Deck 19: The Citric Acid Cycle

Question 1

The acetyl group is carried on lipoic acid as

A) an alcohol.
B) a thioester.
C) a phosphoanhydride.
D) an amide.

Answer 1

a thioester.

Question 2

The enzymes involved in the pyruvate dehydrogenase complex are

A) physically separated from each other
B) crosslinked to each other by lipoic acid linkers
C) covalently bonded to coenzyme A
D) associated with each other in an ordered and complex array

Answer 2

associated with each other in an ordered and complex array

Question 3

The reactions in which succinate is converted to oxaloacetate are, in order

A) an oxidation, a dehydration, and an oxidation
B) three successive oxidation reactions
C) an oxidative decarboxylation, a dehydration, and a condensation
D) a condensation, a dehydration, and an oxidative decarboxylation

Answer 3

an oxidation, a dehydration, and an oxidation

Question 4

Each of the enzymes of the pyruvate dehydrogenase complex requires a different vitamin.

Question 5

The citric acid cycle is the only metabolic pathway that can be used both as an anabolic and as a catabolic pathway.

Question 6

In which cellular location do the majority of the reactions of the citric acid cycle take place?

A) the cytosol.
B) the mitochondrial matrix.
C) the endoplasmic reticulum.
D) lysosomes.
E) none of these

Question 7

The immediate electron acceptor for the majority of the oxidative reactions of the citric acid cycle is

A) ATP.
B) NAD.
C) FAD.
D) coenzyme A.
E) none of these

Question 8

The citric acid cycle produces one high energy phosphate molecule per cycle directly in the form of ____

A) ATP
B) GTP
C) CTP
D) AMP
E) none of these

Question 9

Which of the following vitamins and enzyme cofactors are used by the pyruvate dehydrogenase complex during oxidative decarboxylation?

A) Lipoic Acid.
B) Niacin.
C) Pantothenic Acid.
D) Thiamine.
E) All of these

Question 10

Which of the following does not play a role in the overall aerobic metabolism of glucose?

A) citric acid cycle
B) electron transport
C) oxidative phosphorylation
D) urea cycle
E) all of these play a role in overall aerobic metabolism of glucose

Question 11

Which group of small molecules best fit the boxes associated with the reaction shown?

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

Question 12

The reaction of the citric acid cycle that does not take place in the mitochondrial matrix is the one catalyzed by:

A) fumarase
B) citrate synthase
C) isocitrate dehydrogenase
D) succinate dehydrogenase
E) All of these reactions take place in the matrix

Question 13

Roughly how many more ATP's can be produced via the complete aerobic oxidation of glucose compared to that produced by glycolysis alone?

A) twice as much
B) five times as much
C) fifteen times as much
D) thirty-two times as much
E) none of these

Question 14

Which of the following statements concerning the citric acid cycle as the central metabolic pathway is true ?

A) It is involved in the metabolism of sugars and amino acids.
B) It is involved in the metabolism of amino acids and lipids.
C) It links anaerobic metabolism to aerobic metabolism.
D) Many of its intermediates are starting points for synthesis of a variety of compounds.
E) All of these are reasons why the citric acid cycle is considered to be the central pathway.

Question 15

The citric acid cycle is amphibolic, meaning

A) it plays a role in both anabolism and catabolism.
B) it is essentially irreversible.
C) it can operate both in the presence and absence of oxygen.
D) it can oxidize carbons and nitrogens equally well.
E) none of these

Question 16

Lipoic acid is a required vitamin in the human diet.

Question 17

Which of the following is not a component of the pyruvate dehydrogenase complex?

A) pyruvate dehydrogenase
B) dihydrolipoyl transacetylase
C) dihydrolipoyl dehydrogenase
D) pyruvate dehydrogenase kinase
E) aconitase

Question 18

Thiamine pyrophosphate carries a ____ carbon unit.

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

Question 19

Which of the following is not a reaction occurring during oxidative decarboxylation of pyruvate?

A) Removal of CO₂.
B) Oxidation of an acetate group.
C) Addition of Coenzyme A to a 2-carbon fragment.
D) Reduction of NAD⁺
E) All of these reactions take place during oxidative decarboxylation.

Question 20

Which enzymes in the citric acid cycle catalyze oxidative decarboxylation reactions?

A) isocitrate dehydrogenase and the α -ketoglutarate dehydrogenase complex
B) aconitase and succinate dehydrogenase
C) the α -ketoglutarate dehydrogenase complex and succinate thiokinase
D) fumarase and succinate dehydrogenase

Question 21

Which of the following reactions involves substrate-level phosphorylation?

