Deck 14: Citric Acid Cycle and Glyoxylate Cycle

Question 1

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

Answer 1

C

Question 2

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

Answer 2

C

Question 3

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

Answer 3

lipoic acid; $\alpha$ -ketoglutarate dehydrogenase

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

Question 5

__________ 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

Question 6

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

Question 7

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

Question 8

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

Question 9

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

A)citrate $\rarr$ oxaloacetate + acetyl CoA
B)oxaloacetate $\rarr$ aspartate
C)glutamate $\rarr$ $\alpha$ -ketoglutarate
D)malate $\rarr$ pyruvate
E)all of the above

Question 10

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

Question 11

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

Question 12

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

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

Question 14

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

Question 15

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

Question 16

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

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

Question 18

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

Question 19

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

Question 20

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

Question 21

The glyoxylate cycle reaction catalyzed by isocitrate lyase is a reversible aldol condensation. As it is written below, the reaction is an aldol cleavage. Propose a mechanism for this reaction using the acid and base supplied by the enzyme.

Question 22

What is the source of acetyl-CoA for the glyoxylate cycle?

A)pyruvate
B)amino acids
C)fatty acids
D)cholesterol
E)all of the above

Question 23

The most common reactions of acetyl acetyl-CoA are with a nucleophile or with a base. Show the reactions of acetyl CoA with the generic base and the generic nucleophile:

Question 24

The reaction catalyzed by malate synthase is shown below. Which citric acid cycle enzyme would have a mechanism most similar to malate synthase?

A)malate dehydrogenase
B)citrate synthase
C)fumarase
D)succinyl-CoA synthetase
E)none of the above

Question 25

Draw the enzyme-bound intermediate of the following reaction clearly showing proper stereochemistry: