Deck 19: The Tricarboxylic Acid Cycle

Question 1

The two step reaction catalyzed by isocitrate dehydrogenase involves:
A)β-decarboxylation expelling the α-keto carboxyl as CO₂.
B)oxidation of the C-2 alcohol of isocitrate to form oxalosuccinatE.C.oxidation of the C-2 alcohol to form oxaloacetatE.D.β-elimination expelling the central carboxyl group as CO₂.

A) A and B
B) B and C
C) C and D
D) A and C
E) B and D

Answer 1

E

Question 2

Order the coenzymes according to their involvement in the pyruvate dehydrogenase complex.
A)NAD⁺
B)CoA-SH
C)TPP
D)Lipoate (lipoamide)
E)[FAD]

A) A, B, C, D, E
B) C, B, A, E, D
C) C, D, B, E, A
D) B, D, E, A, C
E) C, E, D, B, A

Answer 2

C

Question 3

Isocitrate dehydrogenase has all of the characteristics EXCEPT:

A) ADP raises the K_m for isocitrate by a factor of 10.
B) virtually inactive in the absence of ADP.
C) sufficiently exergonic to pull the aconitase reaction forward.
D) allosterically inhibited by NADH and ATP.
E) an oxidative-decarboxylation reaction.

Answer 3

A

Question 4

Which enzymes of the TCA cycle catalyze oxidative decarboxylation reactions?

A) malate dehydrogenase and citrate synthase
B) fumarase and succinate dehydrogenase
C) α-ketoglutarate dehydrogenase and succinate dehydrogenase
D) isocitrate dehydrogenase and α-ketoglutarate dehydrogenase
E) aconitase and succinate dehydrogenase

Question 5

The following statements are correct about the reaction catalyzed by the pyruvate dehydrogenase complex EXCEPT:

A) it is the link between glycolysis and the TCA cycle
B) The activity of the complex is regulated by the phosphorylation of E_PDH.
C) NAD⁺ is the direct oxidant of reduced lipoamide.
D) An FAD is covalently linked to the E_DLD, the lipoamide dehydrogenase component.
E) Although E_PDH is called "pyruvate dehydrogenase" it is not a dehydrogenase.

Question 6

Which is NOT true of citrate synthase?

A) it catalyzes the first reaction in the TCA cycle
B) it is not regulated
C) OAA and acetyl-CoA bind to the active sites
D) the citrate synthase reaction liberates a relatively large amount of energy
E) all of the above are true

Question 7

α-Ketoglutarate dehydrogenase is a multi-enzyme complex analogous to:

A) pyruvate kinase.
B) glyceraldehyde-3-phosphate dehydrogenase.
C) isocitrate dehydrogenase.
D) pyruvate dehydrogenase.
E) lactate dehydrogenase.

Question 8

Citrate synthase has a ____ ΔG°′ and is essentially ____ due to the ____ of the CoA thioester.

A) large positive; reversible; oxidation
B) nearly zero; reversible; reduction
C) nearly zero; irreversible; reduction
D) large negative; irreversible; hydrolysis
E) large negative; reversible; hydrolysis

Question 9

Citrate must be isomerized to isocitrate because the ____ group of citrate is not oxidizable, but the ____ group of isocitrate can be oxidized.

A) ketone; primary alcohol
B) tertiary alcohol; secondary alcohol
C) ketone; secondary alcohol
D) aldehyde; primary alcohol
E) secondary alcohol; ketone

Question 10

In eukaryotic cells, glycolysis occurs in the ____, and the TCA cycle reactions take place in ____.

A) mitochondria; mitochondria
B) cytoplasm; mitochondria
C) cytoplasm; cytoplasm
D) mitochondria; ribosomes
E) cytoplasm; ribosomes

Question 11

In the TCA cycle, carbon enters the cycle as ____ and exits as ____ with metabolic energy captured as ____, ____ and ____.

A) malonate; water; NADH; ATP; NADPH
B) acetyl-CoA; CO₂; NADH; ATP; NADPH
C) succinyl-CoA; CO₂; ATP; NADH; NADPH
D) acetyl-CoA; CO₂; ATP; NADH; [FADH₂]
E) malonyl-CoA; water; NADH; [FADH₂]; ATP

Question 12

All are correct statements about the reaction catalyzed by citrate synthase EXCEPT:

A) Citrate synthase is allosterically activated by ATP.
B) The complete chemical equation is: Oxaloacetate + acetyl-CoA + H₂O → citrate + CoA
C) Citryl-CoA is formed as an intermediate.
D) The mechanism involves attack of the carbanion of acetyl-CoA on the carbonyl carbon of oxaloacetate.
E) The enzyme uses general base catalysis to generate the reactive species.

