Chapter 17 Test Bank

Chapter 17 The Citric Acid Cycle

Matching QuestionsUse the following to answer questions 1-10:

Choose the correct answer from the list below. Not all of the answers will be used.a) cytosolb) phosphorylationc) anapleroticd) mitochondriae) cis-aconitatef) arseniteg) metabolonh) oxaloacetatei) inner membranej) flavoproteinsk) carbon dioxidel) glyoxylate cycle

1 ____________ Where does the citric acid cycle take place in the cell?

Ans: dSection: Introduction

2 ____________ These proteins are tightly associated with FAD or FMN.

Ans: jSection: 17.1

3 ____________ This is the intermediate between citrate and isocitrate.

Ans: eSection: 17.2

4 ____________ This is the location of succinate dehydrogenase.

Ans: iSection: 17.2

5 ____________ This TCA intermediate is both at the beginning and at the end of the citric acid cycle.

Ans: hSection: 17.2

6 ____________ This is one of the products of the citric acid cycle.

Ans: k


Section: 17.2

7 ____________ This is a name suggested for associated multienzyme complexes in metabolism.

Ans: gSection: 17.2

8 ____________ This substance is toxic because it reacts with the neighboring sulfhydryl groups of dihydrolipoyl groups and blocks its reoxidation to lipoamide.

Ans: fSection: 17.4

9 ____________ This type of enzyme regulation process inhibits the pyruvate dehydrogenase complex.

Ans: bSection: 17.3

10 ____________ This is the name applied to metabolic reactions that replenish citric acid cycle intermediates that are depleted because they were used for biosynthesis.

Ans: cSection: 17.4

Fill in the Blank Questions

11 Carbons from carbohydrate enter the citric acid cycle in the form of _______________.Ans: acetyl CoA Section: Introduction

12 In the citric acid cycle, the __________ is produced by a substrate-level phosphorylation.Ans: GTP Section: 17.2

13 E1 of the pyruvate dehydrogenase complex requires the coenzyme ________________ for proper activity.Ans: thiamine pyrophospate Section: 17.1

14 E2 of the pyruvate dehydrogenase complex contains a lipoyl group that is covalently attached to a _______________ residue of the enzyme.Ans: lysine Section: 17.1

15 _______________ is a citric acid cycle enzyme that is also an example of an iron-sulfur protein.Ans: Aconitase or succinate dehydrogenase Section: 17.2

16 The ____________ cycle is a process by which plants and some bacteria can convert two-carbon acetyl units into four-carbon units (succinate) for glucose synthesis, energy production, and biosynthesis.Ans: glyoxylate Section: 17.5

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17 During the oxidation of isocitrate, the intermediate that is decarboxylated to form α-ketoglutarate is ______________________. Ans: oxalosuccinate Section: 17.2

18 In general, the citric acid cycle is inhibited under ________ (high, low) energy conditions.Ans: high Section: 17.3

19 ________________ is the first citric acid cycle intermediate to be oxidized.Ans: Isocitrate Section: 17.2

20 Beri-beri is caused by a deficiency of __________________.Ans: thiamine Section: 17.4

Multiple Choice Questions

21 The citric acid cycle is also known as the A) Krebs cycle. D) a and c. B) Cori cycle. E) a, b, and c. C) tricarboxylic acid cycle. Ans: D Section: Introduction

22 What molecule initiates the citric acid cycle by reacting with oxaloacetate? A) pyruvate D) All of the above. B) acetyl CoA E) None of the above. C) oxaloacetate Ans: B Section: 17.2

23 What enzyme(s) is (are) responsible for the following reaction?Pyruvate + CoA + NAD+ acetyl CoA + NADH + H+ + CO2

A) acetyl CoA synthetase D) a and b B) pyruvate decarboxylase E) a, b, and c C) pyruvate dehydrogenase complex Ans: C Section: 17.1

24 What are the steps involved (in order) in the conversion of pyruvate to acetyl CoA?A) decarboxylation, oxidation, transfer to CoA B) decarboxylation, transfer to CoA, oxidation C) oxidation, decarboxylation, transfer to CoA D) oxidation, transfer to CoA, decarboxylation E) None of the above. Ans: A Section: 17.1

