Question 1

Which of the following statements best describes the amphibolic nature of the citric acid cycle?

Accepted Answer

It functions in both catabolic and anabolic processes, providing intermediates for biosynthesis and energy production.

Question 2

What is the net standard free energy change ($\Delta G^{\circ'}$) for the conversion of pyruvate to acetyl-CoA by the pyruvate dehydrogenase complex?

Accepted Answer

$\Delta G^{\circ'} = -33.4\ \mathrm{kJ/mol}$

Question 3

Which coenzyme is NOT required by the pyruvate dehydrogenase complex?

Accepted Answer

Biotin

Question 4

During the citric acid cycle, which enzyme catalyzes the formation of citrate from oxaloacetate and acetyl-CoA?

Accepted Answer

Citrate synthase

Question 5

Which step in the citric acid cycle is thermodynamically unfavorable under standard conditions?

Accepted Answer

Oxidation of malate to oxaloacetate

Question 6

What is the role of lipoate in the pyruvate dehydrogenase complex?

Accepted Answer

It serves as a redox cofactor and acyl carrier, allowing internal disulfide bond formation.

Question 7

Which enzyme in the citric acid cycle is directly inhibited by high levels of ATP?

Accepted Answer

Citrate synthase

Question 8

A patient presents with a deficiency in biotin. Which reaction in the citric acid cycle would be most directly affected?

Accepted Answer

Carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase

Question 9

Which of the following best explains substrate channeling in the pyruvate dehydrogenase complex?

Accepted Answer

Intermediates are passed directly from one active site to another without diffusing away, increasing efficiency.

Question 10

Which intermediate is formed during the conversion of citrate to isocitrate in the citric acid cycle?

Accepted Answer

cis-Aconitate

Question 11

The oxidation of succinate to fumarate is catalyzed by which enzyme, and what is the associated electron carrier?

Accepted Answer

Succinate dehydrogenase; FAD

Question 12

Which of the following is a direct product of the conversion of succinyl-CoA to succinate?

Accepted Answer

GTP

Question 13

Which enzyme catalyzes the oxidative decarboxylation of isocitrate, and what cofactor does it require?

Accepted Answer

Isocitrate dehydrogenase; Mn$^{2+}$

Question 14

What is the main regulatory effect of NADH on the citric acid cycle?

Accepted Answer

It inhibits several enzymes, including pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, via feedback inhibition.

Question 15

Which of the following reactions in the citric acid cycle produces CO$_2$ as a byproduct?

Accepted Answer

Oxidation of isocitrate to α-ketoglutarate

Question 16

A mutation in the gene encoding succinate dehydrogenase would most likely affect which cellular process?

Accepted Answer

Electron transport chain and citric acid cycle

Question 17

Which of the following best describes the role of pyruvate carboxylase in cellular metabolism?

Accepted Answer

It carboxylates pyruvate to form oxaloacetate, replenishing TCA cycle intermediates.

Question 18

Which of the following is NOT a direct inhibitor of pyruvate dehydrogenase?

Accepted Answer

GTP

Question 19

What is the main reason the citric acid cycle is considered a central metabolic hub?

Accepted Answer

It provides intermediates for both energy production and biosynthetic pathways.

The Citric Acid Cycle (TCA/Krebs Cycle): Structure, Function, and Regulation

Study Guide - Practice Questions