Deck 13: The Citric Acid Cycle

Question 1

Compounds like succinate, fumarate and α-ketoglutarate have a catalytic effect on the consumption of oxygen in a cell suspension. The rate of oxygen consumption is far more than that required for their own oxidation. This is evidence that .

A) they are intermediates in glycolysis
B) they act as enzymes to cause the oxidation of other compounds
C) they are involved in a cyclic pathway
D) they must be cofactors for enzymes that are oxidoreductases

Answer 1

they are involved in a cyclic pathway

Question 2

The enzyme pyruvate translocase is located .

A) in the cytosol
B) in the inner mitochondrial membrane
C) in the mitochondrial matrix
D) in the endoplasmic reticulum

Answer 2

in the inner mitochondrial membrane

Question 3

Pyruvate translocase is a/an protein that transports .

A) antiport; pyruvate and H⁺ in opposite directions
B) uniport; only pyruvate
C) symport; pyruvate and H⁺ in the same direction
D) antiport; pyruvate and CO₂ in opposite directions

Answer 3

symport; pyruvate and H⁺ in the same direction

Question 4

Most of the energy released in citric acid cycle reactions is conserved in .

A) GTP
B) ATP
C) NADH and QH₂
D) ADP

Question 5

In eukaryotes the enzymes of the citric acid cycle are found in the .

A) cytosol
B) mitochondria
C) nucleus
D) endoplasmic reticulum

Question 6

When energy-rich acetyl CoA is oxidized to carbon dioxide, eight electrons are released. Most often, electrons are released in such reactions because

A) double bonds formed in CO₂ share electron pairs.
B) there are more electrons in the reactants than in the products.
C) water is not available in the reaction.
D) an electron acceptor is linked to the reaction.

Question 7

The citric acid cycle oxidizes pyruvate, and some of the pathway intermediates are starting materials for many biosynthetic pathways. This means the citric acid cycle is .

A) amplifying
B) catabolic
C) anabolic
D) catabolic and anabolic

Question 8

The arrangement of subunits in the PDHpyruvate dehydrogenase) complex ensures that .

A) ATP is formed
B) the product of one enzyme is delivered to the next in turn
C) the dihydrolipoamide arm can react with NAD⁺
D) acetyl CoA enters into the series of reactions

Question 9

A deficiency in thiamine causes the disease beriberi. Which might you expect to have a higher than normal blood concentration in an individual with this condition?

A) isocitrate
B) pyruvate
C) oxaloacetate
D) acetyl CoA

Question 10

Which carbon atoms) of pyruvate isare) first converted to carbon dioxide by pyruvate dehydrogenase complex?

A) the carboxylate carbon #1)
B) the carbonyl carbon #2)
C) the methyl carbon #3)
D) both carbons #1 and #3 in equal amounts

Question 11

Which step in the citric acid cycle is a rearrangement reaction?

A) succinyl CoA to succinate
B) fumarate to L-malate
C) citrate to isocitrate
D) glucose 1,6-bisphosphate to fructose 1,6-bisphosphate

Question 12

Pyruvate passes through the outer mitochondrial membrane by .

A) porin proteins
B) passive transport
C) pyruvate translocase
D) simple diffusion through the lipid bilayer

Question 13

Some bacteria and anaerobic eukaryotes not using PDHpyruvate dehydrogenase) form acetyl CoA and CO₂ from pyruvate with .

A) PDH modified with methyl groups
B) pyruvate:ferredoxin oxidoreductase
C) 2-oxoacid dehydrogenase
D) an isoenzyme of PDH

Question 14

Which cofactor is not used by the pyruvate dehydrogenase complex?

A) lipoamide
B) thiamine pyrophosphate
C) FAD
D) QH₂

Question 15

The order of prosthetic groups as they act in the three proteins of the PDHpyruvate dehydrogenase) complex is:

A) FAD → thiamine pyrophosphate → NAD⁺
B) FAD → thiamine pyrophosphate → dihydrolipoamide
C) thiamine pyrophosphate → dihydrolipoamide → FAD
D) NAD⁺ → FAD → dihydrolipoamide

Question 16

Which enzyme is the same in both the pyruvate dehydrogenase and α-ketoglutarate complexes?

A) dihydrolipoamide dehydrogenase
B) aconitase
C) pyruvate decarboxylase
D) The two complexes have no components that are similar.

Question 17

Citrate can react asymmetrically in the citric acid cycle because the enzyme aconitase

A) binds citrate asymmetrically.
B) binds either form of citrate.
C) binds both forms of isocitrate.
D) does not distinguish the -CH₂COO^- group.

Question 18

What type of reaction is the conversion of fumarate to malate?

A) oxidative decarboxylation
B) hydration
C) dehydrogenation
D) condensation

Question 19

Carbons from acetyl CoA are transferred to the citric acid cycle. Which is the first round of the citric acid cycle that could possibly release a carbon atom originating from this acetyl CoA?

A) first round
B) second round
C) third round
D) fourth round

Question 20

Which is not a component of the pyruvate dehydrogenase complex?

