Chapter 10: The Citrate Cycle

1. Which molecules function as electron acceptors in the citrate cycle?

a. NAD+, FAD

b. NADH, FADH2

c. GDP, ADP

d. GTP, ATP

ANS: A DIF: Easy REF: 10.1

OBJ: 10.1.a. Describe the role of NAD+ and FAD in the citrate cycle.

MSC: Remembering

2. In two turns of the citrate cycle, how many electrons are transferred from the citrate cycle

intermediates to NAD+ and FAD?

a. 4

b. 8

c. 12

d. 16

ANS: D DIF: Easy REF: 10.1

OBJ: 10.1.a. Describe the role of NAD+ and FAD in the citrate cycle.

MSC: Applying

3. The primary function of NAD+ in the citrate cycle is that it

a. functions as an electron donor.

b. is oxidized to produce GTP.

c. acts as an electron acceptor.

d. phosphorylates ADP.

ANS: C DIF: Easy REF: 10.1

OBJ: 10.1.a. Describe the role of NAD+ and FAD in the citrate cycle.

MSC: Remembering

4. Regeneration of NAD+ and FAD inside the mitochondrial matrix is required because

a. anabolic reactions generally require them.

b. they produce GDP through the citrate cycle.

c. they transport pyruvate through the matrix.

d. they maintain flux through the citrate cycle.

ANS: D DIF: Medium REF: 10.1

OBJ: 10.1.a. Describe the role of NAD+ and FAD in the citrate cycle.

MSC: Analyzing

5. Where do citrate cycle reactions in eukaryotic cells take place?

a. cytosol

b. mitochondrial matrix

c. endoplasmic reticulum

d. nucleus

ANS: B DIF: Easy REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Remembering

6. The primary function of the citrate cycle is to oxidize

a. glucose.

b. pyruvate.

c. acetyl-CoA.

d. citrate.

ANS: C DIF: Easy REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Remembering

7. How many ATP are produced from two turns of the citrate cycle?

a. 9

b. 10

c. 18

d. 20

ANS: D DIF: Medium REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Applying

8. Which of the following is a reactant in the net reaction of the citrate cycle?

a. CO2

b. H2O

c. GTP

d. CoA

ANS: B DIF: Easy REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Remembering

9. Which of the following is a product of the net reaction of the citrate cycle?

a. FAD

b. H2O

c. H+

d. NAD+

ANS: C DIF: Easy REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Remembering

10. Which molecule in the net reaction of the citrate cycle contributes to the inhibition of

pyruvate dehydrogenase?

a. FAD

b. H2O

c. H+

d. NADH

ANS: D DIF: Medium REF: 10.1

OBJ: 10.1.b. State the net reaction of the citrate cycle. MSC: Applying

11. Which enzyme regulates the flux of acetyl-CoA through the citrate cycle?

a. pyruvate dehydrogenase

b. citrate synthase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: A DIF: Easy REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Remembering

12. Which enzyme in the citrate cycle is activated by CoA?

a. pyruvate dehydrogenase

b. citrate synthase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: A DIF: Easy REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Applying

13. Which enzyme of the citrate cycle catalyzes the oxidative decarboxylation reaction that

produces CO2, NADH, and succinyl-CoA?

a. pyruvate dehydrogenase

b. citrate synthase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: D DIF: Easy REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Remembering

14. The poison compound 1080 converts fluoroacetate to fluorocitrate. Which enzyme in the

citrate cycle is inhibited by this poison?

a. aconitase

b. citrate synthase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: A DIF: Difficult REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Applying

15. Which enzyme in the citrate cycle produces NADH?

a. aconitase

b. citrate synthase

c. isocitrate dehydrogenase

d. fumarase

ANS: C DIF: Easy REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Remembering

16. Which enzyme requires CoASH to produce acetyl-CoA?

a. pyruvate dehydrogenase

b. citrate synthase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: A DIF: Easy REF: 10.1

OBJ: 10.1.c. List the functions of the key enzymes of the citrate cycle.

MSC: Remembering

21. In linked metabolic pathways, the oxidants in subsequent reactions must

a. result in negative E° values at each reaction step.

b. result in positive G° values at each reaction step.

c. have progressively lower standard reduction potentials.

d. have progressively higher standard reduction potentials.

