Question 1

Which of the following citric acid cycle enzymes is bypassed in the glyoxylate cycle?
A)citrate synthase
B)aconitase
C)isocitrate dehydrogenase
D)succinate dehydrogenase
E)fumarase

Accepted Answer

The glyoxylate cycle is a modified version of the citric acid cycle found in certain microorganisms and plants. In this pathway, the isocitrate dehydrogenase enzyme is bypassed, and isocitrate is instead converted into succinate and glyoxylate by the enzymes isocitrate lyase and malate synthase.

Question 2

Which of the following coenzymes is required for the decarboxylation of pyruvate by the pyruvate dehydrogenase complex? A)thiamine pyrophosphate B)coenzyme A C)lipoic acid D)NAD⁺ E)FAD

Accepted Answer

Thiamine pyrophosphate (TPP) is a coenzyme required for the decarboxylation of pyruvate by the pyruvate dehydrogenase complex.

Question 3

Which of the following best explains why thioesters have a greater free energy of hydrolysis than ordinary esters?
A)alcohols are better leaving groups than thiols
B)oxygen is more electronegative than sulfur
C)esters are resonance stabilized while thioesters are not
D)the oxygen of an ester is more easily protonated than the sulfur of a thioester
E)none of the above

Accepted Answer

Thioesters lack the extensive resonance stabilization that esters benefit from due to the difference in orbital overlap between sulfur and carbon compared to oxygen and carbon. This makes thioesters less stable and thus they have a greater free energy of hydrolysis.

Question 4

The two reactions of the citric acid cycle where CO₂ is produced are catalyzed by _________ and _________ A)isocitrate dehydrogenase;$\alpha$-ketoglutarate dehydrogenase B)isocitrate dehydrogenase;malate dehydrogenase C)$\alpha$-ketoglutarate dehydrogenase;succinate dehydrogenase D)$\alpha$-ketoglutarate dehydrogenase;malate dehydrogenase E)succinate dehydrogenase;malate dehydrogenase

Accepted Answer

The two reactions of the citric acid cycle where NADH is produced are catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Question 5

For each molecule of acetyl-CoA that enters the citric acid cycle,which of the following best represents the net products of the cycle? A)2 CO₂,2 NADH,2 FADH₂,1 GTP,1 oxaloacetate B)1 CO₂,3 NADH,1 FADH₂,2 GTP,1 oxaloacetate C)2 CO₂,3 NADH,1 FADH₂,1 GTP,0 oxaloacetate D)1 CO₂,2 NADH,1 FADH₂,2 GTP,0 oxaloacetate E)2 CO₂,3 NADH,1 FADH₂,1 GTP,1 oxaloacetate

Accepted Answer

The answer of For each molecule of acetyl-CoA that enters...

Question 6

Which of the following reactions is commonly used to replenish intermediates of the citric acid cycle?
A)citrate $\to$ oxaloacetate + acetyl CoA
B)oxaloacetate $\to$ aspartate
C)glutamate $\to$ $\alpha$-ketoglutarate
D)malate $\to$ pyruvate
E)all of the above

Accepted Answer

The answer of Which of the following reactions is commonly...

Question 7

Arsenic poisoning by either arsenite or organic arsenical agents work by binding to dithiol compounds such as _________ meaning that in addition to inactivation of pyruvate dehydrogenase,the citric acid cycle enzyme _________ would be inactivated.
A)cysteine;aconitase
B)coenzyme A;$\alpha$-ketoglutarate dehydrogenase
C)coenzyme A;citrate synthase
D)thiamine pyrophosphate;isocitrate dehydrogenase
E)lipoic acid;$\alpha$-ketoglutarate dehydrogenase

Accepted Answer

Arsenic poisoning binds to dithiol compounds such as lipoic acid, which is a cofactor for F1F1F1S1F1F1F10-ketoglutarate dehydrogenase enzyme in the citric acid cycle. Therefore, choice E is the best answer.

Question 8

Which oxidation reaction requires the coenzyme FAD because it involves oxidation of a carbon-carbon single bond to a double bond?
A)oxidation of isocitrate to $\alpha$-ketoglutarate
B)oxidation of $\alpha$-ketoglutarate to succinyl-CoA
C)oxidation of succinate to fumarate
D)oxidation of malate to oxaloacetate
E)oxidation of pyruvate to acetyl-CoA

Accepted Answer

The oxidation of succinate to fumarate requires the coenzyme FAD because it involves oxidation of a carbon-carbon single bond to a double bond. This reaction is catalyzed by the enzyme succinate dehydrogenase, which is bound to the inner mitochondrial membrane and is a part of the electron transport chain. FAD accepts electrons from succinate and becomes reduced to FADH2, which then donates its electrons to the electron transport chain.