A) dihydrolipoamide dehydrogenase
B) isocitrate dehydrogenase
C) pyruvate dehydrogenase
D) dihydrolipoamide acetyltransferase

Question 21

Which 5-carbon intermediate of the citric acid cycle is converted to a 4-carbon molecule with the release of carbon dioxide?

A) fumarase
B) α-ketoglutarate
C) succinate
D) isocitrate

Question 22

Which statement is true about lactate dehydrogenase and malate dehydrogenase?

A) Both enzymes are highly specific for their own substrates.
B) A single amino acid change can convert lactate dehydrogenase to a malate dehydrogenase.
C) The naturally occurring enzymes do not have much sequence similarity, but do have closely related three-dimensional structures.
D) All of the above

Question 23

After passing through the citric acid cycle, one mole of pyruvate will result in the formation of moles of carbon dioxide and moles) of ATP or GTP).

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

Question 24

Which enzyme does not catalyze a reaction that releases carbon dioxide?

A) α-ketoglutarate dehydrogenase complex
B) pyruvate dehydrogenase
C) malate dehydrogenase
D) isocitrate dehydrogenase

Question 25

Which of the following is/are true statements about the succinate dehydrogenase reactions?

A) It is stereospecific.
B) Only the cis-isomer of the product is formed.
C) It is not stereospecific.
D) Both B and C

Question 26

Which intermediate of the citric acid cycle has a plane of symmetry?

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

Question 27

The step at which acetyl CoA enters the citric acid cycle is classified as a reaction.

A) condensation
B) substrate-level phosphorylation
C) decarboxylation
D) dehydrogenation

Question 28

Which of the following are components of the succinate dehydrogenase complex?

A) FAD prosthetic group
B) iron-sulfur clusters
C) malonate
D) All of the above
E) A and B only

Question 29

Which is not produced by the citric acid cycle?

A) NADH
B) FMN
C) CO₂
D) QH₂

Question 30

Which of the following inhibits) pyruvate dehydrogenase kinase?

A) pyruvate
B) NADH
C) acetyl-CoA
D) All of the above

Question 31

The succinate dehydrogenase complex catalyzes

A) the formation of a single hydrogen bond in the oxidation of succinate to fumarate.
B) the formation of a double bond in the oxidation of succinate to fumarate.
C) Both A and B
D) None of the above

Question 32

About how many total ATP equivalents are generated by the complete oxidation of one molecule of acetyl CoA?

A) 1.5
B) 2.5
C) 3
D) 10
E) 30

Question 33

Which of the following enzymatic reactions is not an example of substrate level phosphorylation?

A) succinyl Co-A synthetase
B) pyruvate kinase
C) phosphoglycerate kinase
D) All of the above

Question 34

During catalysis, succinyl CoA synthetase generates

A) an energy deficient, unstable phosphoenzyme intermediate.
B) an energy rich, unstable phosphoenzyme intermediate.
C) an energy rich, stable phosphoenzyme intermediate.
D) All of the above

Question 35

A carbon atom that has single bond to four substituents with exactly two of the four substituents identical is called a .

A) prochiral carbon atom
B) tetrahedral stereocenter
C) divalent carbon atom
D) anomeric carbon atom

Question 36

NAD-dependent malate dehydrogenase

A) catalyzes the oxidation of malate to regenerate oxaloacetate.
B) catalyzes the conversion of fumarase to malate.
C) catalyzes a reaction which results in the formation of an NADH molecule.
D) All of the above
E) A and C only

Question 37

Malonate

A) is a structural analogy of succinate.
B) binds to the substrate-binding site of the succinate dehydrogenase complex but does not react.
C) undergoes an oxidation reaction.
D) Both A and B
E) All of the above

Question 38

Pyruvate dehydrogenase kinase catalyzes

A) the dephosphorylation and activation of E₁.
B) the dephosphorylation and inactivation of E₁.
C) the phosphorylation and inactivation of E₁.
D) the phosphorylation and activation of E₁.

Question 39

Elevated calcium levels lead to

A) phosphorylation of the E₁ subunit and an increase in flux through the pyruvate dehydrogenase complex.
B) dephosphorylation of the E₁ subunit and a decrease in flux through the pyruvate dehydrogenase complex.
C) Neither of the above

Question 40

Which enzyme catalyzes the conversion of citrate to isocitrate?

A) aldolase
B) citrate synthase
C) citrate isomerase
D) aconitase

Question 41

The mechanism of pyruvate dehydrogenase complex is an example of channeling.

Question 42

The overall goal of the citric acid cycle is to oxidize pyruvate, form reduced coenzymes, and produce ATP.

Question 43

The following enzymes) is/are unique to the glyoxylate cycle.

A) malate synthase
B) malate dehydrogenase
C) isocitrate lyase
D) All of the above
E) A and C only

Question 44

Each of the following leads to a biosynthetic pathway except

A) a-ketoglutarate.
B) Succinyl CoA.
C) oxaloacetate.
D) citrate.
E) None of the above

Question 45

The carbon atoms that enter the citric acid cycle via acetyl CoA are the same ones released as carbon dioxide during one round of the citric acid cycle.

