Deck 10: The Citrate Cycle

Question 1

Where do citrate cycle reactions in eukaryotic cells take place?

A) cytosol
B) mitochondrial matrix
C) endoplasmic reticulum
D) nucleus

Answer 1

mitochondrial matrix

Question 2

Identify the strongest reductant in the table below.

A) O₂
B) oxaloacetate
C) pyruvate
D) ferredoxin

Answer 2

ferredoxin

Question 3

Calculate the $\Delta$ G $\degree$ ^' of a redox reaction of pyruvate and hydrogen under standard conditions using the table below.

A) 18.33 kJ/mol
B) 36.66 kJ/mol
C) 54.99 kJ/mol
D) 73.32 kJ/mol

Answer 3

36.66 kJ/mol

Question 4

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

Question 5

Identify the strongest oxidant in the table below.

A) O₂
B) oxaloacetate
C) pyruvate
D) ferredoxin

Question 6

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

A) 9
B) 10
C) 18
D) 20

Question 7

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.

Question 8

Calculate the $\Delta$ E $\degree$ ^' of a coupled redox reaction with O₂ and ferredoxin (Fe²⁺) using the table below.

A) "-1.25"
B) "-0.39"
C) "0.39"
D) "1.25"

Question 9

Which enzyme requires CoASH to produce acetyl-CoA?

A) "pyruvate dehydrogenase"
B) "citrate synthase"
C) "isocitrate dehydrogenase"
D) " $\alpha$ -ketoglutarate dehydrogenase"

Question 10

The primary function of the citrate cycle is to oxidize

A) glucose.
B) pyruvate.
C) acetyl-CoA.
D) citrate.

Question 11

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

Question 12

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

A) CO₂
B) H₂O
C) GTP
D) CoA

Question 13

Which enzyme in the citrate cycle produces NADH?

A) aconitase
B) citrate synthase
C) isocitrate dehydrogenase
D) fumarase

Question 14

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

A) FAD
B) H₂O
C) H⁺
D) NAD⁺

Question 15

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) " $\alpha$ -ketoglutarate dehydrogenase"

Question 16

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

A) "pyruvate dehydrogenase"
B) "citrate synthase"
C) "isocitrate dehydrogenase"
D) " $\alpha$ -ketoglutarate dehydrogenase"

Question 17

Which molecules function as electron acceptors in the citrate cycle?

A) NAD⁺, FAD
B) NADH, FADH₂
C) GDP, ADP
D) GTP, ATP

Question 18

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) " $\alpha$ -ketoglutarate dehydrogenase"

Question 19

Which enzyme in the citrate cycle is activated by CoA?

A) "pyruvate dehydrogenase"
B) "citrate synthase"
C) "isocitrate dehydrogenase"
D) " $\alpha$ -ketoglutarate dehydrogenase"

Question 20

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.

Question 21

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

Question 22

Identify the E2 subunit in the reaction schematic of the pyruvate dehydrogenase reaction below.

A) pyruvate dehydrogenase
B) dihydrolipoyl dehydrogenase
C) dihydrolipopyl acetyltransferase
D) flavin adenine dinucleotide

Question 23

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.

Question 24

The disease beriberi is a result of which vitamin deficiency?

A) thiamine
B) pantothenic acid
C) riboflavin
D) niacin

Question 25

Which coenzyme in the citrate cycle is affected by arsenic?

A) coenzyme A
B) thiamine pyrophosphate
C) dihydrolipopyl acetyltransferase
D) flavin adenine dinucleotide

Question 26

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.

Question 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

Question 28

Coenzyme A is derived from which of the following vitamins?

A) thiamine
B) pantothenic acid
C) riboflavin
D) niacin

Question 29

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

Question 30

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.

Question 31

Identify the E1 subunit in the reaction schematic of the pyruvate dehydrogenase reaction below.

A) pyruvate dehydrogenase
B) dihydrolipoyl dehydrogenase
C) dihydrolipopyl acetyltransferase
D) flavin adenine dinucleotide

Question 32

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

A) pyruvate dehydrogenase and succinate dehydrogenase
B) pyruvate dehydrogenase and $\alpha$ -ketoglutarate dehydrogenase
C) malate dehydrogenase and succinate dehydrogenase
D) malate dehydrogenase and $\alpha$ -ketoglutarate dehydrogenase

Question 33

Classify the reaction that occurs at step 5 in the reaction schematic of the pyruvate dehydrogenase reaction below.

A) isomerization
B) addition
C) oxidation reduction
D) decarboxylation

Question 34

Classify the reaction that occurs at step 4 in the reaction schematic of the pyruvate dehydrogenase reaction below.

A) isomerization
B) addition
C) oxidation reduction
D) decarboxylation

Question 35

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.

Question 36

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.

Question 37

In linked metabolic pathways, the oxidants in subsequent reactions must

A) result in negative $\Delta$ E $\degree$ ^' values at each reaction step.
B) result in positive $\Delta$ G $\degree$ ^' values at each reaction step.
C) have progressively lower standard reduction potentials.
D) have progressively higher standard reduction potentials.

