Deck 17: Citric Acid Cycle

Question 1

The reaction catalyzed by pyruvate carboxylase is an example of a(n)______ reaction because it replenishes the citric acid cycle intermediates that have been siphoned off for use in other reactions.

A)amphibolic
B)amphipathic
C)anaplerotic
D)cataplerotic
E)none of the above

Answer 1

anaplerotic

Question 2

In eukaryotes, the citric acid cycle occurs in the _____ and therefore requires that reactants of the citric acid cycle be transported from the _____.

A)cytosol; mitochondrial matrix
B)mitochondrial matrix; cytosol
C)endoplasmic reticulum; mitochondrial matrix
D)inner mitochondrial membrane; mitochondrial matrix
E)outer mitochondrial membrane; cytosol

Answer 2

mitochondrial matrix; cytosol

Question 3

Matching
Choose the correct answer from the list. Not all the answers will be used.
The citric acid cycle is also called the ______ cycle.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Answer 3

Krebs

Question 4

How many NADH are produced when one acetyl group is oxidized in the citric acid cycle?

A)1
B)2
C)3
D)4
E)more than 4

Question 5

The reaction catalyzed by _____ requires an enzyme bound flavin coenzyme.

A)isocitrate dehydrogenase
B)( $\alpha$ -ketoglutarate dehydrogenase)
C)succinyl-CoA synthetase
D)succinate dehydrogenase
E)malate dehydrogenase

Question 6

Matching
Choose the correct answer from the list. Not all the answers will be used.
______ is the first compound that is oxidized in the citric acid cycle.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 7

Matching
Choose the correct answer from the list. Not all the answers will be used.
______ catalyzes a substrate-level phosphorylation reaction within the citric acid cycle.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 8

Matching
Choose the correct answer from the list. Not all the answers will be used.

-The enzyme ______ is unique because it contains an iron-sulfur cluster but does not participate in a redox reaction.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 9

Matching
Choose the correct answer from the list. Not all the answers will be used.
_____ and pyruvate can be combined in an ATP dependent reaction that regenerates one of the key intermediates in the citric acid cycle.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 10

Matching
Choose the correct answer from the list. Not all the answers will be used.
The compound ______ is a structural analog and competitive inhibitor of the enzyme succinate dehydrogenase.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 11

Matching
Choose the correct answer from the list. Not all the answers will be used.
A membrane-bound enzyme of the citric acid cycle that catalyzes an oxidation reaction is ______.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 12

During the oxidation of isocitrate, _____ is decarboxylated to form $\alpha$ -ketoglutarate.

A)hydroxyethyl-TPP
B)carboxybiotin
C)oxalosuccinate
D)succinyl-phosphate
E)none of the above

Question 13

Which of the following utilizes intermediates of the citric acid cycle?
I.gluconeogenesis
II.amino acid biosynthesis
III.fatty acid oxidation
IV.glycolysis

A)I only
B)II only
C)I, II, III
D)I, II
E)I, IV

Question 14

Matching
Choose the correct answer from the list. Not all the answers will be used.
______ is used in the first reaction of the citric acid cycle and regenerated upon completion of one turn of the citric acid cycle.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 15

Matching
Choose the correct answer from the list. Not all the answers will be used.
The enzyme ______ is part of the glyoxylate cycle that occurs in plants, bacteria, and fungi.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 16

Condensation of ____ with oxaloacetate yield citrate.

A)acetyl-CoA
B)α-ketoglutarate
C)isocitrate
D)citrophosphate
E)oxalosuccinate

Question 17

E₂ of the pyruvate dehydrogenase complex contains a lipoyl group that is covalently attached via an amide linkage to a ______ residue on the enzyme?

A)tyrosine
B)serine
C)lysine
D)arginine
E)cysteine

Question 18

Matching
Choose the correct answer from the list. Not all the answers will be used.
The name of a citric acid cycle intermediate that is derived from the Latin word for "apple" is ______.

A)oxaloacetate
B)malate synthase
C)malate
D)malonate
E)Krebs
F)aconitase
G)succinyl-CoA synthetase
H)α-ketoglutarate
I)carbon dioxide
J)malate dehydrogenase
K)isocitrate
L)succinate dehydrogenase

Question 19

Which of the following coenzymes is required by E₁ of the pyruvate dehydrogenase complex for catalytic activity?

A)thiamine pyrophosphate
B)lipoamide
C)coenzyme A
D)NAD⁺
E)FAD

Question 20

Which of the following is an intermediate between citrate and isocitrate in the citric acid cycle?

