Fatty Acid Catabolism
chapter
17
1. Energy in Triacylglycerols On a per-carbon basis, where does the largest amount of biologically
available energy in triacylglycerols reside: in the fatty acid portions or the glycerol portion? Indicate
how knowledge of the chemical structure of triacylglycerols provides the answer.
2. Fuel Reserves in Adipose Tissue Triacylglycerols, with their hydrocarbon-like fatty acids, have the
highest energy content of the major nutrients.
(a) If 15% of the body mass of a 70.0 kg adult consists of triacylglycerols, what is the total available
fuel reserve, in kilojoules and kilocalories, in the form of triacylglycerols? Recall that
1.00 kcal 4.18 kJ.
(b) If the basal energy requirement is approximately 8,400 kJ/day (2,000 kcal/day), how long could
this person survive if the oxidation of fatty acids stored as triacylglycerols were the only source
of energy?
(c) What would be the weight loss in pounds per day under such starvation conditions (1 lb
0.454 kg)?
Answer
3. Common Reaction Steps in the Fatty Acid Oxidation Cycle and Citric Acid Cycle Cells often
use the same enzyme reaction pattern for analogous metabolic conversions. For example, the steps in
the oxidation of pyruvate to acetyl-CoA and of a-ketoglutarate to succinyl-CoA, although catalyzed by
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S-200 Chapter 17 Fatty Acid Catabolism
different enzymes, are very similar. The first stage of fatty acid oxidation follows a reaction sequence
closely resembling a sequence in the citric acid cycle. Use equations to show the analogous reaction
sequences in the two pathways.
4. ␤Oxidation: How Many Cycles? How many cycles of ␤oxidation are required for the complete
oxidation of activated oleic acid, 18:1(
9
)?
5. Chemistry of the Acyl-CoA Synthetase Reaction Fatty acids are converted to their coenzyme A
esters in a reversible reaction catalyzed by acyl-CoA synthetase:
RATPCoA
COO
R
C
O
CoA AMP PP
i
ORPC
O
OO
O
Adenine
HH
HH
OH
CH
2
OH
O
(a) The enzyme-bound intermediate in this reaction has been identified as the mixed anhydride of
the fatty acid and adenosine monophosphate (AMP), acyl-AMP:
Write two equations corresponding to the two steps of the reaction catalyzed by acyl-CoA
synthetase.
(b) The acyl-CoA synthetase reaction is readily reversible, with an equilibrium constant near 1. How
can this reaction be made to favor formation of fatty acyl–CoA?
Chapter 17 Fatty Acid Catabolism S-201
6. Intermediates in Oleic Acid Oxidation What is the structure of the partially oxidized fatty acyl
group that is formed when oleic acid, 18:1(
9
), has undergone three cycles of ␤oxidation? What are
the next two steps in the continued oxidation of this intermediate?
7. ␤Oxidation of an Odd-Chain Fatty Acid What are the direct products of ␤oxidation of a fully
saturated, straight-chain fatty acid of 11 carbons?
8. Oxidation of Tritiated Palmitate Palmitate uniformly labeled with tritium (
3
H) to a specific activ-
ity of 2.48 10
8
counts per minute (cpm) per micromole of palmitate is added to a mitochondrial
preparation that oxidizes it to acetyl-CoA. The acetyl-CoA is isolated and hydrolyzed to acetate. The
specific activity of the isolated acetate is 1.00 10
7
cpm/mmol. Is this result consistent with the
b-oxidation pathway? Explain. What is the final fate of the removed tritium?
9. Compartmentation in b Oxidation Free palmitate is activated to its coenzyme A derivative
(palmitoyl-CoA) in the cytosol before it can be oxidized in the mitochondrion. If palmitate and
[
14
C]coenzyme A are added to a liver homogenate, palmitoyl-CoA isolated from the cytosolic fraction
is radioactive, but that isolated from the mitochondrial fraction is not. Explain.
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10. Comparative Biochemistry: Energy-Generating Pathways in Birds One indication of the rela-
tive importance of various ATP-producing pathways is the V
max
of certain enzymes of these pathways.
The values of V
max
of several enzymes from the pectoral muscles (chest muscles used for flying) of
pigeon and pheasant are listed below.
(a) Discuss the relative importance of glycogen metabolism and fat metabolism in generating ATP in
the pectoral muscles of these birds.
(b) Compare oxygen consumption in the two birds.
(c) Judging from the data in the table, which bird is the long-distance flyer? Justify your answer.