A) isocitrate → α -ketoglutarate
B) citrate → isocitrate
C) succinate → fumarate
D) succinyl-CoA → succinate
E) acetyl-CoA + oxaloacetate → citrate

Question 22

When acetyl-CoA reacts with oxaloacetate to form citrate

A) a new carbon-carbon bond is formed
B) an oxidative decarboxylation reaction takes place
C) a dehydration reaction takes place
D) a rearrangement takes place
E) none of these

Question 23

Which of the following enzymes contains a non-heme iron?

A) citrate synthase
B) succinyl-CoA synthetase
C) succinate dehydrogenase
D) fumarase

Question 24

Which of the reactions of the citric acid cycle requires FAD as a coenzyme?

A) the conversion of isocitrate to α -ketoglutarate
B) the conversion of citrate to isocitrate
C) the conversion of succinate to fumarate
D) the conversion of malate to oxaloacetate
E) none of these

Question 25

Which enzyme catalyzes the reaction shown?

A) succinyl-CoA synthetase
B) succinate dehydrogenase
C) pyruvate dehydrogenase
D) α -ketoglutarate dehydrogenase

Question 26

Release of succinate from succinyl-CoA can be coupled to GTP synthesis because:

A) The amide bond between succinate and CoA has a large − Δ G of hydrolysis.
B) The thioester bond between succinate and CoA has a large − Δ G of hydrolysis.
C) The link between succinate and CoA involves an acid anhydride to phosphate.
D) Coenzyme A is a "high energy" compound, just like GTP.
E) None of these explains why GTP can be formed during this reaction.

Question 27

Which enzyme catalyzes the reaction shown?

A) isocitrate dehydrogenase
B) pyruvate dehydrogenase
C) fumarase
D) succinate dehydrogenase
E) none of these

Question 28

Which group of small molecules best fit the boxes associated with the reaction shown?

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

Question 29

Which group of small molecules best fit the boxes associated with the reaction shown?

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

Question 30

Fluorine is related to the citric acid cycle because:

A) fluoroacetyl-CoA is also a substrate for citrate synthase
B) fluoroacetate, found in poisonous plants, acts as an inhibitor of aconitase
C) fluorocitrate acts as a potent inhibitor of the citric acid cycle
D) all of these

Question 31

In the conversion of succinyl-CoA to succinate, GTP is produced from GDP in a reaction in which the source of the added phosphate is

A) ATP.
B) ADP.
C) phosphenolpyruvate.
D) inorganic phosphate ion.

Question 32

Which of the following enzymes is the only membrane-bound enzyme in the citric acid cycle?

A) Aconitase.
B) Iso Citrate Dehydrogenase.
C) Succinate Dehydrogenase.
D) Malate Dehydrogenase.
E) Alpha -Ketoglutarate Dehydrogenase complex.

Question 33

The enzyme "aconitase" is also known as "condensing enzyme"

Question 34

Which coenzyme listed below is not associated with the α -ketoglutarate dehydrogenase complex?

A) thiamine pyrophosphate
B) lipoic acid
C) biotin
D) NAD⁺

Question 35

All but one of the enzymes of the citric acid cycle are found in this part of the mitochondrion:

A) The outer membrane.
B) The inner membrane.
C) The mitochondrial matrix.
D) The intermembrane space.
E) It is not known where these enzymes are located.

Question 36

The conversion of citrate to isocitrate is remarkable because

A) it is a condensation reaction.
B) a chiral center is introduced in a molecule that did not have one previously.
C) a dehydration reaction is involved.
D) the enzyme that catalyzes it has very little specificity.

Question 37

The only difference between succinate and fumarate is the geometry around their double bonds, one contains a cis double bond and the other contains a trans double bond.

Question 38

The iron ion, which is part of succinate dehydrogenase, is bonded to heme.

Question 39

The order of compounds and intermediates found in the citric acid cycle is as follows:

A) Iso Citrate → Aconitate → α -Ketoglutarate → Fumarate → Malate → Oxaloacetate
B) Aconitate → Iso Citrate → Oxaloacetate → α -Ketoglutarate → Malate → Fumarate
C) Aconitate → Iso Citrate → α -Ketoglutarate → Fumarate → Malate → Oxaloacetate
D) Aconitate → Iso Citrate → α -Ketoglutarate → Malate → Fumarate → Oxaloacetate
E) Iso Citrate → Aconitate → α -Ketoglutarate → Malate → Oxaloacetate → Fumarate

Question 40

Which enzyme catalyzes the reaction shown?

A) isocitrate dehydrogenase
B) malate dehydrogenase
C) fumarase
D) succinate dehydrogenase
E) none of these

Question 41

Which of the following enzymes is allosterically activated by NAD⁺?

