it is phosphorylated and dissociates into
monomeric units.
ACC monomer-P < == > ACC polymer
less active
more active
Allosteric control (enzyme has no covalent P):
polymerization activated by citrate (+)
polymerization inhibited by palmitoylSCoA (-)
Also, increased levels of malonylCoA inhibits CAT I.
Reactions of Fatty Acid Synthesis
Synthesis uses a small protein, 77 amino acid
residues, called acyl carrier protein (ACP) to hold
the growing fatty acid chain. It resembles CoASH.
Acyl-malonyl ACP condensing enzyme =
b-ketoacyl ACP synthase
Acyl Carrier Protein(ACP) & CoA
CoASH
ACP
ACP and Coenzyme A
Reactions of Fatty Acid Synthesis
FA are synthesized by the repetitive
condensation of two-carbon units derived from
malonyl CoA
This occurs in separate steps:
(1) Synthesis of malonylCoA
(2) Loading of acetyl & malonyl using thioesters
(3) Condensation of the precursors
(4) Reduction
(5) Dehydration
(6) Reduction
(7) Cleavage (at the end)
Detail of steps in FAS
1. AcetylCoA ACP transacylase loads ACP,
b-ketoacyl ACP synthase transfers acetyl to Cys,
2. MalonylCoA ACP transacylase loads ACP,
3. b-KAS transfers acetyl to malonyl (loss of CO2),
4. b-KAS now on ACP goes to b-ketoacyl
reductase (NADPH) and forms a hydroxy group,
5. Then b-hydroxyacyl ACP dehydrase forms a
trans double bond,
6. Enoyl ACP reductase (NADPH) reduces the db,
7. b-KAS transfers acyl to Cys & steps 2-6 repeat,
8. Thioesterase cleaves palmitate from ACP.
b-ketoacyl ACP synthase = condensing enzyme
Fatty Acid Synthase
A dimer and multifunctional enzyme in mammals,
a multienzyme complex in yeast and separate
enzymes in Ecoli and plants.
Three domains with
flexible connections
Regulation
Active is polymeric and inactive monomeric.
Regulation
ACC-P (less active) can be allosterically activated
with citrate (+) and inhibited by palmitoylCoA (-).
Elongation and Unsaturation
Palmitate can be elongated either in the mito or the
cytosol but do not use FASase.
Unsaturation occurs in the microsomes. Mammals
can insert double bonds at carbons 4, 5, 6, & 9 but
cannot insert beyond carbon 9. Unsaturation uses
both O2 and NADPH or NADH (two enzymes).
Eicosanoid Outline
Eicosanoid Structures
Biochemistry
Sixth Edition
Berg • Tymoczko • Stryer
End of Chapter 22
Copyright © 2007 by W. H. Freeman and Company