25 Which of the following vitamins are precursors to coenzymes that are necessary for the formation of acetyl CoA from pyruvate? A) thiamine, riboflavin, niacin, lipoic acid, and pantothenic acid B) thiamine, riboflavin, niacin, lipoic acid, pantothenic acid, and biotin C) thiamine, riboflavin, niacin, and biotin D) thiamine, riboflavin, and lipoic acid E) none of the above

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Ans: A Section: 17.1 and Table 17.1

26 Which of the following functions as a “flexible swinging arm” when it transfers the reaction intermediate from one active site to the next?A) FADB) NAD+C) lipoamideD) thiamine pyrophosphateE) coenzyme AAns: C Section: 17.1

27 Formation of citrate from acetyl CoA and oxaloacetate is a(n) _________ reaction. A) oxidation D) ligation B) reduction E) None of the above. C) condensation Ans: C Section: 17.2

28 What is/are the chemical change(s) involved in the conversion of citrate into isocitrate? A) hydration followed by dehydration D) dehydration followed by hydration B) oxidation E) a and b C) oxidation followed by reduction Ans: D Section: 17.2

29 In which reaction is GTP (or ATP) directly formed in the citric acid cycle? A) conversion of succinyl CoA to succinate B) decarboxylation of -ketoglutarate C) conversion of isocitrate to -ketoglutarate D) All of the above. E) None of the above. Ans: A Section: 17.2

30 In which step of the citric acid cycle is FADH2 formed? A) the conversion of succinate to malate B) the conversion of succinate to oxaloacetate C) the conversion of succinate to fumarate D) the conversion of malate to oxaloacetate E) none of the above Ans: C Section: 17.2

31 Which of these compounds is oxidized by a multienzyme complex that requires five different coenzymes?A) D)

B) E)

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C)

Ans: B Section: 17.2

32 Which of the following conditions will activate pyruvate dehydrogenase kinase which catalyzes the phorphorylation and inactivation of E1 in the pyruvate dehydrogenase complex?A) elevated concentrations of NADH and ATPB) elevated concentrations of NAD+ and ADPC) Ca2+

D) insulinE) elevated concentrations of acetyl-CoAAns: A Section: 17.3

33 Approximately how many ATP or GTP equivalents are produced during one turn of the citric acid cycle? A) 10 B) 6 C) 9 D) 12 E) None of the above. Ans: A Section: 17.2

34 In addition to pyruvate dehydrogenase, what other enzymes are key regulatory sites in the citric acid cycle? A) isocitrate dehydrogenase D) a and b B) -ketoglutarate dehydrogenase E) a, b, and c C) citrate synthase (in bacteria) Ans: E Section: 17.3

35 The glyoxylate cycle enables plants to survive using only A) pyruvate. D) all of the above. B) acetate. E) none of the above. C) oxaloacetate. Ans: B Section: 17.4

Short-Answer Questions

36 Give the net equation of the citric acid cycle.Ans: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi

2 CO2 + 3 NADH + 3 H+ + FADH2 + GTP + CoASection: 17.2

37 Why is the isomerization of citrate to isocitrate a necessary step of the citric acid cycle?Ans: Citrate is a tertiary alcohol that cannot be oxidized. The isomerization converts the 3°

alcohol into isocitrate, which is a 2° alcohol that can be oxidized.Section: 17.2

38 List the five coenzymes that are required for the oxidative decarboxylation of pyruvate and α–ketoglutarate and give the essential nutrient (vitamin) that is required for each.Ans: 1. thiamine pyrophosphate: thiamine, vitamin B1

2. lipoamide: lipoic acid3. NAD+: niacin

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4. FAD: riboflavin, vitamin B2

5. coenzyme A: pantothenic acid

Section: 17.1 & 17.2

39 Explain why a GTP is energetically equivalent to an ATP in metabolism.Ans: The enzyme nucleoside diphosphokinase reversibly transfers a phosphoryl group from

GTP to ADP according the reaction:GTP + ADP GDP + ATPConversly, a phosphoryl group can be transferred from ATP to a GDP forming GTP.