Question 13

Allosteric inhibitors of isocitrate dehydrogenase include ____ and ____, whereas ____ acts as an allosteric activator, ____ the K_m for isocitrate.

A) ATP; NADH; ADP; lowering
B) ATP; ADP; AMP; lowering
C) NADH; NADPH; AMP; increasing
D) NADH; ATP; ADP; increasing
E) NADH; ATP; AMP; lowering

Question 14

Fluoroacetate is a potent inhibitor of the TCA cycle. Which step of the TCA cycle is inhibited as a result of fluoroacetate entering the TCA cycle?

A) citrate synthase
B) aconitase
C) isocitrate dehydrogenase
D) α-ketoglutarate dehydrogenase
E) succinyl-CoA synthetase

Question 15

All are true for aconitase EXCEPT:

A) iron atom acts as a Lewis acid.
B) equilibrium favors citrate.
C) contains an iron-sulfur cluster.
D) one Fe³⁺ coordinates with C-3 carbonyl and hydroxyl group of citrate.
E) all are true.

Question 16

The ____ of pyruvate to acetyl-CoA is catalyzed by ____.

A) dehydration; pyruvate dehydration complex
B) decarboxylation; pyruvate dehydrogenase complex
C) decarboxylation; pyruvate decarboxylase
D) transacylation; pyruvate transacylase
E) none of the above.

Question 17

The coenzymes listed below are associated with α-ketoglutarate dehydrogenase complex EXCEPT:

A) [FAD].
B) TPP.
C) lipoamide.
D) NAD⁺.
E) biotin.

Question 18

Citrate synthase is a ____ with binding of ____ inducing a conformational change ____ the binding of acetyl-CoA, and ____ is an allosteric inhibitor.

A) dimer; OAA; increasing; NADH
B) dimer; OAA; decreasing; NADH
C) tetramer; OAA; decreasing; NAD⁺
D) monomer; pyruvate; decreasing; ATP
E) monomer; pyruvate; increasing; ATP

Question 19

The pyruvate dehydrogenase complex contains three multimeric enzymes (E_TA, E_PDH, and E_DLD). All are properties of E_PDH EXCEPT:

A) It uses thiamin pyrophosphate as a catalytic coenzyme.
B) It oxidatively decarboxylates pyruvate.
C) It binds NAD⁺ in its active site.
D) It transfers an acetyl group to lipoamide of E_TA.
E) It forms a hydroxyethyl-TPP intermediate.

Question 20

____ catalyzes citrate isomerization to isocitrate by abstracting ____ from citrate to yield the enzyme-bound intermediate ____, followed by ____ to produce isocitrate.

A) Citrate isomerase; CO₂; trans-aconitate; carboxylation
B) Citrate isomerase; water; trans-aconitate; rehydration
C) Aconitase; water; cis-aconitate; rehydration
D) Aconitase; CO₂; cis-aconitate; carboxylation
E) None are true

Question 21

The succinate dehydrogenase catalyzed reaction involves dehydrogenation ____ to a carbonyl group and is ____ to yield a ____ double bond.

A) β,β; electrophilic; trans
B) α,β; electrophilic; cis
C) α,β; stereospecific; trans
D) β,γ; stereospecific; cis
E) none are true

Question 22

All are principal allosteric regulatory "signals" controlling the TCA cycle activity EXCEPT:

A) acetyl-CoA.
B) NADH.
C) NAD⁺.
D) ATP.
E) all are true.

Question 23

Characteristics of succinate dehydrogenase include all EXCEPT:

A) it is also known as succinate-Coenzyme Q reductase.
B) it has covalently bound FAD.
C) it is a membrane-bound enzyme.
D) it removes hydrogens from C-O bonds.
E) it carries out either 1-electron or 2-electron transfers to/from FAD.

Question 24

The anaplerotic reactions associated with the TCA cycle are a result of the:

A) use of many of the TCA cycle intermediates in biosynthesis.
B) oxidative nature of the TCA cycle.
C) decarboxylation reactions.
D) production of GTP and reduced coenzymes.
E) irreversible nature of some of the TCA cycle reactions.

Question 25

The correct sequence of steps between succinyl-CoA and ATP in mammals to "preserve" the energy of the thioester bond are:
A)succinyl-phosphatE.B.phosphohistidinE.C.GTP.
D)Acetyl-CoA.

A) A, B, C
B) B, C, A
C) A, C, D
D) C, B, A
E) D, A, B

Question 26

The correct sequence of electron transport in the succinate dehydrogenase reaction mechanism is:
A)Coenzyme Q (UQ).
B)[FAD].
C)iron-sulfur clusters.
D)electron transport chain.