Question 46

Which of the following allosterically activates mammalian isocitrate dehydrogenase?

A) ADP
B) NADH
C) calcium
D) All of the above
E) A and C only

Question 47

True statements about the glyoxylate cycle include

A) four carbon atoms of the acetyl group of acetyl CoA are released as carbon dioxide during operation of the glyoxylate cycle.
B) the net formation of a four carbon molecule from two acetyl CoA molecules supplies a precursor that can be converted to glucose.
C) the reaction catalyzed by malate synthase is the first bypass enzyme of the glyoxylate cycle.
D) it is inactive in oily seed plants.

Question 48

The only reaction in the citric acid cycle to generate a carbon-carbon bond is catalyzed by citrate synthase.

Question 49

Pyruvate translocase is specific for transporting pyruvate across the inner mitochondrial membrane.

Question 50

Each of the following catalyzed reactions of the citric acid cycle appears to be regulated except

A) citrate synthase.
B) fumarase.
C) isocitrate dehydrogenase.
D) a-ketoglutarate dehydrogenase complex.

Question 51

The citric acid cycle can be viewed as a multi-step catalyst simply because it returns to its original state after each round of reactions.

Question 52

Which of the following is not a fate of a citric acid cycle intermediate?

A) a-ketoglutarate reversibly converting to glutamate
B) the biosynthesis of porphyrins from succinyl CoAʹs interaction with glycine
C) oxaloacetate as a carbohydrate precursor
D) None of the above

Question 53

ATP is consumed by the pyruvate dehydrogenase complex during the synthesis of acetyl CoA.

Question 54

Which statement is false about the glyoxylate cycle?

A) In mammals the glyoxylate cycle is used to replenish citric acid cycle intermediates.
B) It is an anabolic alternative for the metabolism of acetyl CoA.
C) It can be regarded as a shunt in the citric acid cycle.
D) In eukaryotes, metabolites must be transferred from the mitochondria to the cytosol to be used in the glyoxylate cycle.

Question 55

Which process is not implicated in the evolution of the citric acid cycle pathway?

A) gene duplication
B) palindromic inversion
C) pathway extension
D) pathway reversal
E) enzyme theft

Question 56

The glyoxylate cycle leads from two carbon compounds to glucose in each organism below except

A) animals.
B) plants.
C) bacteria.
D) yeast.
E) None of the above

Question 57

Isocitrate is more easily oxidized than citrate because it has a secondary alcohol group, whereas citrateʹs alcohol group is tertiary.

Question 58

A racemic mixture of the enantiomeric forms of isocitrate is produced during the citric acid cycle reactions.

Question 59

The citric acid cycle is an anaerobic pathway that occurs in the mitochondria of eukaryotes.

Question 60

The glyoxylate cycle is

A) a catabolic pathway in plants and some microorganisms.
B) an anabolic pathway in plants and some microorganisms.
C) regarded as a shunt within the citric acid cycle.
D) A and C only
E) B and C only

Question 61

Please match the following reactions to their respective energy yielding products. Please note that a product may be used once, more than once, or not at all as an answer.

Isocitrate dehydrogenase

Question 62

Malonate is a competitive inhibitor of the succinate dehydrogenase complex.

Question 63

The a-ketoglutarate and succinate dehydrogenase complexes are the only oligomeric enzymes used in the citric acid cycle.

Question 64

Citric acid cycle components are dissolved in the cytosol for all prokaryotic and eukaryotic species.

Question 65

Most bacteria do not have a complete citric acid cycle.

Question 66

In aerobic prokaryotes, the succinate dehydrogenase complex is embedded in the inner mitochondrial membrane.

Question 67

The recycling of NAD allows glycolysis to continue in the absence of oxygen.

Question 68

The activity of dihydrolipoamide acetyltransferase E₂) is inhibited when the concentration of of acetyl CoA is decreased.

Question 69

Please match the following reactions to their respective energy yielding products. Please note that a product may be used once, more than once, or not at all as an answer.

Succinate dehydrogenase complex

Question 70

Please match the following reactions to their respective energy yielding products. Please note that a product may be used once, more than once, or not at all as an answer.

Malate dehydrogenase

Question 71

Please match the following reactions to their respective energy yielding products. Please note that a product may be used once, more than once, or not at all as an answer.

Succinyl CoA synthetase

Question 72

The eukaryotic succinate dehydrogenase complex is dissolved in the mitochondrial matrix.

Question 73

The activity of dihydrolipoamide dehydrogenase E₃) is inhibited by a high NADH/NAD⁺ ratio.

Question 74

Please match the following reactions to their respective energy yielding products. Please note that a product may be used once, more than once, or not at all as an answer.

a-ketoglutarate dehydrogenase complex

Question 75

All members of the aconitase family of proteins contain an iron sulfur cluster.