Chapter Review in Chapter 17 Fatty Acid Catabolism

Chapter Review

KEY TERMS

Terms in bold are defined in the glossary.

Problems

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 in the glycerol portion? Indicate how knowledge of the chemical structure of triacylglycerols provides the answer.
2. Effect of PDE Inhibitor on Adipocytes How would the addition of a cAMP phosphodiesterase (PDE) inhibitor affect the response of an adipocyte to epinephrine? (Hint: See Fig. 12-4.)
3. Compartmentation in β Oxidation The activation of free palmitate to its coenzyme A derivative (palmitoyl-CoA) in the cytosol occurs before it can be oxidized in the mitochondrion. After adding palmitate and $[^{14} C]$ $left-bracket Superscript 14 Baseline zero width space upper C right-bracket$ coenzyme A to a liver homogenate, you find palmitoyl-CoA isolated from the cytosolic fraction is radioactive, but that isolated from the mitochondrial fraction is not. Explain.
4. Mutant Carnitine Acyltransferase What changes in metabolic pattern would result from a mutation in the muscle carnitine acyltransferase 1 in which the mutant protein has lost its affinity for malonyl-CoA but not its catalytic activity?
5. 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. An homogenate of a skeletal muscle specimen from the patient oxidized added oleate more slowly than did control homogenates consisting of muscle specimens from healthy individuals. When the pathologist added carnitine to the patient’s muscle homogenate, the rate of oleate oxidation equaled that in the control homogenates. Based on these results, the attending physician diagnosed the patient as having a carnitine deficiency.
1. Why did added carnitine increase the rate of oleate oxidation in the patient’s muscle homogenate?
2. Why did fasting, exercise, and a high-fat diet aggravate the patient’s symptoms?
3. Suggest two possible reasons for the deficiency of muscle carnitine in this individual.
6. Fuel Reserves in Adipose Tissue Triacylglycerols, with their hydrocarbon-like fatty acids, have the highest energy content of the major nutrients.
1. If 15% of the body mass of a 70.0 kg adult consists of triacylglycerols, what is the total available fuel reserve, in both kilojoules and kilocalories, in the form of triacylglycerols? Recall that $1.00 kcal = 4.18 kJ$ $1.00 kcal equals 4.18 kJ$ .
2. 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?
3. What would be the weight loss in pounds per day under such starvation conditions $(1 lb = 0.454 kg)$ $left-parenthesis 1 lb equals 0.454 kg right-parenthesis$ ?
7. 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 α-ketoglutarate to succinyl-CoA, although catalyzed by 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.
8. β Oxidation: How Many Cycles? How many cycles of β oxidation are required for the complete oxidation of activated oleic acid, $18 : 1 (Δ^{9})$ $18 colon 1 left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ ?
9. 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:
1. 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.
2. 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?
10. Intermediates in Oleic Acid Oxidation What is the structure of the partially oxidized fatty acyl group that is formed when oleic acid, $18 colon 1 left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ $18 colon 1 left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ , has undergone three cycles of β oxidation? What are the next two steps in the continued oxidation of this intermediate?
11. β Oxidation of an Odd-Number Fatty Acid What are the direct products of β oxidation of a fully saturated, straight-chain fatty acid of 11 carbons?
12. Oxidation of Tritiated Palmitate An investigator adds palmitate uniformly labeled with tritium $(^{3} H)$ $left-parenthesis Superscript 3 Baseline upper H right-parenthesis$ to a specific activity of $2.48 \times 10^{8}$ $2.48 times 10 Superscript 8$ counts per minute (cpm) per micromole of palmitate to a mitochondrial preparation that oxidizes it to acetyl-CoA. She then isolates the acetyl-CoA and hydrolyzes it to acetate. The specific activity of the isolated acetate is $1.00 \times 10^{7} cpm/ μ mol$ $1.00 times 10 Superscript 7 Baseline cpm slash mu mol$ . Is this result consistent with the β-oxidation pathway? Explain. What is the final fate of the removed tritium? (Note: Specific activity is a measure of the degree of labeling with a radioactive tracer expressed as radioactivity per unit mass. In a uniformly labeled compound, all atoms of a given type are labeled.)

13. Comparative Biochemistry: Energy-Generating Pathways in Birds One indication of the relative importance of various ATP-producing pathways is the $V_{\max}$ $upper V Subscript max$ of certain enzymes of these pathways. The values of $V_{\max}$ $upper V Subscript max$ of several enzymes from the pectoral muscles (chest muscles used for flying) of pigeon and pheasant are listed below.

