Question 1

Which enzyme complex catalyzes the conversion of pyruvate to acetyl-CoA in the mitochondria?

Accepted Answer

Pyruvate dehydrogenase complex

Question 2

What is the net reaction of the citric acid cycle for one turn, starting from acetyl-CoA?

Accepted Answer

$$\mathrm{Acetyl{-}CoA} + 3\mathrm{NAD}^+ + \mathrm{FAD} + \mathrm{GDP} + \mathrm{P}_i + 2\mathrm{H}_2O \rightarrow 2\mathrm{CO}_2 + 3\mathrm{NADH} + 3\mathrm{H}^+ + \mathrm{FADH}_2 + \mathrm{GTP} + \mathrm{CoA}$$

Question 3

Which cofactor is directly involved in the decarboxylation of pyruvate by pyruvate dehydrogenase?

Accepted Answer

Thiamine pyrophosphate (TPP)

Question 4

In the regulation of pyruvate dehydrogenase in mammals, which molecule acts as an allosteric inhibitor?

Accepted Answer

Acetyl-CoA

Question 5

Which of the following statements about the mitochondrial transport of pyruvate is correct?

Accepted Answer

Pyruvate is transported into the mitochondria by a specific translocase protein.

Question 6

During the conversion of pyruvate to acetyl-CoA, which of the following is released as a byproduct?

Accepted Answer

CO$$_2$$

Question 7

Which subunit of the pyruvate dehydrogenase complex is responsible for the transfer of the acetyl group to Coenzyme A?

Accepted Answer

E$$_2 ($$Dihydrolipoyl transacetylase)

Question 8

Which molecule is the final electron acceptor in the reaction catalyzed by E$$_3 ($$dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex?

Accepted Answer

NAD$$^+$$

Question 9

A patient with a deficiency in thiamine (vitamin B$$_1) $$would most likely have impaired activity of which enzyme complex?

Accepted Answer

Pyruvate dehydrogenase complex

Question 10

Which of the following best describes the role of lipoamide in the pyruvate dehydrogenase complex?

Accepted Answer

It serves as a swinging arm to transfer the acetyl group between active sites.

Question 11

Which of the following reactions is catalyzed by the E$$_3$$ subunit of the pyruvate dehydrogenase complex?

Accepted Answer

Oxidation of reduced lipoamide by FAD and transfer of electrons to NAD$$^+$$

Question 12

Which of the following would most likely increase the activity of pyruvate dehydrogenase in the mitochondrial matrix?

Accepted Answer

High concentrations of pyruvate and ADP

Question 13

What is the role of pyruvate dehydrogenase kinase in the regulation of the pyruvate dehydrogenase complex?

Accepted Answer

It inactivates the complex by phosphorylation.

Question 14

Which of the following is a direct product of the pyruvate dehydrogenase reaction?

Accepted Answer

Acetyl-CoA

Question 15

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

Accepted Answer

It functions in both catabolic and anabolic pathways.

Question 16

Which of the following statements about the fate of electrons from NADH and FADH$$_2$$ produced in the citric acid cycle is correct?

Accepted Answer

They are transferred to the electron transport chain to generate ATP.

Question 17

A mutation that impairs the function of dihydrolipoyl dehydrogenase (E$$_3) $$would most directly affect which step in the pyruvate dehydrogenase reaction?

Accepted Answer

Regeneration of oxidized lipoamide

Question 18

Which of the following is NOT a product of one turn of the citric acid cycle?

Accepted Answer

Glucose

Question 19

Which of the following best describes the role of FAD in the pyruvate dehydrogenase complex?

Accepted Answer

It accepts electrons from reduced lipoamide and transfers them to NAD$$^+.$$

The Aerobic Fate of Pyruvate and the Citric Acid Cycle

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