(d) Why were these particular enzymes selected for comparison? Would the activities of triose phos-
phate isomerase and malate dehydrogenase be equally good bases for comparison? Explain.
11. Mutant Carnitine Acyltransferase What changes in metabolic pattern would result from a muta-
tion in the muscle carnitine acyltransferase I in which the mutant protein has lost its affinity for
malonyl-CoA but not its catalytic activity?
12. Effect of Carnitine Deficiency An individual developed a condition characterized by progressive
muscular weakness and aching muscle cramps. The symptoms were aggravated by fasting, exercise,
and a high-fat diet. The homogenate of a skeletal muscle specimen from the patient oxidized added
oleate more slowly than did control homogenates, consisting of muscle specimens from healthy indi-
viduals. When carnitine was added to the patient’s muscle homogenate, the rate of oleate oxidation
equaled that in the control homogenates. The patient was diagnosed as having a carnitine deficiency.
(a) Why did added carnitine increase the rate of oleate oxidation in the patient’s muscle homogenate?
V
max
(mmol substrate/min/g tissue)
Enzyme Pigeon Pheasant
Hexokinase 3.0 2.3
Glycogen phosphorylase 18.0 120.0
Phosphofructokinase-1 24.0 143.0
Citrate synthase 100.0 15.0
Triacylglycerol lipase 0.07 0.01
(b) Why were the patient’s symptoms aggravated by fasting, exercise, and a high-fat diet?
(c) Suggest two possible reasons for the deficiency of muscle carnitine in this individual.
13. Fatty Acids as a Source of Water Contrary to legend, camels do not store water in their humps,
which actually consist of large fat deposits. How can these fat deposits serve as a source of water?
Calculate the amount of water (in liters) that a camel can produce from 1.0 kg of fat. Assume for
simplicity that the fat consists entirely of tripalmitoylglycerol.
14. Petroleum as a Microbial Food Source Some microorganisms of the genera Nocardia and
Pseudomonas can grow in an environment where hydrocarbons are the only food source. These
bacteria oxidize straight-chain aliphatic hydrocarbons, such as octane, to their corresponding
carboxylic acids:
CH
3
(CH
2
)
6
CH
3
NAD
O
2
88z
y88 CH
3
(CH
2
)
6
COOH NADH H
How could these bacteria be used to clean up oil spills? What would be some of the limiting factors to
the efficiency of this process?
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15. Metabolism of a Straight-Chain Phenylated Fatty Acid A crystalline metabolite was isolated from
the urine of a rabbit that had been fed a straight-chain fatty acid containing a terminal phenyl group:
COO
(CH
2
)
n
CH
2
A 302 mg sample of the metabolite in aqueous solution was completely neutralized by 22.2 mL of
0.100
M
NaOH.
(a) What is the probable molecular weight and structure of the metabolite?
(b) Did the straight-chain fatty acid contain an even or an odd number of methylene (OCH
2
O)
groups (i.e., is neven or odd)? Explain.
16. Fatty Acid Oxidation in Uncontrolled Diabetes When the acetyl-CoA produced during boxida-
tion in the liver exceeds the capacity of the citric acid cycle, the excess acetyl-CoA forms ketone
bodies—acetone, acetoacetate, and
D
–b-hydroxybutyrate. This occurs in severe, uncontrolled diabetes:
because the tissues cannot use glucose, they oxidize large amounts of fatty acids instead. Although
acetyl-CoA is not toxic, the mitochondrion must divert the acetyl-CoA to ketone bodies. What problem
would arise if acetyl-CoA were not converted to ketone bodies? How does the diversion to ketone
bodies solve the problem?
17. Consequences of a High-Fat Diet with No Carbohydrates Suppose you had to subsist on a diet
of whale blubber and seal blubber, with little or no carbohydrate.
(a) What would be the effect of carbohydrate deprivation on the utilization of fats for energy?
(b) If your diet were totally devoid of carbohydrate, would it be better to consume odd- or even-
numbered fatty acids? Explain.
Answer
18. Even- and Odd-Chain Fatty Acids in the Diet In a laboratory experiment, two groups of rats are
fed two different fatty acids as their sole source of carbon for a month. The first group gets heptanoic
acid (7:0), and the second gets octanoic acid (8:0). After the experiment, a striking difference is seen
between the two groups. Those in the first group are healthy and have gained weight, whereas those
in the second group are weak and have lost weight as a result of losing muscle mass. What is the
biochemical basis for this difference?