Enzyme	Pigeon	Pheasant
	$V_{\max}$ $bold-italic upper V Subscript bold max$ (μmol substrate/min/g tissue)
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

Discuss the relative importance of glycogen metabolism and fat metabolism in generating ATP in the pectoral muscles of these birds.
Compare oxygen consumption in the two birds.
Judging from the data in the table, which bird is the long-distance flyer? Justify your answer.
Why were these particular enzymes selected for comparison? Would the activities of triose phosphate isomerase and malate dehydrogenase be equally good bases for comparison? Explain.

14. 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.
15. Metabolism of a Straight-Chain Phenylated Fatty Acid Investigators isolate a crystalline metabolite from the urine of a rabbit that has been fed a straight-chain fatty acid containing a terminal phenyl group:

The addition of 22.2 mL of 0.100 m NaOH completely neutralized a 302 mg sample of the metabolite in aqueous solution.
1. What is the probable molecular weight and structure of the metabolite?
2. Did the straight-chain fatty acid contain an even or an odd number of methylene $(— {CH}_{2} —)$ $left-parenthesis em-dash CH Subscript 2 Baseline em-dash right-parenthesis$ groups (i.e., is n even or odd)? Explain.
16. Fatty Acid Oxidation in Uncontrolled Diabetes When the acetyl-CoA produced during β oxidation in the liver exceeds the capacity of the citric acid cycle, the excess acetyl-CoA forms ketone bodies — acetone, acetoacetate, and d-β-hydroxybutyrate. This occurs in people with severe, uncontrolled diabetes; because their 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.
1. What would be the effect of carbohydrate deprivation on the utilization of fats for energy?
2. If your diet were totally devoid of carbohydrate, would it be better to consume odd- or even-number fatty acids? Explain.
18. Even- and Odd-Number Fatty Acids in the Diet In a laboratory experiment, investigators feed two groups of rats 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, 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 ω-Fluorooleate The shrub Dichapetalum toxicarium, native to Sierra Leone, produces ω-fluorooleate, which is highly toxic to warm-blooded animals.

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: Review Chapter 16, Problem 21.)
20. Mutant Acetyl-CoA Carboxylase What would be the consequences for fat metabolism of a mutation in acetyl-CoA carboxylase that replaced the Ser residue normally phosphorylated by AMPK with an Ala residue? What might happen if the same Ser were replaced by Asp? (Hint: Compare the structures of phosphoserine, alanine, aspartate; see Fig. 17-13.)
21. Role of FAD as Electron Acceptor Acyl-CoA dehydrogenase uses enzyme-bound FAD as a prosthetic group to dehydrogenate the α and β carbons of fatty acyl–CoA. What is the advantage of using FAD as an electron acceptor rather than ${NAD}^{+}$ $NAD Superscript plus$ ? Explain in terms of the standard reduction potentials for the ${Enz-FAD/FADH}_{2} (E^{'} ° = −0.219 V)$ $Enz hyphen FAD slash FADH Subscript 2 Baseline left-parenthesis upper E Superscript prime Baseline degree equals Number minus 0 period 2 1 9 upper V right-parenthesis$ and ${NAD}^{+} /NADH (E^{'} ° = −0.320 V)$ $NAD Superscript plus Baseline slash NADH left-parenthesis upper E Superscript prime Baseline degree equals Number minus 0 period 3 2 0 upper V right-parenthesis$ half-reactions.
22. β Oxidation of Arachidic Acid How many turns of the fatty acid oxidation cycle are required for complete oxidation of arachidic acid (20:0) to acetyl-CoA?
23. Fate of Labeled Propionate Adding $[3 -^{14} C] propionate$ $left-bracket 3 hyphen zero width space upper C right-bracket propionate$ ( $^{14} C$ $Superscript 14 Baseline upper C$ in the methyl group) to a liver homogenate leads to the rapid production of $^{14} C$ $Superscript 14 Baseline upper C$ -labeled oxaloacetate. Draw a flowchart for the pathway by which propionate is transformed to oxaloacetate, and indicate the location of the $^{14} C$ $Superscript 14 Baseline upper C$ in oxaloacetate.
24. Phytanic Acid Metabolism A mouse fed phytanic acid uniformly labeled with $^{14} C$ $Superscript 14 Baseline upper C$ produces detectable levels of radioactive malate, a citric acid cycle intermediate, within minutes. Draw a metabolic pathway that could account for this. Which of the carbon atoms in malate would contain $^{14} C$ $Superscript 14 Baseline upper C$ label?
25. Sources of $H_{2} O$ $bold upper H bold 2 bold upper O$ Produced in β Oxidation The complete oxidation of palmitoyl-CoA to carbon dioxide and water is represented by the overall equation