Section: 17.2

40 Give the reaction in the citric acid cycle by which the energy is conserved in the formation of a phosphoanhydride bond by substrate level phosphorylation. Give the name of the enzyme that catalyzes this reaction and give the structures of the reactants and products of this reaction.Ans:

Section: 17.2

41 What reaction serves to link glycolysis and the citric acid cycle? Ans: Pyruvate + CoA + NAD+ acetyl CoA + NADH + H++ CO2

Section: 17.1

42 Why is the observed electron transfer from FADH2 to NAD+ unusual? Ans: It is unusual because the electrons are passed to the NAD+ from the FADH2. The transfer

is usually in the other direction. Section: 17.1

43 What is the energy source that drives the condensation of oxaloacetate and and acetyl CoA to produce citrate?Ans: Citrate synthase catalyzes the condensation of acetyl CoA and oxalacetate to form citryl

CoA. This reaction is easily reversible. The hydrolysis of the thioester of citryl CoA forms citrate and regenerates the CoA. The hydrolysis of the high energy thioester drives the reaction toward citrate.

Section: 17.2

44 How does the decarboxylation of -ketoglutatarate resemble that of pyruvate decarboxylation? Ans: Both are -ketoacids, which are decarboxylated, and involve formation of a thioester

with CoA, which has high transfer potential. The enzymatic complexes and mechanisms are similar, and the dihydrolipoyl dehydrogenase components are identical.

Section: 17.1 & 17.2

45 How many ATP equivalents are produced from the total oxidation of one pyruvate to 3 CO2.Ans: The total oxidation of one pyruvate by pyruvate dehydrogenase and the citric acid cycle

produces 4 NADH, 1 FADH2 and one GTP. 2.5 ATP are produced when two electrons are transferred from NADH to oxygen by the electron transport chain. 1.5 ATP are produced when two electrons are transferred from FADH2 to oxygen by the electron transport chain. Energetically, a GTP is equal to an ATP. So a total of 12.5 ATP equivalents are produced. (4 2.5 + 1.5 + 1 = 12.5)

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Section: 17.2

46 The ΔG˚′ = 21 kJ/mol for the reaction catalyzed by isocitrate dehydrogenase yet the ΔG˚′ = +29.7 kJ/mol for the reaction catalyzed by malate dehydrogenase. Both of these reactions involve the oxidation of a secondary alcohol. Give an explanation as to why the oxidation of isocitrate is so exergonic.Ans: The oxidation of isocitrate produces oxalosuccinate. The decarboxylation of

oxalosuccinate produces CO2 gas, which essentially eliminates the reverse reaction. The conversion of malate to oxaloacetate does not produce CO2 and is endergonic. In contrast, the loss of CO2 makes the conversion of isocitrate to α-ketoglutarate very favorable.

Section: 17.2

47 How is succinate dehydrogenase unique when compared to the other enzymes in the citric acid cycle? Ans: It is the only enzyme embedded in the mitochondrial membrane, and it is directly

associated with the electron transport chain. Section: 17.2

48 Are the acetyl carbons that enter the citric acid cycle the exact same carbons that leave as CO2? Briefly explain.Ans: No, the carbons are different. The carbons that leave as CO2 come from oxaloacetate that

condensed with acetyl CoA. However, since succinate is symmetrical, and the carbons randomize, eventually all carbons are turned over.

Section: 17.2

49 How does the term “mad as a hatter” realistically reflect the condition? Ans: Hatters used mercury in their craft. Frequently, the mercury would be absorbed and

would react with sulfhydryls, such as those on the dihydrolipoyl groups. This resulted in neurological pathologies.

Section:17.3

50 Give a sequence of metabolic reactions by which all six carbons in citrate could be obtained from two pyruvate molecules.Ans:

Section: 17.1 & 17.2

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Chapter 17 Test Bank

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