Question 1

Which of the following statements best describes the primary function of the tricarboxylic acid (TCA) cycle in cellular metabolism?

Accepted Answer

It oxidizes acetyl CoA to CO$_2$, generating NADH and FADH$_2$ for ATP production via the electron transport chain.

Question 2

Which of the following is NOT a coenzyme required by the pyruvate dehydrogenase complex (PDHC)?

Accepted Answer

Biotin

Question 3

A patient presents with neurologic symptoms and lactic acidosis after exposure to arsenic. Which enzyme complex is most likely inhibited, and what is the mechanism?

Accepted Answer

Pyruvate dehydrogenase complex; arsenic binds to the thiol groups of lipoic acid, inhibiting its function.

Question 4

Which regulatory enzyme of the TCA cycle is allosterically activated by ADP and Ca$^{2+}$, and inhibited by ATP and NADH?

Accepted Answer

Isocitrate dehydrogenase

Question 5

During one complete turn of the TCA cycle, how many molecules of NADH, FADH$_2$, and GTP are produced, respectively?

Accepted Answer

3 NADH, 1 FADH$_2$, 1 GTP

Question 6

Which of the following best explains why the TCA cycle is considered amphibolic?

Accepted Answer

It is involved in both catabolic and anabolic processes, providing intermediates for biosynthesis as well as energy production.

Question 7

A mutation causes a deficiency in the E1 subunit of the pyruvate dehydrogenase complex. Which of the following would most likely accumulate in the cell as a result?

Accepted Answer

Pyruvate

Question 8

Which reaction in the TCA cycle is an example of substrate-level phosphorylation?

Accepted Answer

Conversion of succinyl CoA to succinate by succinate thiokinase (succinyl CoA synthetase)

Question 9

Which of the following best describes the regulation of the pyruvate dehydrogenase complex (PDHC)?

Accepted Answer

PDH kinase phosphorylates and inactivates E1, while PDH phosphatase dephosphorylates and activates E1.

Question 10

Calculate the total number of ATP molecules generated from the complete oxidation of one molecule of pyruvate to CO$_2$ via the TCA cycle and associated processes. (Assume 1 NADH = 3 ATP, 1 FADH$_2$ = 2 ATP, and 1 GTP = 1 ATP.)

Accepted Answer

$15$

Question 11

Which enzyme of the TCA cycle is embedded in the inner mitochondrial membrane and also functions as Complex II of the electron transport chain?

Accepted Answer

Succinate dehydrogenase

Question 12

A patient with a deficiency in lipoic acid would have impaired activity of which of the following enzyme complexes?

Accepted Answer

Pyruvate dehydrogenase complex and α-ketoglutarate dehydrogenase complex

Question 13

Which of the following statements about citrate synthase is TRUE?

Accepted Answer

It is inhibited by its product, citrate.

Question 14

Which of the following best explains why the TCA cycle does not lead to a net production or consumption of its intermediates?

Accepted Answer

The cycle regenerates oxaloacetate at the end of each turn, allowing the cycle to continue.

Question 15

Which of the following is a potent inhibitor of PDH kinase, thereby promoting the active (dephosphorylated) form of the pyruvate dehydrogenase complex?

Accepted Answer

Pyruvate

Question 16

A patient with congenital lactic acidosis is found to have a mutation in the gene encoding the E1 subunit of the PDHC. Which of the following is the most likely biochemical consequence?

Accepted Answer

Accumulation of pyruvate and increased conversion to lactate

Question 17

Which of the following best describes the role of calcium ions (Ca$^{2+}$) in the regulation of the TCA cycle?

Accepted Answer

They activate PDH phosphatase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase, increasing energy production during muscle contraction.

Question 18

Which of the following reactions in the TCA cycle directly produces CO$_2$?

Accepted Answer

Isocitrate to α-ketoglutarate and α-ketoglutarate to succinyl CoA

Question 19

Which of the following best explains why arsenic poisoning leads to neurologic symptoms and death?

Accepted Answer

Arsenic binds to lipoic acid, inhibiting the PDHC and α-ketoglutarate dehydrogenase, leading to energy failure in the brain.

Tricarboxylic Acid Cycle (TCA Cycle) and Pyruvate Dehydrogenase Complex: Structure, Function, and Regulation

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