19. Metabolic Consequences of Ingesting q-Fluorooleate The shrub Dichapetalum toxicarium,
native to Sierra Leone, produces q-fluorooleate, which is highly toxic to warm-blooded animals.
Chapter 17 Fatty Acid Catabolism S-205
(CH
2
)
7
(CH
2
)
7
␻-Fluorooleate
FCH
2
CC
HH
COO
This substance has been used as an arrow poison, and powdered fruit from the plant is sometimes used
as a rat poison (hence the plant’s common name, ratsbane). Why is this substance so toxic? (Hint: re-
view Chapter 16, Problem 22.)
20. Mutant Acetyl-CoA Carboxylase What would be the consequences for fat metabolism of a muta-
tion in acetyl-CoA carboxylase that replaced the Ser residue normally phosphorylated by AMPK to an
Ala residue? What might happen if the same Ser were replaced by Asp? (Hint: See Fig. 17–13.)
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21. Effect of PDE Inhibitor on Adipocytes How would an adipocyte’s response to epinephrine be
affected by the addition of an inhibitor of cAMP phosphodiesterase (PDE)? (Hint: See Fig. 12–4.)
22. Role of FAD as Electron Acceptor Acyl-CoA dehydrogenase uses enzyme-bound FAD as a pros-
thetic group to dehydrogenate the aand bcarbons of fatty acyl–CoA. What is the advantage of using
FAD as an electron acceptor rather than NAD
? Explain in terms of the standard reduction potentials
for the Enz-FAD/FADH
2
(E  0.219 V) and NAD
/NADH (E  0.320 V) half-reactions.
23. b Oxidation of Arachidic Acid How many turns of the fatty acid oxidation cycle are required for
complete oxidation of arachidic acid (see Table 10–1) to acetyl-CoA?
24. Fate of Labeled Propionate If [3-
14
C]propionate (
14
C in the methyl group) is added to a liver ho-
mogenate,
14
C-labeled oxaloacetate is rapidly produced. Draw a flow chart for the pathway by which
propionate is transformed to oxaloacetate, and indicate the location of the
14
C in oxaloacetate.
25. Phytanic Acid Metabolism When phytanic acid uniformly labeled with
14
C is fed to a mouse, ra-
dioactivity can be detected in malate, a citric acid cycle intermediate, within minutes. Draw a meta-
bolic pathway that could account for this. Which of the carbon atoms in malate would contain
14
C
label?
26. Sources of H
2
O Produced in bOxidation The complete oxidation of palmitoyl-CoA to carbon
dioxide and water is represented by the overall equation
Palmitoyl-CoA 23O
2
108P
i
108ADP 88n CoA 16CO
2
108ATP 23H
2
O
Water is also produced in the reaction
ADP P
i
88n ATP H
2
O
but is not included as a product in the overall equation. Why?
27. Biological Importance of Cobalt In cattle, deer, sheep, and other ruminant animals, large amounts
of propionate are produced in the rumen through the bacterial fermentation of ingested plant matter.
Propionate is the principal source of glucose for these animals, via the route propionate noxaloac-
etate nglucose. In some areas of the world, notably Australia, ruminant animals sometimes show
symptoms of anemia with concomitant loss of appetite and retarded growth, resulting from an inability
to transform propionate to oxaloacetate. This condition is due to a cobalt deficiency caused by very
low cobalt levels in the soil and thus in plant matter. Explain.
ciency in animals would result in coenzyme B
12
deficiency.
28. Fat Loss during Hibernation Bears expend about 25 10
6
J/day during periods of hibernation,
which may last as long as seven months. The energy required to sustain life is obtained from fatty acid
oxidation. How much weight loss (in kilograms) has occurred after seven months? How might ketosis be
minimized during hibernation? (Assume the oxidation of fat yields 38 kJ/g.)
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Data Analysis Problem
29. ␤Oxidation of Trans Fats Unsaturated fats with trans double bonds are commonly referred to as
“trans fats.” There has been much discussion about the effects of dietary trans fats on health. In their
investigations of the effects of trans fatty acid metabolism on health, Yu and colleagues (2004) showed
that a model trans fatty acid was processed differently from its cis isomer. They used three related
18-carbon fatty acids to explore the difference in ␤oxidation between cis and trans isomers of the
same-size fatty acid.
The researchers incubated the coenzyme A derivative of each acid with rat liver mitochondria for
5 minutes, then separated the remaining CoA derivatives in each mixture by HPLC (high-performance
liquid chromatography). The results are shown below, with separate panels for the three experiments.