ANS: D DIF: Medium REF: 10.1

OBJ: 10.1.d. Explain how change in free energy can be found from differences in reduction

potential. MSC: Understanding

22. Coenzyme A is derived from which of the following vitamins?

a. thiamine

b. pantothenic acid

c. riboflavin

d. niacin

ANS: B DIF: Easy REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Remembering

23. Which vitamin is the precursor to the coenzyme that functions as a reductant in the pyruvate

dehydrogenase complex in the final step of the reaction?

a. thiamine

b. pantothenic acid

c. riboflavin

d. niacin

ANS: C DIF: Medium REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Remembering

24. Pantothenic acid is essential for life because it is the vitamin precursor to the molecule that

a. provides a reactive disulfide that can participate in redox reactions within the enzyme

active site of pyruvate dehydrogenase.

b. can accept one or two electrons in redox reactions in the cell.

c. is involved in at least 200 redox reactions in the cell.

d. is a cofactor in the biosynthetic pathways that produce fatty acids, acetylcholine,

heme, and cholesterol.

ANS: D DIF: Medium REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Analyzing

25. Thiamine pyrophosphate functions as a coenzyme in which reactions in the citrate cycle?

a. pyruvate dehydrogenase and succinate dehydrogenase

b. pyruvate dehydrogenase and a-ketoglutarate dehydrogenase

c. malate dehydrogenase and succinate dehydrogenase

d. malate dehydrogenase and a-ketoglutarate dehydrogenase

ANS: B DIF: Difficult REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Remembering

26. The disease beriberi is a result of which vitamin deficiency?

a. thiamine

b. pantothenic acid

c. riboflavin

d. niacin

ANS: A DIF: Easy REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Remembering

27. Identify the coenzyme that provides a reactive disulfide that participates in the redox reaction

in the active site of pyruvate dehydrogenase.

a. NAD+

b. coenzyme A

c. lipoamide

d. thiamine pyrophosphate

ANS: C DIF: Easy REF: 10.2

OBJ: 10.2.a. Explain the importance of the vitamins niacin, riboflavin, pantothenic acid, and

thiamine to the citrate cycle. MSC: Understanding

33. What is the purpose of the first 3 steps in the pyruvate dehydrogenase reaction?

a. regenerate the oxidized form of lipoamide

b. form NADH

c. transfer electrons

d. form acetyl-CoA

ANS: D DIF: Medium REF: 10.2

OBJ: 10.2.b. Classify the organic reaction that converts pyruvate into acetyl-CoA.

MSC: Applying

34. Which coenzyme in the citrate cycle is affected by arsenic?

a. coenzyme A

b. thiamine pyrophosphate

c. dihydrolipopyl acetyltransferase

d. flavin adenine dinucleotide

ANS: C DIF: Easy REF: 10.2

OBJ: 10.2.c. Summarize the effect of arsenic on pyruvate dehydrogenase.

MSC: Remembering

35. Which of the following is the cause of the irreversible blockage of the catalytic activity of

lipoamide-containing enzymes?

a. cheilosis

b. pellagra

c. beriberi

d. arsenic poisoning

ANS: D DIF: Easy REF: 10.2

OBJ: 10.2.c. Summarize the effect of arsenic on pyruvate dehydrogenase.

MSC: Understanding

36. A patient seeks medical attention for ulcerous skin lesions. The patient is diagnosed with

a. arsenic exposure.

b. a deficiency in vitamin B3.

c. beriberi.

d. cheilosis.

ANS: A DIF: Medium REF: 10.2

OBJ: 10.2.c. Summarize the effect of arsenic on pyruvate dehydrogenase.

MSC: Applying

37. Which two enzymes in the citrate cycle are affected by arsenic poisoning?

a. pyruvate dehydrogenase and succinate dehydrogenase

b. pyruvate dehydrogenase and a-ketoglutarate dehydrogenase

c. malate dehydrogenase and succinate dehydrogenase

d. malate dehydrogenase and a-ketoglutarate dehydrogenase

ANS: B DIF: Medium REF: 10.2

OBJ: 10.2.c. Summarize the effect of arsenic on pyruvate dehydrogenase.

MSC: Remembering

38. Arsenite affects the lipoamide coenzymes of the pyruvate dehydrogenase reaction by

a. acting as a competitive inhibitor.

b. covalently modifying the coenzyme.

c. being a noncompetitive inhibitor.

d. modifying the coenzyme through electrostatic interactions.

ANS: B DIF: Medium REF: 10.2

OBJ: 10.2.c. Summarize the effect of arsenic on pyruvate dehydrogenase.