A) A, B, C, D
B) B, C, D, A
C) B, C, A, D
D) C, B, D, A
E) C, D, A, B

Question 27

All are characteristics of succinyl-CoA synthetase EXCEPT:

A) succinyl-CoA can be used to drive phosphorylation of GDP or ADP.
B) the enzyme is named for the reverse reaction.
C) it provides an example of substrate-level phosphorylation.
D) succinyl-phosphate is an intermediate in the reaction catalyzed by succinyl-CoA synthetase.
E) all of the above are true.

Question 28

In the TCA cycle, ____ oxidation(s) use(s) NAD⁺ as the oxidizing agent while ____ oxidation(s) use(s) FAD as the oxidizing agent.

A) 0; 4
B) 1; 3
C) 2; 2
D) 3; 1
E) 4; 0

Question 29

Mammalian pyruvate dehydrogenase is regulated by phosphorylation using the enzyme ____ which is allosterically activated by high levels of ____ and ____.

A) pyruvate kinase; ATP; acetyl-CoA
B) pyruvate kinase; NADH; NADPH
C) pyruvate carboxylase; acetyl-CoA; ATP
D) pyruvate dehydrogenase kinase; NADH; acetyl-CoA
E) pyruvate dehydrogenase kinase; ADP; NAD⁺

Question 30

The only reaction of the citric acid cycle that provides substrate-level phosphorylation is catalyzed by:

A) malate dehydrogenase.
B) citrate synthase.
C) isocitrate dehydrogenase.
D) succinyl-CoA synthetase.
E) nucleotide triphosphate kinase.

Question 31

Fumarase catalyzes a reaction for which each of the following is true EXCEPT:

A) fumarate is hydrated
B) the reaction is stereospecific for a trans product
C) the reaction involved is similar to that carried out by aconitase
D) L-malate is the produced
E) all are true

Question 32

The oxidation of malate to oxaloacetate is not thermodynamically favored under standard conditions. It occurs because:

A) it involves substrate-level phosphorylation.
B) it is coupled with a strong reduction.
C) it is coupled with ATP hydrolysis.
D) oxaloacetate is used in the next reaction, which has a negative ΔG.
E) the previous reaction has a large negative ΔG.

Question 33

All are true for pyruvate carboxylase EXCEPT:

A) an anaplerotic reaction.
B) in mitochondria of plants, but not animals.
C) is tetrameric.
D) contains covalently bound biotin.
E) absolute allosteric requirement for acetyl-CoA.

Question 34

Which of the following represents a reaction that could be used to refill the CAC?:

A) OAA → PEP
B) citrate → OAA + acetyl-CoA
C) Asp → α-ketoglutarate
D) pyruvate → OAA
E) none of the above

Question 35

Characteristics of the oxidation of malate to oxaloacetate (OAA) include all EXCEPT:

A) catalyzed by malate dehydrogenase.
B) uses NAD⁺ as an electron acceptor.
C) is very exergonic.
D) results in higher levels of malate than oxaloacetate.
E) is structurally and functionally similar to lactate dehydrogenase.

Question 36

It is crucial that regulation occur at pyruvate dehydrogenase because:

A) pyruvate kinase is reversible.
B) lactate dehydrogenase is the only other enzyme to use pyruvate.
C) the product acetyl-CoA is committed to oxidation in the citric acid cycle or fatty acid biosynthesis.
D) alanine aminotransferase would use the pyruvate.
E) all of the above.

Question 37

The succinate dehydrogenase mechanism involves the ____ of succinate, which is exergonic and can be used to provide for the ____.

A) oxidation; reduction of NAD⁺
B) reduction; oxidation of NAD⁺
C) oxidation; reduction of [FAD]
D) reduction; oxidation of [FAD]
E) none are true

Question 38

The reaction CO₂ + PEP + GDP OAA + GTP is catalyzed by:

A) PEP carboxylase.
B) PEP carboxykinase.
C) malic enzyme.
D) pyruvate carboxylase.
E) None are true.

Question 39

When acetyl-CoA levels exceed the ____ supply, allosteric activation of ____ by ____ raises the oxaloacetate (OAA) levels for condensation with acetyl-CoA to form ____.

A) citrate; citrate synthase; acetyl-CoA; citrate
B) malate; malate dehydrogenase; ATP; citrate
C) OAA; citrate synthase; acetyl-CoA; isocitrate
D) OAA; pyruvate carboxylase; acetyl-CoA; citrate
E) Acetyl-CoA; pyruvate carboxylase; citrate; acetyl-CoA

Question 40

The three reactions of the citric acid cycle with large negative ΔG values include:
A)citrate synthasE.B.aconitasE.C.isocitrate dehydrogenasE.D.α-ketoglutarate dehydrogenasE.E.succinyl-CoA synthetasE.F.
Succinyl-CoA dehydrogenasE.G.
FumarasE.H.
Malate dehydrogenase.