Question 38

Classify the reaction that occurs at step 1 in the reaction schematic of the pyruvate dehydrogenase reaction below.

A) isomerization
B) addition
C) oxidation reduction
D) decarboxylation

Question 39

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

Question 40

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

A) pyruvate dehydrogenase and succinate dehydrogenase
B) pyruvate dehydrogenase and $\alpha$ -ketoglutarate dehydrogenase
C) malate dehydrogenase and succinate dehydrogenase
D) malate dehydrogenase and $\alpha$ -ketoglutarate dehydrogenase

Question 41

Which reaction in the citrate cycle produces NADH?

A) 4
B) 5
C) 6
D) 7

Question 42

Why is the $\Delta$ G $\degree$ ^' 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 $\Delta$ G $\degree$ ^' .
D) The hydrolysis of the thioester bond in citryl-CoA results in the highly exergonic reaction.

Question 43

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

Question 44

Which reaction in the citrate cycle produces FADH₂?

A) 3
B) 4
C) 5
D) 6

Question 45

An in vitro study shows that $\alpha$ -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

Question 46

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

A) activation of citrate synthase.
B) inhibition of $\alpha$ -ketoglutarate dehydrogenase.
C) inhibition of fumarase.
D) activation of $\alpha$ -ketoglutarate dehydrogenase.

Question 47

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

A) "citrate synthase"
B) "fumarase"
C) "isocitrate dehydrogenase"
D) " $\alpha$ -ketoglutarate dehydrogenase"

Question 48

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

A) AMP
B) ADP
C) NAD⁺
D) ATP

Question 49

Which reactions in the citrate cycle produce CO₂?

A) 1 and 2
B) 2 and 3
C) 3 and 4
D) 4 and 5

Question 50

Which reaction in the citrate cycle produces CO₂?

A) 1
B) 2
C) 3
D) 5

Question 51

Which reaction in the citrate cycle produces GTP?

A) 3
B) 4
C) 5
D) 6

Question 52

According to the $\Delta$ G $\degree$ ^' , 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) " $\alpha$ -ketoglutarate dehydrogenase"

Question 53

Which reactions in the citrate cycle produce NADH?

A) 1, 2, and 4
B) 3, 4, and 8
C) 3, 6, and 7
D) 5, 6, and 8

Question 54

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

A) fumarase
B) succinate dehydrogenase
C) malate dehydrogenase
D) aconitase

Question 55

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

A) "isocitrate dehydrogenase"
B) " $\alpha$ -ketoglutarate dehydrogenase"
C) "citrate synthase"
D) "succinyl-CoA synthetase"

Question 56

The reaction catalyzed by is likely to be reversible under cellular conditions according to the $\Delta$ G $\degree$ ^' .

A) "malate dehydrogenase"
B) "citrate synthase"
C) "succinate dehydrogenase"
D) " $\alpha$ -ketoglutarate dehydrogenase"

Question 57

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.

Question 58

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.

Question 59

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

Question 60

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

Question 61

Use the figure below to answer the following questions.
A. The electrochemical cell shown here is measuring the reduction potential of Fe²⁺/Fe³⁺ using a reference hydrogen half reaction. Write each half reaction on the figure and show the flow of electrons.
B. Do these oxidants have a higher or lower affinity for electrons compared with H⁺? Explain how you know this using information from the experiment.

Question 62

Which citrate cycle intermediate is also used in gluconeogenesis?

A) "oxaloacetate"
B) " $\alpha$ -ketoglutarate"
C) "fumarate"
D) "succinate"

Question 63

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 $\alpha$ -ketoglutarate"
C) " $\alpha$ -ketoglutarate and citrate"
D) "citrate and malate"

Question 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

Question 65

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

A) anabolic
B) catabolic
C) anaplerotic
D) amphibolic

Question 66

Describe the fate of NADH and FADH₂ produced in the citrate cycle.

Question 67

Use the overall net reaction of the citrate cycle to explain why excess NADH inhibits the pyruvate dehydrogenase reaction.

Question 68

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

Question 69

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

A) "succinyl-CoA"
B) "malate"
C) " $\alpha$ -ketoglutarate"
D) "fumarate"

Question 70

What two features of the citrate cycle make it unique compared with linear metabolic pathways?

Question 71

The anaplerotic reaction catalyzed by pyruvate carboxylase requires which coenzyme?

A) niacin
B) biotin
C) riboflavin
D) thiamine

Question 72

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

Question 73

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

Question 74

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

A) pyruvate carboxylase
B) malic enzyme
C) phosphoenolpyruvate carboxylase
D) malate dehydrogenase

Question 75

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

A) "succinyl Co-A"
B) "oxaloacetate"
C) " $\alpha$ -ketoglutarate"
D) "malate"

Question 76

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.

Question 77

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

Question 78

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

Question 79

Predict the fate of $\alpha$ -ketoglutarate when it is not used in the citrate cycle.

A) cholesterol synthesis
B) heme synthesis
C) gluconeogenesis
D) amino acid synthesis

Question 80

Compare and contrast the functions of NAD⁺ and FAD in the citrate cycle.