A) isocitrate dehydrogenase
B) succinyl-CoA synthetase
C) succinate dehydrogenase
D) fumarase
E) none of these

Question 42

Exhibit 19A The following compounds are all intermediates in the citric acid cycle. Refer to Exhibit 19A. Which intermediate becomes bonded to Coenzyme A during the cycle?

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

Question 43

Exhibit 19A The following compounds are all intermediates in the citric acid cycle. Refer to Exhibit 19A. Which intermediate releases CO₂ concurrent with oxidation?

A) 1
B) 2
C) 3
D) Both 1 and 3
E) Both 2 and 3

Question 44

One round of the citric acid cycle generates about ten equivalents of ATP.

Question 45

Exhibit 19A The following compounds are all intermediates in the citric acid cycle. Refer to Exhibit 19A. Which intermediate is formed from acetyl-CoA and oxaloacetate.

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

Question 46

Which of the following enzymes is allosterically inhibited by ATP?

A) pyruvate dehydrogenase complex
B) succinyl-CoA synthetase
C) succinate dehydrogenase
D) fumarase

Question 47

A control point outside the citric acid cycle is the reaction catalyzed by

A) the pyruvate dehydrogenase complex.
B) citrate synthetase.
C) isocitrate dehydrogenase.
D) the α -ketoglutarate dehydrogenase complex.

Question 48

Which of these enzymes is most similar to pyruvate dehydrogenase?

A) Alpha -Ketoglutarate Dehydrogenase complex.
B) Iso Citrate Dehydrogenase.
C) Succinate Dehydrogenase.
D) Malate Dehydrogenase.
E) None of these enzymes is similar to pyruvate dehydrogenase.

Question 49

The conversion of malate to oxaloacetate has a high + Δ G (it is endergonic). It can take place because:

A) It is coupled to hydrolysis of the GTP produce earlier in the cycle.
B) It is coupled to hydrolysis of ATP from other sources.
C) It involves a substrate level phosphorylation.
D) The oxaloacetate product is used up in the subsequent reaction.
E) It is coupled to a strong reduction reaction.

Question 50

Which of the following is true regarding the control of pyruvate dehydrogenase?

A) It is inhibited by ATP
B) It is inhibited by NAD⁺
C) It is activated by acetyl-CoA
D) It is inhibited by succinyl-CoA
E) none of these are true

Question 51

The reaction in which malate is oxidized to oxaloacetate is not thermodynamically favored. It takes place because

A) it is coupled to ATP hydrolysis.
B) it involves substrate-level phosphorylation.
C) the product is continuously used up in the next reaction of the cycle, which is thermodynamically favored.
D) it is coupled to a strong reduction.

Question 52

Exhibit 19A The following compounds are all intermediates in the citric acid cycle. Refer to Exhibit 19A. Which intermediate does FAD oxidize?

A) 2
B) 3
C) 4
D) 5
E) More than one of these is oxidized by FAD.

Question 53

Exhibit 19A The following compounds are all intermediates in the citric acid cycle. Refer to Exhibit 19A. Which intermediate is formed from fumarate?

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

Question 54

A cell in an active metabolic state has

A) a high (ATP\ADP) and a high (NADH\NAD⁺) ratio.
B) a high (ATP\ADP) and a low (NADH\NAD⁺) ratio.
C) a low (ATP\ADP) and a low (NADH\NAD⁺) ratio.
D) a low (ATP\ADP) and a high (NADH\NAD⁺) ratio.

Question 55

The "energy charge" in a cell is important in the control of metabolism.

Question 56

"Energy charge" in a cell is a measure of

A) ATP\NAD⁺ ratios.
B) ATP\NADH ratios.
C) ATP\ADP ratios.
D) NADH\NAD⁺ ratios.
E) NAD⁺\ADP ratios.

Question 57

Which of the following enzymes is not a control point of the citric acid cycle?

A) citrate synthase
B) isocitrate dehydrogenase
C) aconitase
D) the α -ketoglutarate dehydrogenase complex

Question 58

Which of the following enzymes does not use NAD⁺ for oxidation?

A) Alpha -Ketoglutarate Dehydrogenase complex.
B) Iso Citrate Dehydrogenase.
C) Succinate Dehydrogenase.
D) Malate Dehydrogenase.
E) All of these enzymes use NAD⁺

Question 59

The glyoxylate cycle occurs in

A) plants and animals.
B) bacteria and animals.
C) plants and bacteria.
D) plants, animals, and bacteria.

Question 60

In muscle cells, the following reaction proceeds as written, i.e. , from left to right, despite having Δ G ° ' ≈ +30 kJ\mol. How can this occur?
Malate + NAD⁺ → oxaloacetate + NADH + H⁺

A) It is obviously thermodynamically favored under standard conditions.
B) In the cell, it is kinetically favored, even though it's thermodynamically unfavored.
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 Δ G ° ' to Δ G ° .