A) A, B, C
B) A, C, D
C) C, E, F
D) B, C, E
E) A, C, H

Question 41

Inhibition of the citric acid cycle at isocitrate dehydrogenase increases the levels of citrate and isocitrate that may increase the production of:

A) amino acids.
B) pyruvate and oxaloacetate.
C) glyoxylate and cytosolic acetyl-CoA.
D) succinate and fumarate.
E) lactate and alanine.

Question 42

What is the function of thiamine pyrophosphate (TPP) in the reaction mechanism of the pyruvate dehydrogenase complex in the tricarboxylic acid (TCA) cycle?​

Question 43

Which of the following is correctly paired with its inhibitor?

A) pyruvate dehydrogenase: phosphorylation
B) isocitrate dehydrogenase: AMP
C) α-ketoglutarate dehydrogenase: NAD⁺
D) citrate synthase: acetyl CoA
E) none of the above are correct

Question 44

All are true statements for the glyoxylate pathway EXCEPT:

A) the glyoxylate pathway enzymes are found in the glyoxysomes.
B) although glyoxysomes are found in germinating seeds, they disappear after a plant begins photosynthesis.
C) glyoxysomes contain all of the enzymes for the glyoxylate cycle.
D) bypasses the oxidative-decarboxylation steps of the TCA cycle.
E) isocitrate is routed through isocitrate lyase and malate synthase.

Question 45

The glyoxylate cycle reaction that is catalyzed by malate synthase involves the condensation of acetyl CoA with _____. The mechanism is most similar to the TCA cycle enzyme _____.

A) succinate; fumarase
B) isocitrate; succinyl-CoA synthetase
C) glyoxylate; citrate synthase
D) oxaloacetate; aconitase
E) acetyl CoA; isocitrate lyase

Question 46

Which of the following statements regarding the glyoxylate cycle is true?

A) the process takes place in the mitochondria
B) two molecules of acetyl CoA can be converted to one molecule of oxaloacetate
C) isocitrate dehydrogenase and malate dehydrogenase are not used
D) the primary role is to allow for the synthesis of amino acids from fatty acids
E) none of the above

Question 47

Discuss the regulation of tricarboxylic acid cycle enzymes by acetylation.​

Question 48

All of the following are inhibitors of citrate synthase EXCEPT:

A) acetyl-CoA
B) NADH
C) succinyl-CoA
D) ATP
E) all are inhibitors

Question 49

Explain the role of the iron-sulfur cluster of aconitase in the reversible conversion of citrate to isocitrate in the tricarboxylic acid cycle.​

Question 50

Discuss the role of fumarase in the tricarboxylic acid cycle.​

Question 51

In mammalian tissues, isocitrate dehydrogenase is allosterically stimulated by:

A) high NAD⁺/NADH and high ADP/ATP ratios.
B) low ADP/ATP and low NAD⁺/NADH ratios.
C) high NAD⁺/NADH and low ADP/ATP ratios.
D) low NAD⁺/NADH and high ADP/ATP ratios.
E) none of the above.

Question 52

Discuss the metabolites that can be synthesized from citrate in the tricarboxylic acid cycle.​

Question 53

An essential feature of the glyoxylate cycle, which is not associated with the TCA cycle is:

A) it utilizes condensation of acetyl-CoA with glyoxylate
B) each complete cycle produces a net gain of two 4-carbon molecules
C) it occurs in chloroplasts and the mitochondria of plant seeds
D) mammals use the glyoxylate cycle for gluconeogenesis
E) it is a reductive pathway that utilizes NADPH as an electron source

Question 54

The isocitrate lyase catalyzed reaction cleaves isocitrate into:

A) glyoxylate and fumarate.
B) succinate and acetyl-CoA.
C) malate and acetyl-CoA.
D) succinate and glyoxylate.
E) glyoxylate and acetyl-CoA.

Question 55

Which of the following coenzymes is NOT used by pyruvate dehydrogenase?

A) NAD⁺
B) FAD
C) thiamine pyrophosphate
D) tetrahydrofolate
E) lipoic acid

Question 56

Which of the following enzymes catalyzes an endergonic step of the citric acid cycle?

A) citrate synthase
B) malate dehydrogenase
C) α-ketoglutarate dehydrogenase
D) isocitrate dehydrogenase
E) none of the above