Deck 13: The Citric Acid Cycle

Question 1

Which carbon atom(s)of pyruvate is(are)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.

Answer 1

The carboxylate carbon (#1).

Question 2

Pyruvate passes through the outer mitochondrial membrane by ________.

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

Answer 2

porin proteins

Question 3

Which is not a component of the pyruvate dehydrogenase complex?

A)Dihydrolipoamide dehydrogenase.
B)Isocitrate dehydrogenase.
C)Pyruvate dehydrogenase.
D)Dihydrolipoamide acetyltransferase.

Answer 3

Isocitrate dehydrogenase.

Question 4

Pyruvate translocase is a(n)________ 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

Question 5

What are the fewest number of turns of the citric acid cycle needed before carbon radioactive in both carbons of the acetyl group in acetyl Co A are incorporated into all 4 carbons in every molecule of oxaloacetate produced?

A)1
B)2
C)3
D)none

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

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 8

The pyruvate dehydrogenase complex is larger than a ribosome.In bacteria these complexes are located in the cytosol while they are found in ________ of eukaryotes

A)cell membranes
B)mitochondrial membranes
C)mitochondrial matrices
D)nuclei

Question 9

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

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

Question 10

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

Question 11

Some bacteria and anaerobic eukaryotes not using PDH(pyruvate 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 12

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

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

Question 13

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

A)GTP
B)ATP
C)NADH and QH²
D)ADP

Question 14

The arrangement of subunits in the PDH(pyruvate 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 15

Which cofactor is not used by the pyruvate dehydrogenase complex?

A)Lipoamide.
B)Thiamine pyrophosphate.
C)FAD.
D)QH₂.

Question 16

The net effect of the eight steps of the citric acid cycle is to

A)completely oxidize an acetyl group to carbon dioxide.
B)convert pyruvate to succinate.
C)produce NAD+ and Q.
D)produce 8 ATP for every pass through the cycle.

Question 17

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 18

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

Question 19

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

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

Question 20

Why were muscle cells and extracts the best choice for Krebs,Johnson and Szent-Györgyi to elucidate the cyclic nature of the citric acid cycle?

A)The reactions of the cycle in muscle cells are more highly regulated,and therefore easier to study,than in other tissues.
B)Muscle cells have an unusually high concentration of the starting material,pyruvate.
C)In muscle the cycle intermediates are used almost exclusively for energy metabolism by the cycle and not for other reactions.
D)Muscle cells can more easily uptake oxygen than other cells,so monitoring the consumption of oxygen is easier for these cells.

Question 21

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 22

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 23

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 24

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 25

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 26

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 27

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 28

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 29

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 30

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

A)Succinate.
B)Citrate.
C)Succinyl CoA.
D)α-ketoglutarate.

Question 31

The succinate dehydrogenase reaction is unusual for a dehydrogenase because it uses ________ as an electron acceptor (oxidizing agent)that is regenerated by ubiquinone.

A)ATP
B)NADP
C)Malonate
D)FAD

Question 32

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 33

Which is not produced by the citric acid cycle?

A)NADH
B)FMN
C)CO₂
D)QH₂

Question 34

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 35

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

A)Oxidative decarboxylation.
B)Hydration.
C)Dehydrogenation.
D)Condensation.

Question 36

Which of the following enzymatic reactions are examples of substrate level phosphorylation?

A)Succinyl Co-A synthetase.
B)Pyruvate kinase.
C)Phosphoglycerate kinase.
D)All of the above.

Question 37

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 38

Which enzyme catalyzes the conversion of citrate to isocitrate?

A)Aldolase.
B)Citrate synthase.
C)Citrate isomerase.
D)Aconitase.

Question 39

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 40

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

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

Question 41

The following enzyme(s)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 42

Which product of the citric acid cycle produces the most ATP equivalents?

A)NADH.
B)QH₂.
C)GTP.
D)CO₂.

Question 43

What is the role of mitochondrial PEPCK?

A)It converts oxaloacetate to PEP,which can then be transported to the cytoplasm.
B)It aids in the transport of pyruvate across the inner mitochondrial membrane.
C)It converts oxaloacetate to malate,which feeds into the malate-aspartate shuttle.
D)It converts acetyl CoA to citrate,which can then be transported to the cytoplasm.

Question 44

Pyruvate dehydrogenase is inactivated by phosphorylation.This can be prevented by a chemical,dichloroacetate,and this disrupts many cancer cells because they are

A)aerobic.
B)anaerobic.
C)fast growing.
D)non-fermenting.

Question 45

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 46

Mammalian pyruvate dehydrogenase is inactivated when pyruvate dehydrogenase kinase is activated by

A)NADH.
B)ADP.
C)pyruvate.
D)AMP.

Question 47

Malonate

A)is a structural analog 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 48

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 49

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 50

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 51

The pyruvate dehydrogenase complex is located inside mitochondria .Pyruvate is formed by glycolysis in the cytoplasm.Pyruvate is brought into the mitochondrion by

A)diffusion.
B)TCA transporter.
C)PEP transporter.
D)Pyruvate transporter.

Question 52

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 53

Which of the following inhibit(s)pyruvate dehydrogenase kinase?

A)Pyruvate.
B)NADH.
C)Acetyl-CoA.
D)All of the above.

Question 54

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 55

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 CoA's interaction with glycine.
C)Oxaloacetate as a carbohydrate precursor.
D)None of the above.

Question 56

A metabolic pathway that is both catabolic and anabolic is ________.An example of such a pathway is ________.

A)ambibolic;glycolysis
B)amphibolic;the citric acid cycle
C)duplibolic;the pentose phosphate pathway
D)Pathways cannot be both anabolic and catabolic,so there are no example pathways.

Question 57

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 58

When fumarate is positioned in the active site of fumarase,the double bond of the substrate can be attacked from only one side.Therefore,fumarate is a ________ molecule.

A)asymmetric
B)prochiral
C)isomeric
D)duplicate

Question 59

Aqueous channels in cell membranes that small molecules such as pyruvate can pass through are formed by transmembrane proteins called

A)openings.
B)porins.
C)pores.
D)locase.

Question 60

Metabolites moved across the mitochondrial membrane by transporters include

A)pyruvate.
B)PEP.
C)citrate and malate.
D)All of the above.

Question 61

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

Question 62

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

Question 63

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 64

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 65

The citric acid cycle produces a nucleotide triphosphate directly by substrate level phosphorylation.

Question 66

In citrus fruits,the levels of citric acid are related to its ability to conduct a citric acid cycle of reactions and not to its ability to act as a preservative or a reservoir of carbon.

Question 67

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

Question 68

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

Question 69

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

Question 70

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 71

Synthases do not require ATP as a cofactor while synthetases are coupled to ATP (or GTP)hydrolysis.Both types of enzymes are found in the citric acid cycle.

Question 72

Dehydrogenases that act on α-ketoglutarate (in the citric acid cycle)and on branch chain alpha-keto acids (in amino acid metabolism)closely resemble pyruvate dehydrogenase in structure and function.

Question 73

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

Question 74

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

Question 75

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

Question 76

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 77

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

Question 78

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

Question 79

Citrate is chemically symmetric but is a prochiral molecule because the two acetyl groups are specifically oriented by the enzyme aconitase.

Question 80

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.