Question 61

The anaplerotic reactions associated with the citric acid cycle are the result of

A) the oxidative nature of the citric acid cycle
B) the use of many of the citric acid cycle intermediates in anabolism
C) the decarboxylation reactions
D) the production of GTP and reduced coenzymes

Question 62

The production of malate in the glyoxylate pathway is important, since it can be readily converted to phosphoenolpyruvate and then to sugars.

Question 63

The glyoxylate pathway bypasses part of the citric acid cycle by converting isocitrate to glyoxylate and

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

Question 64

The citric acid cycle uses anaplerotic reactions to get rid of the many intermediates of the cycle that accumulate during catabolism of amino acids.

Question 65

Weight loss in humans can be difficult to achieve, since we lack the ability to convert our fats to sugars, and it is difficult to change our metabolism to using fats as a primary energy source.

Question 66

The citric acid cycle is considered part of aerobic metabolism even though oxygen does not appear explicitly in any reaction because

A) the NADH and FADH₂ produced are reoxidized in the electron transport chain linked to oxygen
B) the reoxidation of NADH and FADH₂ leads to the production of considerable quantities of ATP
C) it takes place in the mitochondrion
D) it contains oxidation reactions

Question 67

Most of the products of the catabolism of sugars, fats and amino acids enter the citric acid cycle as:

A) pyruvate
B) acetyl-CoA
C) malate
D) all of these
E) none of these

Question 68

In the glyoxylate cycle, acetyl-CoA reacts with glyoxylate to produce

A) succinyl-CoA
B) succinate
C) fumarate
D) malate

Question 69

There is a cyclic reaction in which pyruvate becomes oxaloacetate. The oxaloacetate is converted to malate and then back to pyruvate. This cycle is important because:

A) There is no net use or fixation of CO₂ in this cycle.
B) NADH is converted to NADPH in this cycle.
C) There is no net oxidation or reduction in this cycle.
D) NADPH is converted to NADH in this cycle.
E) This is actually a wasteful pathway with no practical use.

Question 70

The intracellular site of the glyoxylate cycle is

A) glyoxysomes only.
B) glyoxysomes and lysosomes.
C) glyoxysomes and Golgi apparatus.
D) glyoxysomes and smooth endoplasmic reticulum.

Question 71

A unique feature of the glyoxylate cycle is that it allows the organisms that possess this pathway to

A) produce fats from carbohydrates.
B) produce carbohydrates from fats.
C) convert acetyl-CoA to pyruvate.
D) do all of the above.

Question 72

When the citric acid cycle is not functioning, the most common fate of acetyl-CoA from sugar metabolism in humans is the formation of fatty acids or cholesterol.

Question 73

An organism that undergoes the glyoxylate cycle can make sugar from fat because:

A) there is a specific isomerase that converts a six carbon fatty acid to glucose
B) the unique reactions of the glyoxylate cycle bypass the two decarboyxlation reactions of the citric acid cycle
C) glyoxysomes lack succinate dehydrogenase
D) none of these

Question 74

In the classical equation for respiration:
Glucose + 6O₂ --> 6CO₂ + 6H₂O, the following molecules are found directly in the citric acid cycle:

A) O₂
B) Glucose and O₂
C) CO₂ and H₂O
D) all of these arae found directly in the citric acid cycle

Question 75

Which of the following statements concerning the glyoxylate pathway is false ?

A) It utilizes one mole of acetyl-CoA per cycle.
B) It can produce a net synthesis of 4-carbon fragments that are intermediates of the citric acid cycle.
C) It usually occurs in the mitochondria
D) It is the main pathway that allows for synthesis of sugars from acetyl-CoA.

Question 76

Glyoxysomes are named for the fact that they contain the glyoxylate pathway.

Question 77

Intermediates of the citric acid cycle are especially important in the synthesis of fatty acids and amino acids.

Question 78

Which of the following cannot cross the inner mitochondrial membrane?

A) malate
B) phosphoenolpyruvate
C) succinyl-CoA
D) oxaloacetate

Question 79

Which of the following describes a use for acetyl-CoA as an important intermediate in metabolism?

A) Breakdown to CO₂ and water, yielding much energy.
B) Synthesis of terpenes and steroids.
C) Synthesis of oxaloacetate in plants.
D) Synthesis of fatty acids.
E) All of these are reasons why acetyl-CoA is a central molecule in metabolism.

Question 80

Which of the following is a source of NADPH?

A) the pentose phosphate pathway
B) a series of reactions in which oxaloacetate is reduced to malate followed by oxidative decarboxylation of the malate to pyruvate
C) both of the above
D) neither of these