1. Berg • Tymoczko • Stryer
Biochemistry
Sixth Edition
Chapter 22:
Fatty Acid Metabolism
Copyright © 2007 by W. H. Freeman and Company

4. Fatty acid metabolism
 An overview
 TAG are highly concentrated energy source
 Steps in fa oxidation
 Additional steps are required for certain fas
 Fa synthesis
 Controlling fa metabolism
 Elongation and unsaturation of fas

8. Electron
Micrograph
of Adipocyte

13. Utilization of fas
3 stages
1. Mobilization
‒ TAG  fa + glycerol
2. Fa activation and transportation into
mitochondria where oxidation takes
place.
3. Fa breakdown to Acetyl-CoA

15. FIGURE 17-3 Mobilization of triacylglycerols stored in adipose
tissue. When low levels of glucose in the blood trigger the release of
glucagon, 1 the hormone binds its receptor in the adipocyte membrane
and thus 2 stimulates adenylyl cyclase, via a G protein, to produce cAMP.
This activates PKA, which phosphorylates 3 the hormone-sensitive lipase
and 4 perilipin (a family of proteins that restrict access to lipid droplets,
preventing untimely lipid mobilization)molecules on the surface of the
lipid droplet. Phosphorylation of perilipin permits hormone-sensitive
lipase access to the surface of the lipid droplet, where 5 it hydrolyzes
triacylglycerols to free fatty acids. 6 Fatty acids leave the adipocyte, bind
serum albumin in the blood, and are carried in the blood; they are
released from the albumin and 7 enter a myocyte via a specific fatty acid
transporter. 8 In the myocyte, fatty acids are oxidized to CO2, and the
energy of oxidation is conserved in ATP, which fuels muscle contraction
and other energy-requiring metabolism in the myocyte.

17. Hormones that induce lipolysis
 Epinephrine
 Norepinephrine
 Glucagon
 Adrenocorticotropic hormones
IN CONTRAST,
INSULIN INHIBITS LIPOLYSIS!

25. Carnitine carries
activated fatty acids
into the
mitochondrial
matrix

29. Entry of electrons from fatty acid oxidation
in the mitochondrial respiratory chain

32. Fatty Acid Oxidation
 In one pass through the fatty acid oxidation, one mol. Of
Acetyl CoA, two pairs of electrons, and four hydrogen ions
are removed.
─ CH3 - - - - COOH
each time it is shorter by 2 C units
 The equation for one pass:
(C16) palmitoyl-CoA + CoA + FAD + NAD + H2O 
myristoyl CoA + Acetyl-CoA + FADH2 + NADh + H+
 Myristoyl CoA enters the -oxidation sequence, another set
of four reactions, to give a second Acetyl-CoA.

33. Fatty Acid Oxidation Continued
 The overall reaction is:
palmitoyl CoA + 7CoA + 7NAD + 7FAD + 7H2O 
8 Acetyl CoA + 7 FADH2 + 7NADH + 7H+
 These four steps are repeated (n/2 – 1) times for even
numbered carbon chains.
─ FADH2  ETFP (electron transferring flavoprotein),
provides 1.5 ATP
─ NADH  complex I, provides 2.5ATP
─ Thus, four mol. of ATP are formed for each 2-C unit
removed in one pass.

44. First 3 rounds
in the
degradation
of palmitate

48. In hibernating animals, fatty acid oxidation provides energy, heat,
and water. Fatty acid oxidation is also important for the camel.
I) Palmitoyl-CoA + 7CoA + 7O2 + 28Pi + 28ADP 
8Acetyl-CoA + 28 ATP + 7H2O
II) Acetyl-CoA oxidized in the TCA cycle
1Acetyl-CoA  10ATP
8Acetyl-CoA  80ATP
8Acetyl-CoA + 16O2 + 80Pi + 80ADP 
8 CoA + 80ATP + 16H2O + 16CO2
Combine I and II
Palmitoyl-CoA + 23O2 + 108Pi + 108ADP 
CoA + 108ATP + 16CO2 + 23H2O

51. Certain fatty acids require additional steps for
degradation
 Even numbered are fully saturated and are completely
oxidized
 Not all fa are simple and even numbered
 The oxidation of fa containing double bonds require
additional steps
 Odd numbered fa yield propionyl CoA at the final
thiolysis step. We need to metabolize propionyl CoA.

52. Oxidation of unsaturated fatty acids
 This is somewhat difficult
 Two additional enzymes are required:
• Isomerase
• Reductase
 Let’s analyze the oxidation of palmitoleate
• Activated
• Transported
• Undergoes 3 cycles of degradation by the same enzymes as in the
oxidation of saturated fas. However, the cis-d3-enoyl CoA formed
in the third round is not a substrate for acylCoA dehydrogenase.
• This should be converted to trans by the ISOMERASE enzyme.