A)succinyl-CoA
B)citryl-CoA
C)oxalosuccinate
D)succinyl-phosphate
E)cis-aconitate

Question 21

Germinating plant seeds can convert acetyl-CoA (obtained from fatty acids stored as oils)into carbohydrates, whereas animals are incapable of converting fatty acids into glucose.This difference is due to the fact that:

A)animals have glycogen and don't need to make glucose from fatty acids.
B)plants use the glyoxylate cycle to convert two acetyl CoA to oxaloacetate, a precursor for gluconeogenesis.
C)plant seeds use photosynthesis to make sugar.
D)animals use the citric acid cycle selectively for energy production, whereas plants primarily use glycolysis.
E)B and D

Question 22

If acetyl-CoA labeled with ¹⁴C, as shown in the figure to the right, were used as the substrate for the citric acid cycle, which of the following intermediates would be produced during the first round of the cycle?

A)
B)
C)
D)
E)

Question 23

Which of the following coenzymes is required by the pyruvate dehydrogenase complex for reoxidation of E3?

A)thiamine pyrophosphate
B)pyridoxal phosphate
C)coenzyme A
D)FAD
E)NAD⁺

Question 24

Which of the following is oxidized by a multienzyme complex that requires five different coenzymes?

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

Question 25

How many electrons are transferred from one acetyl group during the citric acid cycle?

A)2
B)4
C)6
D)8
E)10

Question 26

Which of the following amino acid residues facilitates the transfer of a phosphate group in the reaction producing succinate and GTP?

A)aspartate
B)glutamate
C)histidine
D)lysine
E)serine

Question 27

Which of the following causes pyruvate dehydrogenase kinase to catalyze the phosphorylation and inactivation of E₁ in the pyruvate dehydrogenase complex?

A)elevated concentrations of NADH and ATP
B)elevated concentrations of NAD⁺ and ADP
C)insulin
D)elevated concentrations of acetyl-CoA
E)C and D

Question 28

Which of the following citric acid cycle intermediates can be converted to aspartate via transamination?

A)citrate
B)( $\alpha$ -ketoglutarate)
C)succinate
D)fumarate
E)oxaloacetate

Question 29

Which of the following can be said about the isocitrate product of aconitase?

A)The product always has the R configuration.
B)The product always has the S configuration.
C)The product is not chiral.
D)The product can exist in either the R or S configuration in eukaryotes.
E)If the product is in the R configuration it has by passed the cis-aconitate intermediate.

Question 30

Consider the following reaction which has a ∆G˚' ≈ +30 kJ/mol.
malate + NAD⁺ → oxaloacetate + NADH + H⁺
In muscle cells, the reaction proceeds as written.Which of the following explains how the reaction occurs as written despite the positive value for $\Delta$ G^°'?

A)It is thermodynamically favored under standard conditions.
B)In the cell, it is kinetically favored, even though it's thermodynamically unfavorable.
C)The concentration of malate must be higher than oxaloacetate for this reaction to occur in the cell.
D)[H⁺] must be higher in muscle than under standard conditions, thus altering ∆G˚ to ∆G˚'.
E)None of the above

Question 31

The isomerization of citrate to isocitrate

A)is the reaction of the citric acid cycle that occurs spontaneously without enzymatic catalysis.
B)protects cells from the toxic effects of arsenite ion.
C)converts a tertiary alcohol, which cannot easily be oxidized, to a secondary alcohol that can be oxidized.
D)is one major regulatory step for the citric acid cycle because it functions as a rate limiting step.
E)A and B

Question 32

Which of the following enzymes catalyzes a reaction with the pictured compound as an intermediate?

A)( $\alpha$ -ketoglutarate dehydrogenase)
B)succinyl-CoA synthetase
C)succinate dehydrogenase
D)fumarase
E)malate dehydrogenase

Question 33

Which of the following is(are)TRUE concerning the metabolic reaction shown below? Pyruvate + NAD⁺ + CoA-SH SYMBOL 174 \f "Symbol" Acetyl-CoA + NADH + H⁺ + CO₂
I.The enzyme that catalyzes this reaction contains a pyridoxal phosphate prosthetic group.
II.This reaction is an oxidative decarboxylation.
III.The enzyme that catalyzes this metabolic conversion is activated by high concentrations of ATP.

A)I, II, III
B)II, III
C)II only
D)III only
E)I, II

Question 34

Which of the following structures represents a molecule that is added to succinate to form CoA and malate?

A)
B)
C)
D)
E)

Question 35

Considering the reactions within the citric acid cycle, which of the following molecules will be produced from the intermediate pictured on the right?

A)citrate
B)isocitriate
C)succinate
D)succinyl-CoA
E)α-ketoglutarate

Question 36

Which of the following uses inherent length and flexibility to facilitate transfer of an acetyl group?

A)FAD
B)NAD⁺
C)lipoamide
D)thiamine pyrophosphate
E)coenzyme A

Question 37

The figure at right represents an intermediate in the reaction catalyzed by which of the following enzymes?

A)enolase
B)aldolase
C)pyruvate decarboxylase
D)glyceraldehyde-3-phosphate dehydrogenase
E)lactate dehydrogenase

Question 38

Which of the following best describes the importance of the citric acid cycle as a central pathway of metabolism?