MSC: Understanding

39. If acetyl-CoA is not metabolized by the citrate cycle, the molecule in the cell

a. undergoes fatty acid metabolism.

b. is transported across the cell membrane.

c. is used to synthesize amino acids.

d. is used during glycolysis.

ANS: A DIF: Easy REF: 10.2

OBJ: 10.2.d. Hypothesize why the activity of pyruvate dehydrogenase is so tightly controlled

by the cell. MSC: Analyzing

40. How would an increase in Ca2+ be expected to affect the pyruvate dehydrogenase reaction?

a. The pyruvate dehydrogenase kinase enzyme activity would increase, resulting in an

inhibition of pyruvate dehydrogenase activity.

b. The last step of the pyruvate dehydrogenase reaction is blocked, resulting in a

decrease in activity.

c. The E1 subunit is phosphorylated by pyruvate dehydrogenase kinase, and the

catalytic activity of pyruvate dehydrogenase decreases.

d. The pyruvate dehydrogenase phosphatase-1 enzyme would increase, resulting in

pyruvate dehydrogenase activation at an accelerated rate.

ANS: D DIF: Medium REF: 10.2

OBJ: 10.2.d. Hypothesize why the activity of pyruvate dehydrogenase is so tightly controlled

by the cell. MSC: Applying

41. How is the pyruvate dehydrogenase reaction regulated?

a. pH and enzyme conformation

b. pH and the protonation state of the active site

c. allosteric control and covalent modification

d. product inhibition

ANS: C DIF: Easy REF: 10.2

OBJ: 10.2.d. Hypothesize why the activity of pyruvate dehydrogenase is so tightly controlled

by the cell. MSC: Understanding

42. How would a high NADH to NAD+ ratio be expected to affect the pyruvate dehydrogenase

reaction?

a. The pyruvate dehydrogenase kinase enzyme activity would increase, resulting in an

increase in pyruvate dehydrogenase activity.

b. The last step of the pyruvate dehydrogenase reaction is blocked, resulting in a

decrease in activity.

c. The E1 subunit is phosphorylated by pyruvate dehydrogenase kinase, and the

catalytic activity of pyruvate dehydrogenase decreases.

d. The pyruvate dehydrogenase phosphatase-1 enzyme would increase, resulting in

pyruvate dehydrogenase activation at an accelerated rate.

ANS: B DIF: Medium REF: 10.2

OBJ: 10.2.d. Hypothesize why the activity of pyruvate dehydrogenase is so tightly controlled

by the cell. MSC: Applying

43. How would an increased level of acetyl-CoA be expected to affect the pyruvate dehydrogenase

reaction?

a. The pyruvate dehydrogenase kinase enzyme activity would increase, resulting in an

inhibition of pyruvate dehydrogenase activity.

b. The last step of the pyruvate dehydrogenase reaction would be blocked, resulting in a

decrease in activity.

c. The E1 subunit would be phosphorylated by pyruvate dehydrogenase kinase, and the

catalytic activity of pyruvate dehydrogenase would decrease.

d. The pyruvate dehydrogenase phosphatase-1 enzyme would increase, resulting in

pyruvate dehydrogenase activation at an accelerated rate.

ANS: A DIF: Medium REF: 10.2

OBJ: 10.2.d. Hypothesize why the activity of pyruvate dehydrogenase is so tightly controlled

by the cell. MSC: Applying

44. The reaction catalyzed by __________is the most endergonic reaction in the citrate cycle.

a. fumarase

b. succinate dehydrogenase

c. malate dehydrogenase

d. aconitase

ANS: C DIF: Easy REF: 10.3

OBJ: 10.3.a. Classify the eight reactions of the citrate cycle as either exergonic or

endergonic.

MSC: Remembering

45. The reaction catalyzed by __________ is the most exergonic reaction in the citrate cycle.

a. citrate synthase

b. fumarase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: D DIF: Easy REF: 10.3

OBJ: 10.3.a. Classify the eight reactions of the citrate cycle as either exergonic or

endergonic.

MSC: Remembering

46. Why is the G° of the condensation of oxaloacetate and acetyl-CoA highly favorable?

a. The addition of inorganic phosphate provides energy, resulting in the highly

favorable nature of this reaction.

b. Allosteric regulations of the reaction cause a release of energy, making the reaction

thermodynamically favorable.

c. The iron-sulfur cluster in the enzyme reduces the activation energy of the reaction,

resulting in the favorable G°.

d. The hydrolysis of the thioester bond in citryl-CoA results in the highly exergonic

reaction.