$\begin{matrix} Palmitoyl-CoA + 23 O_{2} + 108 P_{i} + 108 ADP \to \\ CoA + 16 {CO}_{2} + 108 ATP + 23 H_{2} O \end{matrix}$ $StartLayout 1st Row Palmitoyl hyphen CoA plus 23 upper O Subscript 2 Baseline plus 108 upper P Subscript i Baseline plus 108 ADP right-arrow 2nd Row CoA plus 16 CO Subscript 2 Baseline plus 108 ATP plus 23 upper H Subscript 2 Baseline upper O EndLayout$

Water also forms in the reaction

$ADP + P_{i} \to ATP + H_{2} O$ $ADP plus upper P Subscript i Baseline right-arrow ATP plus upper H Subscript 2 Baseline upper O$

but is not included as a product in the overall equation. Why?
26. Biological Importance of Cobalt Cattle, deer, sheep, and other ruminant animals produce large amounts of propionate 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 $\to$ $right-arrow$ oxaloacetate $\to$ $right-arrow$ glucose. 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.
27. Fat Loss during Hibernation Bears expend about $25 \times 10^{6} J/day$ $25 times 10 Superscript 6 Baseline upper J slash day$ 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 (in kilograms) do bears lose after 7 months of hibernation? How could a bear’s body minimize ketosis during hibernation? (Assume the oxidation of fat yields 38 kJ/g.)

Data Analysis Problem

28. β Oxidation of Trans Fats Unsaturated fats with trans double bonds are commonly referred to as “trans fats.” 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 β oxidation between cis and trans isomers of the same-size fatty acid.

Stearic acid (octadecenoic acid) is an 18-carbon zigzag chain with C 1 double bonded to O and bonded to O H. Oleic acid (italicized cis end italics Greek letter delta-superscript 9-octadecenoic acid) is an 18-carbon zigzag chain with C 1 double bonded to O and bonded to O H and a cis double bond between C 9 and C 10. Elaidic acid (italicized trans end italics Greek letter delta superscript 9-octadecenoic acid) is an 18-carbon zigzag chain with C 1 double bonded to O and bonded to O H and a trans double bond between C 9 and C 10.

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.

The horizontal axis is labeled time (min) and is divided into three panels to represent the three different experiments. In each case, the horizontal axis ranges from 12 to 30, labeled in increments of 9. The vertical axis is labeled absorbance at 254 n m for all panels. The first panel is labeled stearoyl-Co A plus mitochondria. There are many small peaks from 21 to 22, then a tall peak labeled I S at 22 that extends to almost half the height of the vertical axis and an even taller peak labeled C subscript 18-Co A at 27 that extends to three-quarters of the height of the vertical axis. The second panel is labeled oleoyl-Co A plus mitochondria. There are many small peaks. At 13 and 15 on the horizontal axis, the peaks are at almost a quarter of the height of the vertical axis. At 20 on the horizontal axis, there is a peak labeled c Greek letter delta superscript 5 C subscript 14 – Co A at one-quarter of the height of the vertical axis. There is a peak labeled I S at 23 at almost three-quarters of the height of the vertical axis and a peak labeled c Greek letter delta superscript 9 C subscript 18-Co A at 24 that almost reaches the top of the vertical axis. The third panel is labeled elaidoyl-Co A plus mitochondria. It has many small peaks, including one at 13 on the horizontal axis that approaches one-quarter of the height of the vertical axis. There is a peak labeled t Greek letter delta superscript 5 C subscript 14-Co A at 19 on the horizontal axis and almost three-quarters of the height of the vertical axis, a peak labeled I S at 23 that is only slightly shorter, and a peak labeled t Greek letter delta superscript 9 C subscript 18-Co A at 25 on the horizontal axis that approaches the top of the vertical axis. All data are approximate.

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, $C_{18} -CoA$ $upper C Subscript 18 Baseline hyphen CoA$ ; cis- $Δ^{5}$ $upper Delta Superscript 5$ -tetradecenoyl-CoA, $c Δ^{5} C_{14} -CoA$ $c upper Delta Superscript 5 Baseline upper C Subscript 14 Baseline hyphen CoA$ ; oleoyl-CoA, $c Δ^{9} C_{18} -CoA$ $c upper Delta Superscript 9 Baseline upper C Subscript 18 Baseline hyphen CoA$ ; trans- $Δ^{5}$ $upper Delta Superscript 5$ -tetradecenoyl-CoA, $t Δ^{5} C_{14} -CoA$ $t upper Delta Superscript 5 Baseline upper C Subscript 14 Baseline hyphen CoA$ ; and elaidoyl-CoA, $t Δ^{9} C_{18} -CoA$ $t upper Delta Superscript 9 Baseline upper C Subscript 18 Baseline hyphen CoA$ .