57. Another problem arises with the oxidation of pufas
 What if we have two double bonds?
• Linoleate (18:9,12)
 Steps
• Cis double bond is formed after 3 rounds of beta oxidation
• This is coverted to a trans by ISOMERASE
• The acylCoA produced by another round of beta oxidation contains
a cis-delta 4 double bond.
• Dehydrogenation of this yields a 2,4-dienoyl intermediate, which is
not a substrate for the next enzyme in the b-oxidation pathway.
• This problem is solved by 2,4-DIENOYL CoA REDUCTASE!
 Thus, 2 extra enzymes are needed for the oxidation of even
numbered pufas: isomerase and reductase

67. Oxidation of odd chain fatty acids
 Odd chain fas oxidation is the same except the last step.
• The last step produces Acetyl CoA (2C) and Propionyl CoA (3C)
 How do we deal with 3C compound?
• It is converted into succinyl CoA in a reaction that requires Vit B12.
• Succinyl CoA is an intermediate in the TCA cycle.

76. Fas are also oxidized in peroxisomes
 Peroxisomes are membrane-enclosed cellular compartments.
• Hydrogen peroxide is produced by fa oxidation and then destroyed
enzymatically.
• Peroxisomes have high levels of catalase.
 The process consists of 4 steps:
1) Dehydrogenation
2) Hydration
3) Oxidation
4) Thiolytic cleavage
 The differences:
• In peroxisomes, the flavoprotein dehydrogenase passes electrons
directly to oxygen.
• The NADH formed in peroxisomes cannot be reoxidized, and the
peroxisome must export reducing equivalents to the cytosol.

79. Peroxisomal degradation
 Fa oxidation in these organelles stop at octanyl CoA
• Meaning, peroxisomes serve to shorten long chains to make
them better substrates for beta oxidation in mitochondria.
 Zelweger syndrome, which results from the absence of
functional peroxisomes, is characterized by liver,
kidney, and muscle abnormalities and usually results in
death by age 6.

81. Ketone bodies are formed from Acetyl CoA
when fat breakdown predominates
 Acetyl CoA enters the TCA cycle only if fat and ch
degradation are balanced.
 If oxaloacetate is decreased (if ch is unavailable) then acetyl
CoA will not enter TCA cycle.
 Also, in fasting and diabetes, oxalocetate is consumed to
make glc by the gluconeogenic pathway; therefore, acetyl
CoA increases.
 Under these conditions increased acetyl CoA makes
KETON BODIES
• Acetoacetate
• Beta hydroxybutyrate
• Acetone

86. Utilization of ketone bodies
by extrahepatic tissues

90. Fatty Acid Synthesis
 Production of cytoplasmic Acetyl-CoA
 Carboxylation of Acetyl-CoA to form
malonyl-CoA
 Fatty acid synthesis by a multi-enzyme
complex
 Regulation of fatty acid synthesis
 Metabolism of unsaturated fatty acids and
eicosanoids

91. Fatty Acid Synthesis
 It is not a reversal of fa oxidation. It occurs in the cytoplasm of the cells of
the liver, fat tissue, and mammary gland and, to a lesser extent, in the
cytoplasm of the cells of the kidney.
 The process incorporates carbons from Acetyl CoA (the immediate
substrate) into growing a fa chain, using ATP and NADPH as cofactors. Fa
synthesis is increased by insulin.
 Production of cytoplasmic acetyl CoA:
• Transfer of acetate units from mitochondrial acetylCoA to cytoplasm, forming
cytoplasmic acetyl CoA, since CoA can not cross the mitochondrial membrane.
• Carboxylation of AcetylCoA to form malonyl CoA
• The irreversible formation of malonyl CoA from acetylCoA is catalyzed by
acetylCoA carboxylase. Biotin, prosthetic group (The 2 step reaction, similar to
other biotin-dependent carboxylation reactions. Others: Pyruvate carboxylase,
propionylCoA carboxylase)
• The CO2 derived from HCO3 is first transferred to biotin in an ATP-dependent
reaction. The biotinyl group serves as a temporary carrier of CO2.

92. Citrate carries acetyl groups from
mitochondria to the cytosol for fa synthesis
 The synthesis of palmitate requires 8 mols of
acetyl CoA, 14 mols of NADPH, and 7 mols of
ATP.
 Fa synthesis is in the cytosol.
 Acetyl CoA is made from pyruvate in the
mitochondria. So, Acetyl CoA must be
transferred from the mitochondria to the cytosol.
 Solution: acetyl groups are carried as citrate.
• When citrate is high, it is transported to the cytosol. In the cytosol,
it is cleaved by ATP-citrate lyase.