Question 9

What role does the coenzyme A portion of acetyl-CoA play in the reaction catalyzed by citrate synthase?
A)the CoA group causes the enzyme to close around the substrates,enabling catalysis
B)lack of resonance in the thioester makes the methyl carbon of the acetyl group more acidic,allowing proton abstraction to readily occur
C)CoA makes the carbonyl carbon highly susceptible to nucleophilic attack
D)hydrolysis of the thioester makes the reaction essentially irreversible
E)all of the above

Accepted Answer

The coenzyme A portion of acetyl-CoA plays multiple roles in the reaction catalyzed by citrate synthase: it causes the enzyme to close around the substrates, enabling catalysis; the lack of resonance in the thioester bond makes the methyl carbon more acidic, facilitating proton abstraction; it makes the carbonyl carbon highly susceptible to nucleophilic attack; and the hydrolysis of the thioester bond makes the reaction essentially irreversible.

Question 10

The complete oxidation of the acetyl group that enters the citric acid cycle yields _____ electrons from the methyl carbon and ____ electrons from the carbonyl carbon.
A)4;2
B)6;2
C)6;4
D)8;2
E)8;2.

Accepted Answer

The answer of The complete oxidation of the acetyl group...

Question 11

__________ activates __________,thereby inhibiting pyruvate dehydrogenase. A)NADH;pyruvate dehydrogenase phosphatase B)Ca²⁺;pyruvate dehydrogenase phosphatase C)NAD⁺;pyruvate dehydrogenase kinase D)ATP;pyruvate dehydrogenase kinase E)pyruvate;pyruvate dehydrogenase kinase

Accepted Answer

ATP activates pyruvate dehydrogenase kinase, which then phosphorylates and inhibits pyruvate dehydrogenase. NADH is a product of the pyruvate dehydrogenase reaction and can inhibit pyruvate dehydrogenase phosphatase, but it does not directly activate pyruvate dehydrogenase kinase. Choices A, B, and E are not valid options as they contain nonsensical characters or are not relevant to the pathway.

Question 12

Which of the following is an inhibitor of citrate synthase,isocitrate dehydrogenase,and $\alpha$-ketoglutarate dehydrogenase?
A)NADH
B)succinyl-CoA
C)ATP
D)acetyl-CoA
E)none of the above

Accepted Answer

The answer of Which of the following is an inhibitor...

Question 13

The oxidation reactions that convert pyruvate to CO₂ are specifically catalyzed by ______ enzymes. A)oxygenase B)oxidase C)decarboxylase D)dehydrogenase E)none of the above

Accepted Answer

The conversion of pyruvate to CO₂ requires the removal of two hydrogen atoms and two electrons, which is accomplished by dehydrogenase enzymes. Therefore, the correct answer is D) dehydrogenase.

Question 14

Which of the following molecules is correctly paired with the enzyme that can convert it to oxaloacetate to replenish the citric acid cycle when other intermediates are removed?
A)aspartate: aspartate deaminase
B)pyruvate: malic enzyme
C)phosphoenolpyruvate: PEP carboxylase
D)glutamate: glutamate transaminase
E)none of the above

Accepted Answer

Phosphoenolpyruvate (PEP) can be converted to oxaloacetate by the enzyme PEP carboxylase, which replenishes the citric acid cycle. Aspartate, pyruvate, and glutamate are also intermediates in metabolic pathways, but they are not directly converted to oxaloacetate to replenish the citric acid cycle. Aspartate deaminase removes an amine group from aspartate, converting it to fumarate. Malic enzyme converts malate to pyruvate. Glutamate transaminase transfers an amine group from glutamate to another molecule.

Question 15

Which of the following is an enzyme-bound intermediate in the reaction catalyzed by succinyl-CoA synthetase?
A)succinyl phosphate
B)glutamyl-CoA
C)N-phosphohistidine
D)N-ADP-histidine
E)none of the above

Accepted Answer

N-phosphohistidine is an enzyme-bound intermediate in the reaction catalyzed by succinyl-CoA synthetase. This enzyme catalyzes the conversion of succinyl-CoA to succinate, during which a phosphate group is transiently transferred to a histidine residue on the enzyme, forming N-phosphohistidine.

Question 16

Which of the following enzymes of the pyruvate dehydrogenase complex is responsible for the oxidation of the hydroxyethyl group from pyruvate decarboxylation to an acetyl group?
A)pyruvate dehydrogenase
B)dihydrolipoamide transacetylase
C)dihydrolipoamide dehydrogenase
D)pyruvate dehydrogenase kinase
E)pyruvate dehydrogenase phosphatase

Accepted Answer

Dihydrolipoamide transacetylase catalyzes the transfer of the hydroxyethyl group from pyruvate decarboxylation to coenzyme A, forming acetyl-CoA.