ANS: D DIF: Medium REF: 10.3

OBJ: 10.3.a. Classify the eight reactions of the citrate cycle as either exergonic or

endergonic.

MSC: Understanding

47. The reaction catalyzed by __________ is likely to be reversible under cellular conditions

according to the G°.

a. malate dehydrogenase

b. citrate synthase

c. succinate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: C DIF: Medium REF: 10.3

OBJ: 10.3.a. Classify the eight reactions of the citrate cycle as either exergonic or

endergonic.

MSC: Applying

48. According to the G°, which of the following exergonic reactions is most likely irreversible

under normal cellular conditions and is considered to be the rate-limiting step of the citrate

cycle?

a. citrate synthase

b. fumarase

c. isocitrate dehydrogenase

d. a-ketoglutarate dehydrogenase

ANS: C DIF: Easy REF: 10.3

OBJ: 10.3.a. Classify the eight reactions of the citrate cycle as either exergonic or

endergonic.

MSC: Remembering

55. A high concentration of which molecule would inhibit citrate synthase in the citrate cycle?

a. AMP

b. ADP

c. NAD+

d. ATP

ANS: D DIF: Easy REF: 10.4

OBJ: 10.4.a. Hypothesize whether the citrate cycle would be activated or inhibited by high

concentrations of AMP, ADP, or ATP. MSC: Understanding

56. What enzyme in the citrate cycle is activated by high concentrations of AMP?

a. isocitrate dehydrogenase

b. a-ketoglutarate dehydrogenase

c. citrate synthase

d. succinyl-CoA synthetase

ANS: B DIF: Easy REF: 10.4

OBJ: 10.4.a. Hypothesize whether the citrate cycle would be activated or inhibited by high

concentrations of AMP, ADP, or ATP. MSC: Understanding

57. An in vitro study shows that isocitrate dehydrogenase is activated in the citrate cycle. What is

a possible explanation for the activation?

a. high levels of ATP

b. low levels of ATP

c. high levels of NADH

d. low levels of AMP

ANS: B DIF: Medium REF: 10.4

OBJ: 10.4.a. Hypothesize whether the citrate cycle would be activated or inhibited by high

concentrations of AMP, ADP, or ATP. MSC: Applying

58. An in vitro study shows that a-ketoglutarate dehydrogenase is inhibited in the citrate cycle.

What is a possible explanation for this inhibition?

a. high levels of ATP

b. low levels of Ca2+

c. low levels of NADH

d. high levels of ADP

ANS: A DIF: Medium REF: 10.4

OBJ: 10.4.a. Hypothesize whether the citrate cycle would be activated or inhibited by high

concentrations of AMP, ADP, or ATP. MSC: Applying

59. High levels of ATP would result in the inhibition of which enzyme in the citrate cycle?

a. succinate dehydrogenase

b. isocitrate dehydrogenase

c. malate dehydrogenase

d. fumarase

ANS: B DIF: Easy REF: 10.4

OBJ: 10.4.a. Hypothesize whether the citrate cycle would be activated or inhibited by high

concentrations of AMP, ADP, or ATP. MSC: Applying

60. In the citrate cycle, a high concentration of NADH would result in

a. activation of citrate synthase.

b. inhibition of a-ketoglutarate dehydrogenase.

c. inhibition of fumarase.

d. activation of a-ketoglutarate dehydrogenase.

ANS: B DIF: Medium REF: 10.4

OBJ: 10.4.b. Describe the effect of high concentrations of NADH on the citrate cycle.

MSC: Applying

61. An in vitro study shows that citrate synthase is inhibited in the citrate cycle. What is a possible

explanation for this inhibition?

a. high levels of ADP

b. low levels of succinyl-CoA

c. high levels of NADH

d. high levels of AMP

ANS: C DIF: Easy REF: 10.4

OBJ: 10.4.b. Describe the effect of high concentrations of NADH on the citrate cycle.

MSC: Applying

62. A high NADH to NAD+ ratio would inhibit which enzyme in the citrate cycle?

a. succinate dehydrogenase

b. succinyl-CoA synthetase

c. aconitase

d. isocitrate dehydrogenase

ANS: D DIF: Easy REF: 10.4

OBJ: 10.4.b. Describe the effect of high concentrations of NADH on the citrate cycle.