94. Carboxylation of acetyl CoA to form malonyl CoA
by Acetyl-CoA carboxylase
 Acetyl-CoA carboxylase has 3 functional subunits:
1) Biotin carrier protein.
2) Biotin carboxylase,
• which activates CO2 by attaching it to a nitrogen in the biotin ring
in an ATP-dependent reaction.
3) Transcarboxylase
• which transfers activated CO2 from biotin to acetyl-CoA
producing malonyl CoA. The long-flexible biotin arm makes this.

100. The formation of Malonyl-CoA
 The formation of Malonyl-CoA is the committed step in
fa synthesis.
 Fa synthesis starts with carboxylation of acetyl-CoA to
malonyl-CoA.
• Irreversible reaction
• Biotin is a cofactor
• ATP is also required.

101. Important facts about the synthesis:
1) Synthesis takes place in the cytosol; oxidation, in the
mitochondria.
2) Intermediates in fa synthesis are covalently linked to the ACP
(acyl carrier protein), whereas in oxidation, they are bonded to
CoA.
3) Fa Synthesis enzymes are joined in a single pp chain called fa
synthase. Oxidation is not like that.
4) Growing of the chain is done by adding 2C units obtained from
Acetyl-CoA. Activated donor of 2C units is Malonyl-CoA.
5) The reductant in fa synthesis is NADPH.
6) Elongation stops when C16 is made.

102. Fatty Acid Synthesis
The remaining series of reactions are catalyzed by a
multienzyme complex.
• In eukaryotes this enzyme complex, fatty acid synthase, consists
of 2 subunits that, together, have 7 enzymatic activities.
• They are polypeptides.
• The proteins act together to catalyze the formation of fatty acids
from acetyl coA and malonyl CoA.
• The fundamental reaction order by which the long chains of
carbon atoms in fas are assembled consists of 4 steps.
1) Condensation of Acetyl group with malonyl group.
2) Reduction of the Carbonyl Group
3) Dehydration
4) Reduction of the Double Bond

103. Fatty Acid Synthesis
 This is one passage. With each passage through the
cycle, the fatty acyl chain is extended by 2 carbons.
When the chain length is 16, the product (palmitate 16:0)
leaves the cycle.
 The fa synthase complex has 7 different active sites
polypeptides.
• Throughout the process, the intermediates remain covalently
attached to one of two thiol groups of the complex.
• One point of attachment is the -SH group of a Cys residue in one
of the seven proteins.
• Beta-ketoacyl ACP the other is the -SH group of acyl carrier
protein (ACP).
 ACP is a small protein (Mr 8860) containing the
prosthetic group 4 phosphopantetheine, an intermediate
in the synthesis of coenzyme A.

104. Fatty Acid Synthesis
 Biosynthesis of fa is a 4-step sequence that lengthens a growing f.
acyl chain by two carbons.
1) Condensation
• CO2 is eliminated from malonyl group.
• Net effect is extension of the acyl chain by two carbons.
• The beta group is then reduced in three more steps nearly identical to the
reactions of beta oxidation, but in the reverse sequence.
2) The beta-keto group is reduced to an alcohol.
3) The elimination of H2O creates a double bond
4) The double bond is reduced to form the corresponding saturated f.acyl
group.
 The fa chain grows by two-carbon units that are donated by
activated malonate, with loss of CO2.
 After each two-carbon addition, reductions convert the growing chain
to a saturated fatty acid of 4, then 6, 8, and so on.
 The final product is palmitate (16:0).

108. Intermediates in fa synthesis
are attached to ACP
 Intermediates are linked to ACP
• Specifically, to the -SH terminus of a phosphopantetheine
group which is linked to a Ser residue of ACP
 ACP is a single polypeptide chain of 77 residues.
 ACP can be regarded as a giant prosthetic group
(macro CoA).
 Both ACP and CoA include phosphopantetheine as
their reactive units

129. Fa synthase inhibitors
 Fa synthase is overexpressed in some breast cancers.
 Some inhibitors were tested on mice and a great
weight loss was observed.
Therefore, fa synthase inhibitors are exciting candidates,
both as anti-tumor and as anti-obesity drugs!

130. Fa biosynthesis requirements
 The biosynthesis of fa require Acetyl CoA, ATP
and NADPH.
• The ATP is required to attach CO2 to acetylCoA to make
malonyl CoA
• NADPH is required to reduce the double bonds.
1. NADPH supplied from HMP or cytoplasmic conversion
of malate to pyruvate.
2. Malate is oxidized and decarboxylated by a
cytoplasmic NADP-dependent malate dehydrogenase
or malic enzyme to form pyruvate.