Time (min)
21
IS
Stearoyl-CoA
+ mitochondria
Oleoyl-CoA
+ mitochondria
Elaidoyl-CoA
+ mitochondria
C18-CoA
cΔ5C14-CoA
tΔ5C14-CoA
cΔ9C18-CoA tΔ9C18-CoA
IS
IS
3012 21 3012 21 3012
Absorbance at 254 nm
O
OH
O
OH
O
OH
Stearic acid
(octadecenoic acid)
Oleic acid
(cis–
9
-octadecenoic acid)
Elaidic acid
(trans–
9
-octadecenoic acid)
In the figure, IS indicates an internal standard (pentadecanoyl-CoA) added to the mixture, after
the reaction, as a molecular marker. The researchers abbreviated the CoA derivatives as follows:
stearoyl-CoA, C
18
-CoA; cis–
5
-tetradecenoyl-CoA, c
5
C
14
-CoA; oleoyl-CoA, c
9
C
18
-CoA;
trans–
5
-tetradecenoyl-CoA, t
5
C
14
-CoA; and elaidoyl-CoA, t
9
C
18
– CoA.
Chapter 17 Fatty Acid Catabolism S-209
(a) Why did Yu and colleagues need to use CoA derivatives rather than the free fatty acids in these
experiments?
(b) Why were no lower molecular weight CoA derivatives found in the reaction with stearoyl-CoA?
(c) How many rounds of ␤oxidation would be required to convert the oleoyl-CoA and the elaidoyl-
CoA to cis–
5
-tetradecenoyl-CoA and trans–
5
-tetradecenoyl-CoA, respectively?
There are two forms of the enzyme acyl-CoA dehydrogenase (see Fig. 17–8a): long-chain
acyl-CoA dehydrogenase (LCAD) and very-long-chain acyl-CoA dehydrogenase (VLCAD). Yu and
coworkers measured the kinetic parameters of both enzymes. They used the CoA derivatives of
three fatty acids: tetradecanoyl-CoA (C
14
-CoA), cis–
5
-tetradecenoyl-CoA (c
5
C
14
-CoA), and
trans–
5
-tetradecenoyl-CoA (t
5
C
14
-CoA). The results are shown below. (See Chapter 6 for
definitions of the kinetic parameters.)
(d) For LCAD, the K
m
differs dramatically for the cis and trans substrates. Provide a plausible expla-
nation for this observation in terms of the structures of the substrate molecules. (Hint: You may
want to refer to Fig. 10–2.)
(e) The kinetic parameters of the two enzymes are relevant to the differential processing of these
fatty acids only if the LCAD or VLCAD reaction (or both) is the rate-limiting step in the path-
way. What evidence is there to support this assumption?
(f) How do these different kinetic parameters explain the different levels of the CoA derivatives
found after incubation of rat liver mitochondria with stearoyl-CoA, oleoyl-CoA, and elaidoyl-CoA
(shown in the three-panel figure)?
Yu and coworkers measured the substrate specificity of rat liver mitochondrial thioesterase, which
hydrolyzes acyl-CoA to CoA and free fatty acid (see Chapter 21). This enzyme was approximately
twice as active with C
14
-CoA thioesters as with C
18
-CoA thioesters.
(g) Other research has suggested that free fatty acids can pass through membranes. In their experi-
ments, Yu and colleagues found trans–
5
-tetradecenoic acid outside mitochondria (i.e., in the
medium) that had been incubated with elaidoyl-CoA. Describe the pathway that led to this ex-
tramitochondrial trans–
5
-tetradecenoic acid. Be sure to indicate where in the cell the various
transformations take place, as well as the enzymes that catalyze the transformations.
cis–
5
-Tetradecenoyl-CoA
S-Co
A
O
O
S-Co
A
trans–
5
-Tetradecenoyl-CoA
LCAD VLCAD
C
14
-c⌬
5
C
14
-t⌬
5
C
14
-C
14
-c⌬
5
C
14
-t⌬
5
C
14
–
CoA CoA CoA CoA CoA CoA
V
max
3.3 3.0 2.9 1.4 0.32 0.88
K
m
0.41 0.40 1.6 0.57 0.44 0.97
k
cat
9.9 8.9 8.5 2.0 0.42 1.12
k
cat
/
K
m
24 22 5 4 1 1
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(h) It is often said in the popular press that “trans fats are not broken down by your cells and instead
accumulate in your body.” In what sense is this statement correct and in what sense is it an over-
simplification?
Answer