Deck 14: The Citric Acid Cycle

Question 1

What amino acid is covalently linked to lipoic acid in E2?

A) Lys
B) Ser
C) His
D) Gln
E) Thr

Answer 1

Lys

Question 2

What makes the reaction catalyzed by citrate synthase highly exergonic?

A) hydrolysis of ATP
B) hydrolysis of GTP
C) decarboxylation reaction
D) conversion from keto to enol tautomer
E) hydrolysis of thioester

Answer 2

hydrolysis of thioester

Question 3

Which of the following explains the proper binding of the two substrates for citrate synthase?

A) the binding is random
B) acetyl-CoA binds, then oxaloacetate
C) oxaloacetate binds, then acetyl-CoA
D) oxaloacetate binds to one subunit, acetyl-CoA binds to the other
E) in one subunit, acetyl-CoA binds, then oxaloacetate; on the other subunit, this is reversed

Answer 3

oxaloacetate binds, then acetyl-CoA

Question 4

Which of the following is observed in the mechanism of citrate synthase?

A) a histidine residue acts as a base, deprotonating oxaloacetate
B) an aspartic acid residue acts as a base, deprotonating oxaloacetate
C) a histidine residue hydrogen bonds with the carbonyl oxygen of oxaloacetate to enable a nucleophilic attack
D) removal of the CoA is accomplished by transfer to a serine residue, then hydrolysis
E) all of the above

Question 5

Which of the following describes the isomerization of citrate to isocitrate?

A) a major regulatory step for the citric acid cycle
B) an oxidation reaction
C) the only unnecessary step of the citric acid cycle
D) protects cells from the toxic effects of arsenite ion
E) converts a tertiary alcohol, which cannot easily be oxidized, to a secondary alcohol that can be oxidized

Question 6

During the oxidation of isocitrate, the intermediate that is decarboxylated to form $\alpha$ -ketoglutarate is _____.

A) oxalosuccinate
B) succinyl-phosphate
C) cis-aconitate
D) carboxybiotin
E) hydroxyethyl-TPP

Question 7

Which of the following is an enzyme-associated intermediate that is similar in energy to 1,3-bisphosphoglycerate?

A) phosphohistidine
B) succinyl phosphate
C) oxalosuccinate
D) carboxybiotin
E) hydroxyethyl-TPP

Question 8

If acetyl-CoA labeled on the methyl carbon were used as the substrate for the citric acid cycle, where would the labeled carbon appear in oxaloacetate produced from one round of the cycle?

A) C1
B) C4
C) C1 or C2
D) C1 or C4
E) C2 or C3

Question 9

If all reduced electron carriers are reoxidized to run oxidative phosphorylation, how many ATP can be generated from the oxidation of one acetyl-CoA?

A) 6
B) 10
C) 15
D) 20
E) 32

Question 10

Which of the following can be converted into glucose using parts of the citric acid cycle?

A) malate
B) isocitrate
C) glutamic acid
D) succinate
E) all of the above

Question 11

What citric acid cycle intermediate is effectively used as a mechanism for the transport of acetyl-CoA from the mitochondria to the cytosol?

A) citrate
B) isocitrate
C) fumarate
D) malate
E) oxaloacetate

Question 12

When acetyl-CoA and NADPH are needed for lipid synthesis, which of the following reactions is used to produce NADPH in the cytosol?

A) reduction of oxaloacetate to malate
B) oxidation of malate to pyruvate
C) reduction of fumarate to succinate
D) oxidation of malate to oxaloacetate
E) oxidation of isocitrate to $\beta$ -ketoglutarate

Question 13

Which of the following terms indicates that the citric acid cycle is both catabolic and anabolic in nature?

A) cataplerotic
B) amphipathic
C) anaplerotic
D) diabolic
E) amphibolic

Question 14

Which of the following enzymes allows for a net increase of citric acid cycle metabolites without expending any energy in the form of ATP or its equivalents?

A) alanine aminotransferase
B) pyruvate dehydrogenase
C) pyruvate carboxylase
D) citrate lyase
E) none of the above

Question 15

The mechanism of malate synthase that catalyzes the conversion of glyoxylate and acetyl-CoA to malate would be most similar to which citric acid cycle enzyme?

A) fumarase
B) aconitase
C) citrate synthase
D) isocitrate dehydrogenase
E) succinyl-CoA synthetase

Question 16

Why can germinating plant seeds convert acetyl-CoA from fatty acids into carbohydrates, while animals are incapable of converting fatty acids into glucose?

A) animals have glycogen and don't need to make glucose from fatty acids
B) plants, not animals, use the glyoxylate cycle to convert acetyl CoA to oxaloacetate
C) plant seeds use photosynthesis to make sugar
D) animals use the citric acid cycle exclusively for energy production, plants only use glycolysis for energy
E) none of the above

Question 17

If acetyl-CoA labeled with ¹⁴C, as shown in the figure, were used as the substrate for the citric acid cycle, which of the following intermediates would be produced during the first round of the cycle?

A)

B)

C)

D)

E)