131. Fatty acid biosynthesis is tightly regulated
 If we have more energy than our needs, fatty acids are stored.
 Acetyl coA carboxylase is the rate limiting step in the biosynthesis of fatty
acids (Important site regulation).
 The reaction is catalyzed by acetyl coA carboxylase
• Switched off by phosphorylation
• Activated by dephosphorylation
 The carboxylase is regulated by 3 signals:
1. Glucagon – inhibits carboxylase activity
2. Epi – inhibits carboxylase activity
3. Insulin – stimulates fas synthesis
 Control is also done by the levels, within the cell, of
1. Citrate – activates carboxylase
• Energy and building blocks are abundant, and we can go ahead and store them
2. Palmitoyl CoA – inhibits carboxylase
3. AMP – inhibits carboxylase

132. More regulation
 The proportion of active carboxylase depends on the catalytic
rates of these opposing enzymes.
• Protein kinase A inhibits phosphatase by phosphorylating it.
- Carboxylase stays in its inactive form.
• The inactive form also predominates when the energy level of the cell
is low.
- Phosphorylation is stimulated by high AMP levels.
• Insulin stimulates carboxylase perhaps by activating protein
phosphatase 2A.
 How about citrate’s role?
• Citrate is high when acetyl CoA and ATP are high.
• When citrate is high and ATP is available, we can start making fa.
• Citrate stimulates carboxylase.
• Palmitoyl CoA inhibits this action of citrate on the carboxylase
enzyme.

137. Dependence of the catalytic activity of acetyl CoA
carboxylase on the concentration of citrate
 No citrate: Dephosphorylated form is predominant
 The presence of citrate partly reverses the inhibition produced
by phosphorylation for the enzyme (Acetyl CoA carboxylase)
 Citrate facilitates the polymerization of the inactive octamers
into active filaments
• Acetyl CoA carboxylase exists as an octamer
 The level of citrate is high when acetyl CoA and ATP are
abundant. Hence, increased level of citrate signifies that 2-C
units and ATP are available for the fa synthesis

140. More regulation
 The stimulatory effect of citrate on the carboxylase is
antagonized by palmitoyl CoA, which is abundant when there
is an excess of fatty acids
 Palmitoyl CoA causes the filaments to disassemble into the
inactive octamers
 Response to diet: In starvation, ffa are increased because Epi,
Glucagon stimulate lipase. Insulin, in contrast, inhibits
lipolysis
 Also, malonyl CoA inhibits carnitine shuttle preventing
excess of fatty acyl CoAs to the mitochondrial matrix in times
plenty.

141. Elongation and unsaturation
 The major product of fa synthesis is palmitate
 Longer ones are formed by elongation reactions in ER
 2C units are added, the donor is still malonyl CoA
 ER enzymes also introduce double bonds.
There is an electron-transport chain in the desaturation of
fatty acids which includes NADH-cytochrome b5-reductase,
cytb5, and desaturase.

143. pufas
 Some pufas can not be synthesized by mammals and are
nutritionally essential
• C20, 22, and 24 fa may be detected in the tissues. They are derived from
oleic (in plants but not animals) and linoleic acid by chain elongation.
• Palmitoleic and oleic acids are not essential in the diet because animals
can make a double bond at the 9 position.
• Linoleic and linolenic acids are essential fas because animals cannot
synthesize them.
• Arachidonic acid can be formed from linoleic acid in most animals.
 In animals, double bonds can be introduced at the 4,5,6 and 9
positions but never beyond the 9 position.
 Plants can introduce double bonds beyond 9 position and can,
therefore, synthesize essential fas.

145. More about pufas
 Monounsaturated fatty acids are synthesized by a desaturase system.
 Many tissues, including liver tissues, can make monounsaturated
forms. The first double bond introduced into a saturated fa is almost
always at the 9 position. Enzyme is desaturase.
 Synthesis of pufas involves desaturase and elongase systems.
 In animals, the additional double bonds all introduced between the
existing double bond and the -COOH group, but in plants, they may
also be introduced between the 9 and omega carbon.
• Linoleate 18:2(9,12) and linolenate 18:3(9,12,15) cannot be synthesized
by mammals, but plants can synthesize both.
• The plant desaturases that introduce double bonds at 12 and 15 positions
are located in SER.

147. Eicosanoids
 Eicosanoids are formed from C20 pufas
 TXA, LT and PGs are called eicosanoids.
 Arachidonate gives rise to Pgs, TX and LT.
• A family of very potent biological molecules are made from arachidonate.
• They act as short-range messengers, affecting neighboring tissues.
• In response to a hormonal or other stimulus, a specific phospholipase affects
membrane phospholipids, releasing arachidonate.
 SER enzymes then convert arachidonate into prostaglandins,
beginning with the formation of PGG2, very first PGs