Question 1

Which of the following statements best describes the role of the pyruvate dehydrogenase complex in cellular metabolism?

Accepted Answer

It catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle.

Question 2

Which coenzyme is NOT directly involved in the pyruvate dehydrogenase complex?

Accepted Answer

NADP+

Question 3

The reaction catalyzed by citrate synthase is considered irreversible in the cell. What is the main reason for this irreversibility?

Accepted Answer

The reaction has a large negative standard free energy change ($$\Delta G^\circ' = -32.2\ \mathrm{kJ/mol}).$$

Question 4

Which enzyme in the citric acid cycle contains an iron-sulfur cluster that is essential for its catalytic activity?

Accepted Answer

Aconitase

Question 5

Which of the following best explains why the oxidation of malate to oxaloacetate proceeds in the forward direction in the cell, despite a positive $$\Delta G^\circ'$$?

Accepted Answer

The concentration of oxaloacetate is kept extremely low by its rapid consumption in the next step.

Question 6

Which of the following is a primary allosteric activator of isocitrate dehydrogenase in the citric acid cycle?

Accepted Answer

ADP

Question 7

A patient has a genetic deficiency in the malate-aspartate shuttle. Which of the following would most likely be observed in their liver cells?

Accepted Answer

Decreased ATP yield from cytosolic NADH produced during glycolysis

Question 8

Which of the following reactions is an example of an anaplerotic reaction in animal cells?

Accepted Answer

$$\mathrm{Pyruvate} + \mathrm{CO}_2 + \mathrm{ATP} \rightarrow \mathrm{Oxaloacetate} + \mathrm{ADP} + \mathrm{P_i}$$

Question 9

Which of the following best describes the fate of the two carbons from acetyl-CoA after one turn of the citric acid cycle?

Accepted Answer

They are not released as CO$$_2 in $$the first turn but are incorporated into the cycle intermediates.

Question 10

Which of the following is the correct net equation for the oxidation of one molecule of glucose through glycolysis, pyruvate dehydrogenase, and the citric acid cycle?

Accepted Answer

$$\mathrm{Glucose} + 10\ \mathrm{NAD}^+ + 2\ \mathrm{FAD} + 4\ \mathrm{ADP} + 4\ \mathrm{P_i} + 2\ \mathrm{H}_2\mathrm{O} \rightarrow 6\ \mathrm{CO}_2 + 10\ \mathrm{NADH} + 2\ \mathrm{FADH}_2 + 4\ \mathrm{ATP} + 6\ \mathrm{H}^+$$

Question 11

Which of the following best explains why fatty acids with chain lengths greater than 14 carbons require the carnitine shuttle for mitochondrial entry?

Accepted Answer

Carnitine is smaller and more easily transported across the mitochondrial membrane than CoA.

Question 12

Which of the following is a committed and regulated step in fatty acid synthesis?

Accepted Answer

Conversion of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase (ACC)

Question 13

Which of the following statements about the regulation of the citric acid cycle is CORRECT?

Accepted Answer

High concentrations of NADH inhibit isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Question 14

Which of the following best describes the role of the Q-cycle in the electron transport chain?

Accepted Answer

It transfers electrons from CoQH$$_2 to $$cytochrome c, while translocating protons across the inner mitochondrial membrane.

Question 15

Which of the following is a direct product of the succinyl-CoA synthetase reaction in the citric acid cycle?

Accepted Answer

GTP (or ATP)

Question 16

Which of the following best explains why mammals cannot convert acetyl-CoA derived from fatty acid oxidation into glucose?

Accepted Answer

The two carbons of acetyl-CoA are lost as CO$$_2 in $$the citric acid cycle, preventing net synthesis of glucose.

Question 17

Which of the following is a key regulatory effect of malonyl-CoA in fatty acid metabolism?

Accepted Answer

It inhibits carnitine acyltransferase I, preventing fatty acid entry into mitochondria.

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

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