A)It allows recovery of energy from carbohydrates only.
B)It allows recovery of energy from fatty acids only.
C)It allows recovery of energy from amino acids only.
D)It allows recovery of energy from several metabolic fuels that are broken down to acetyl-CoA.
E)A and B

Question 39

Which of the following organisms does not utilize the glyxolate cycle?

A)trees
B)bacteria
C)fungi
D)mammals
E)All of the above utilize the glyxolate cycle when necessary.

Question 40

Which of the following enzymes of the citric acid cycle is inhibited by ATP?

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

Question 41

Which of the following enzymatic reactions is favorable both under physiological conditions and under standard conditions?

A)The reaction catalyzed by isocitrate dehydrogenase.
B)The reaction catalyzed by malate dehydrogenase.
C)The reaction catalyzed by succinate dehydrogenase.
D)all of the above
E)none of the above

Question 42

One turn of the citric acid cycle generates

A)2 FADH₂, 3 ATP, 1 NADH
B)1 NAD+, 2FADH₂, 1 ATP
C)1 GTP, 3 NADH, 1 FADH₂
D)1 FAD, 2 ATP, 3 NADH
E)1 FADH₂, 1GTP, 2 NADH

Question 43

Which reaction below produces GTP in the citric acid cycle?

A)isocitrate $\rarr$$\alpha$ -ketoglutarate
B)( $\alpha$ -ketoglutarate $\rarr$ succinyl CoA)
C)succinyl CoA $\rarr$ succinate
D)fumarate $\rarr$ malate
E)malate $\rarr$ oxaloacetate

Question 44

Which of the following would decrease activity of the citric acid cycle overall?
I.high concentration of NADH
II.high concentration of Ca²⁺
III.high concentration of ATP
IV.high concentration of citrate

A)I only
B)I, II, III, IV
C)I, III
D)I, III, IV
E)I, IV

Question 45

An individual with a shortage of B vitamins (which include thiamine and riboflavin)may feel fatigued as a result of decreased pyruvate dehydrogenase activity.Which of the following would be TRUE regarding this shortage?

A)The amount of acetyl CoA produced from carbohydrate metabolism that enters the citric acid cycle would be decreased.
B)A decrease in the amount of NADH would likely decrease activity of citrate synthase.
C)Flux through the citric acid cycle would cease until adequate pyruvate is available.
D)none of the above
E)all of the above

Question 46

Bioinformatics has identified some enzymes used in the citric acid cycle in primitive bacteria.Once such a cycle utilized many of the citric acid cycle intermediates but occurred in the reverse direction.Researchers suspect which of the following based on this information?

A)Researchers suspect that the "original" citric acid cycle was used by all species.
B)Researchers suspect that portions of the reverse cycle may have been responsible for fixing CO₂
C)Researchers suspect that early methods for converting pyruvate to acetyl CoA did not exist.
D)Researchers suspect that the current "clockwise" version of the citric acid cycle could be used to help humans feed off CO₂.
E)Researchers suspect that primitive prokaryotic use of the citric acid enzymes were focused on the production of CO₂ rather than the use of CO₂.

Question 47

Which of the following is TRUE regarding the discovery of the citric acid cycle?
I.One of the first discoveries was the involvement of coenzyme A in the production of GTP.
II.Krebs had knowledge of the sequence of reactions before he was able to demonstrate the link of the cycle to glucose metabolism.
III.The citric acid cycle was the first cyclic pathway discovered.
IV.Krebs demonstrated the involvement of the citric acid cycle in oxidation of amino acid and fatty acids.

A)I, II, III, IV
B)I, II, III
C)II, III
D)III, IV
E)II, IV

Question 48

Which of the following statements correctly and most accurately describes the function of FAD in the pyruvate dehydrogenase enzyme complex?

A)NADH passes electrons to FAD to form FADH₂.
B)Lipoamide passes electrons through FADH2, which almost instantly passes them to NAD thus forming NADH.
C)FADH₂ donates electrons to lipoamide thus regenerating FAD.
D)Lipoamide oxidizes NADH to NAD by passing electrons to FAD.
E)NAD accepts electrons directly from lipoamide, which has gained them via oxidation of FADH₂.

Question 49

Which of the following steps of the citric acid cycle conserves the energy of a high energy thioester bond?

A)the reaction catalyzed by succinate dehydrogenase.
B)the reaction that produces oxalacetate.
C)the condensation reaction that produces citrate.
D)the reaction catalyzed by α-ketoglutarate dehydrogenase.
E)the conversion of citrate to isocitrate.

Question 50

Which of the following represents a point of regulation for the citric acid cycle?
I.pyruvate dehydrogenase complex
II.aconitase
III.isocitrate dehydrogenase
IV.fumarase

A)I, II, III, IV
B)I, II, IV
C)II, III
D)I, III
E)I, IV