Italicized cis end italics-Greek letter delta superscript 5-tetradecenoyl-CoA is a 14-carbon zigzag chain with C 1 double bonded to O and bonded to S further bonded to Co A. It has a cis double bond between C 5 and C 6. Italicized trans end italics-Greek letter delta superscript 5-tetradecenoyl-Co A is a 14-carbon zigzag chain with C 1 double bonded to O and bonded to S further bonded to Co A and a trans double bond between C 5 and C 6.
1. Why did Yu and colleagues need to use CoA derivatives rather than the free fatty acids in these experiments?
2. Why were no lower molecular weight CoA derivatives found in the reaction with stearoyl-CoA?
3. How many rounds of β oxidation would be required to convert the oleoyl-CoA and the elaidoyl-CoA to $c i s - Δ^{5}$ $c i s hyphen upper Delta Superscript 5$ -tetradecenoyl-CoA and $t r a n s - Δ^{5}$ $t r a n s hyphen upper Delta Superscript 5$ -tetradecenoyl-CoA, respectively?
  Yu and coworkers measured the kinetic parameters of two forms of the enzyme acyl-CoA dehydrogenase: long-chain acyl-CoA dehydrogenase (LCAD) and very-long-chain acyl-CoA dehydrogenase (VLCAD). They used the CoA derivatives of three fatty acids: tetradecanoyl-CoA $left-parenthesis upper C Subscript 14 Baseline hyphen CoA right-parenthesis$ $left-parenthesis upper C Subscript 14 Baseline hyphen CoA right-parenthesis$ , $c i s - Δ^{5}$ $c i s hyphen upper Delta Superscript 5$ -tetradecenoyl-CoA $left-parenthesis c upper Delta Superscript 5 Baseline upper C Subscript 14 Baseline hyphen CoA right-parenthesis$ $left-parenthesis c upper Delta Superscript 5 Baseline upper C Subscript 14 Baseline hyphen CoA right-parenthesis$ , and $t r a n s - Δ^{5}$ $t r a n s hyphen upper Delta Superscript 5$ -tetradecenoyl-CoA $(t Δ^{5} C_{14} -CoA)$ $left-parenthesis t upper Delta Superscript 5 Baseline upper C Subscript 14 Baseline hyphen CoA right-parenthesis$ . The results are shown below. (See Chapter 6 for definitions of the kinetic parameters.)

	LCAD			VLCAD
	$C_{14} - CoA$ $bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$	$c Δ^{5} C_{14} - CoA$ $bold c upper Delta Superscript bold 5 Baseline bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$	$t Δ^{5} C_{14} - CoA$ $bold t upper Delta Superscript bold 5 Baseline bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$	$C_{14} - CoA$ $bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$	$c Δ^{5} C_{14} - CoA$ $bold c upper Delta Superscript bold 5 Baseline bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$	$t Δ^{5} C_{14} - CoA$ $bold t upper Delta Superscript bold 5 Baseline bold upper C Subscript bold 1 bold 4 Baseline zero width space bold minus bold CoA$
$V_{\max}$ $upper V Subscript max$	3.3	3.0	2.9	1.4	0.32	0.88
$K_{m}$ $upper K Subscript m$	0.41	0.40	1.6	0.57	0.44	0.97
$k_{cat}$ $k Subscript cat$	9.9	8.9	8.5	2.0	0.42	1.12
$k_{cat} / K_{m}$ $k Subscript cat Baseline zero width space slash upper K Subscript m Baseline$	24	22	5	4	1	1

For LCAD, the $K_{m}$ $upper K Subscript m$ differs dramatically for the cis and trans substrates. Provide a plausible explanation for this observation in terms of the structures of the substrate molecules. (Hint: You may want to refer to Fig. 10-1.)
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 pathway. What evidence is there to support this assumption?
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. This enzyme was approximately twice as active with $C_{14} -CoA$ $upper C Subscript 14 Baseline hyphen CoA$ thioesters as with $C_{18} -CoA$ $upper C Subscript 18 Baseline hyphen CoA$ thioesters.
Other research has suggested that free fatty acids can pass through membranes. In their experiments, Yu and colleagues found $t r a n s - Δ^{5}$ $t r a n s hyphen upper Delta Superscript 5$ -tetradecenoic acid outside (i.e., in the medium surrounding) mitochondria that had been incubated with elaidoyl-CoA. Describe the pathway that led to this extramitochondrial $t r a n s - Δ^{5}$ $t r a n s hyphen upper Delta Superscript 5$ -tetradecenoic acid. Be sure to indicate where in the cell the various transformations take place, as well as the enzymes that catalyze the transformations.
It is sometimes 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 oversimplification?

Reference

Yu, W., X. Liang, R. Ensenauer, J. Vockley, L. Sweetman, and H. Schultz. 2004. Leaky β-oxidation of a trans-fatty acid. J. Biol. Chem. 279:52,160–52,167.