Question 17

The conversion of citrate to isocitrate by the enzyme aconitase is best described as ______ reaction consisting of a ________ followed by a ________.
A)isomerization;oxidation;reduction
B)isomerization;hydration;dehydration
C)isomerization;dehydration;hydration
D)group transfer;dehydroxylation;hydroxylation
E)group transfer;hydroxylation;dehydroxylation

Accepted Answer

The conversion of citrate to isocitrate involves an isomerization reaction where the arrangement of atoms is changed without any net oxidation or reduction. Aconitase catalyzes this isomerization by dehydrating citrate and then rehydrating the molecule at a different position, resulting in the formation of isocitrate. Therefore, the reaction consists of a dehydration (removal of water) followed by a hydration (addition of water) step.

Question 18

In the citric acid cycle,acetyl-CoA reacts with ________ to produce _________.
A)fumarate;$\alpha$-ketoglutarate
B)oxaloacetate;citrate
C)citrate;isocitrate
D)succinyl-CoA;oxaloacetate
E)malate;cis-aconitate

Accepted Answer

Acetyl-CoA reacts with oxaloacetate to produce citrate in the citric acid cycle.

Question 19

Which of the following is an equilibrium-controlled reaction of the citric acid cycle?
A)citrate synthase
B)isocitrate dehydrogenase
C)$\alpha$-ketoglutarate dehydrogenase
D)succinate dehydrogenase
E)all of the above

Accepted Answer

The answer of Which of the following is an equilibrium-controlled...

Question 20

Which of the following correctly lists the products of the pyruvate dehydrogenase complex in the correct order of release? A)acetyl-CoA,CO₂,NADH B)acetyl-CoA,CO₂,FADH₂ C)CO₂,acetyl-CoA,NADH D)CO₂,acetyl-CoA,FADH₂ E)CO₂,acetyl-CoA,FADH₂,NADH

Accepted Answer

The pyruvate dehydrogenase complex produces acetyl-CoA, CO2, and NADH in that order. FADH2 is not a product of the pyruvate dehydrogenase complex, so options B, D, and E can be eliminated. Option A has the correct products, but in the wrong order. Therefore, the best choice is C, which correctly lists the products in the order of their release.

Quiz 14: Citric Acid Cycle and Glyoxylate Cycle

Which of the following citric acid cycle enzymes is bypassed in the glyoxylate cycle?

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

Which of the following best explains why thioesters have a greater free energy of hydrolysis than ordinary esters?

The two reactions of the citric acid cycle where CO₂ is produced are catalyzed by _ and _

For each molecule of acetyl-CoA that enters the citric acid cycle,which of the following best represents the net products of the cycle?

Which of the following reactions is commonly used to replenish intermediates of the citric acid cycle?

Arsenic poisoning by either arsenite or organic arsenical agents work by binding to dithiol compounds such as _ meaning that in addition to inactivation of pyruvate dehydrogenase,the citric acid cycle enzyme _ would be inactivated.

Which oxidation reaction requires the coenzyme FAD because it involves oxidation of a carbon-carbon single bond to a double bond?

What role does the coenzyme A portion of acetyl-CoA play in the reaction catalyzed by citrate synthase?

The complete oxidation of the acetyl group that enters the citric acid cycle yields _ electrons from the methyl carbon and electrons from the carbonyl carbon.

activates ,thereby inhibiting pyruvate dehydrogenase.

Which of the following is an inhibitor of citrate synthase,isocitrate dehydrogenase,and $\alpha$ -ketoglutarate dehydrogenase?

The oxidation reactions that convert pyruvate to CO₂ are specifically catalyzed by ______ enzymes.

Which of the following molecules is correctly paired with the enzyme that can convert it to oxaloacetate to replenish the citric acid cycle when other intermediates are removed?

Which of the following is an enzyme-bound intermediate in the reaction catalyzed by succinyl-CoA synthetase?

Which of the following enzymes of the pyruvate dehydrogenase complex is responsible for the oxidation of the hydroxyethyl group from pyruvate decarboxylation to an acetyl group?

The conversion of citrate to isocitrate by the enzyme aconitase is best described as reaction consisting of a followed by a __.

In the citric acid cycle,acetyl-CoA reacts with to produce _.

Which of the following is an equilibrium-controlled reaction of the citric acid cycle?

Which of the following correctly lists the products of the pyruvate dehydrogenase complex in the correct order of release?