MSC: Applying

63. The regulatory mechanism in the citrate cycle involving the NADH to NAD+ ratio is

considered to be an example of regulation by

a. pH and protonation state.

b. product inhibition.

c. covalent modification.

d. pH and enzyme conformation.

ANS: B DIF: Medium REF: 10.4

OBJ: 10.4.b. Describe the effect of high concentrations of NADH on the citrate cycle.

MSC: Applying

64. A low NADH to NAD+ ratio would activate which enzyme in the citrate cycle?

a. succinyl-CoA synthetase

b. fumarase

c. citrate synthase

d. succinate dehydrogenase

ANS: C DIF: Medium REF: 10.4

OBJ: 10.4.b. Describe the effect of high concentrations of NADH on the citrate cycle.

MSC: Applying

65. The citrate cycle is considered to be a(n) __________ pathway.

a. anabolic

b. catabolic

c. anaplerotic

d. amphibolic

ANS: D DIF: Easy REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Remembering

66. Which anaplerotic reaction balances the input of oxaloacetate with acetyl-CoA in the citrate

cycle by converting pyruvate into oxaloacetate?

a. pyruvate carboxylase

b. malate dehydrogenase

c. malic enzyme

d. pyruvate kinase

ANS: A DIF: Easy REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Understanding

67. The anaplerotic reaction catalyzed by pyruvate carboxylase requires which coenzyme?

a. niacin

b. biotin

c. riboflavin

d. thiamine

ANS: B DIF: Easy REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Remembering

68. When the citrate cycle is inhibited, which two metabolites are exported to the cytosol for

fatty acid and cholesterol synthesis?

a. malate and succinyl-CoA

b. succinyl-CoA and a-ketoglutarate

c. a-ketoglutarate and citrate

d. citrate and malate

ANS: D DIF: Easy REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Understanding

69. Which citrate cycle intermediate is siphoned off the citrate cycle during starvation?

a. succinyl-CoA

b. malate

c. a-ketoglutarate

d. fumarate

ANS: B DIF: Difficult REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Applying

70. Which anaplerotic reactions do plants, yeast, and bacteria use to generate oxaloacetate?

a. pyruvate carboxylase

b. malic enzyme

c. phosphoenolpyruvate carboxylase

d. malate dehydrogenase

ANS: C DIF: Medium REF: 10.5

OBJ: 10.5.a. List the anaplerotic reactions of the citrate cycle.

MSC: Remembering

71. Which citrate cycle metabolite is used as a precursor for heme biosynthesis?

a. succinyl Co-A

b. oxaloacetate

c. a-ketoglutarate

d. malate

ANS: A DIF: Easy REF: 10.5

OBJ: 10.5.b. Describe the potential fates of oxaloacetate, alpha-ketoglutarate,

and succinyl-CoA in the cell. MSC: Understanding

72. What is the fate of oxaloacetate when it is not used in the citrate cycle?

a. cholesterol synthesis

b. heme synthesis

c. amino acid synthesis

d. gluconeogenesis

ANS: C DIF: Medium REF: 10.5

OBJ: 10.5.b. Describe the potential fates of oxaloacetate, alpha-ketoglutarate,

and succinyl-CoA in the cell. MSC: Applying

73. Predict the fate of a-ketoglutarate when it is not used in the citrate cycle.

a. cholesterol synthesis

b. heme synthesis

c. gluconeogenesis

d. amino acid synthesis

ANS: D DIF: Easy REF: 10.5

OBJ: 10.5.b. Describe the potential fates of oxaloacetate, alpha-ketoglutarate,

and succinyl-CoA in the cell. MSC: Applying

74. Which citrate cycle intermediate is also used in gluconeogenesis?

a. oxaloacetate

b. a-ketoglutarate

c. fumarate

d. succinate

ANS: A DIF: Medium REF: 10.5

OBJ: 10.5.b. Describe the potential fates of oxaloacetate, alpha-ketoglutarate,

and succinyl-CoA in the cell. MSC: Analyzing

75. Predict the fate of succinyl-CoA in the cell when it is not in the citrate cycle.

a. cholesterol synthesis

b. heme synthesis

c. gluconeogenesis

d. amino acid synthesis

ANS: B DIF: Easy REF: 10.5

OBJ: 10.5.b. Describe the potential fates of oxaloacetate, alpha-ketoglutarate,

and succinyl-CoA in the cell. MSC: Applying