Q: Which of the following equations correctly represents the net reaction for one turn of the citric acid cycle?

$$\mathrm{Acetyl\text{-}CoA} + 2\,\mathrm{H_2O} + 3\,\mathrm{NAD^+} + \mathrm{FAD} + \mathrm{ADP} + \mathrm{P_i} \rightarrow 2\,\mathrm{CO_2$} + 3\,\mathrm{NADH} + 3\,\mathrm{H^+} + \mathrm{$$FADH_2$$} + \mathrm{HS\text{-}CoA} + \mathrm{ATP}$$

Question 1

Which of the following best describes the primary function of the citric acid cycle in cellular metabolism?

Accepted Answer

To oxidize acetyl-CoA, generating reduced electron carriers and a small amount of ATP

Question 2

During the conversion of pyruvate to acetyl-CoA by the pyruvate dehydrogenase (PDH) complex, which coenzyme is responsible for accepting the acetyl group from lipoamide?

Accepted Answer

Coenzyme A (CoA)

Question 3

Which of the following equations correctly represents the net reaction for one turn of the citric acid cycle?

Accepted Answer

$$\mathrm{Acetyl	ext{-}CoA} + 2\,\mathrm{H_2O} + 3\,\mathrm{NAD^+} + \mathrm{FAD} + \mathrm{ADP} + \mathrm{P_i} ightarrow 2\,\mathrm{CO_2$} + 3\,\mathrm{NADH} + 3\,\mathrm{H^+} + \mathrm{$$FADH_2$$} + \mathrm{HS	ext{-}CoA} + \mathrm{ATP}$$

Question 4

A patient is diagnosed with a mutation that inactivates the E1 subunit of the pyruvate dehydrogenase complex. Which of the following metabolic consequences is most likely?

Accepted Answer

Impaired conversion of pyruvate to acetyl-CoA, leading to lactic acidosis

Question 5

Which coenzyme of the PDH complex is directly involved in stabilizing the carbanion intermediate during the decarboxylation of pyruvate?

Accepted Answer

Thiamine pyrophosphate (TPP)

Question 6

In the citric acid cycle, which enzyme catalyzes a substrate-level phosphorylation to generate ATP (or GTP)?

Accepted Answer

Succinyl-CoA synthetase

Question 7

Which of the following best explains why vertebrates cannot convert fatty acids into glucose?

Accepted Answer

The conversion of acetyl-CoA to pyruvate is highly exergonic and irreversible

Question 8

Which regulatory mechanism most directly increases the activity of isocitrate dehydrogenase in the citric acid cycle?

Accepted Answer

Allosteric activation by ADP

Question 9

A defect in which citric acid cycle enzyme is most likely to result in the accumulation of succinate and is associated with certain types of cancer?

Accepted Answer

Succinate dehydrogenase

Question 10

Which of the following statements about the glyoxylate cycle is correct?

Accepted Answer

It allows plants and some microorganisms to convert acetyl-CoA into glucose by bypassing the decarboxylation steps of the citric acid cycle

Question 11

Which of the following best describes an anaplerotic reaction in the context of the citric acid cycle?

Accepted Answer

A reaction that replenishes citric acid cycle intermediates

Question 12

Which of the following is a correct statement about the fate of carbon atoms entering the citric acid cycle as acetyl-CoA?

Accepted Answer

The two carbons from acetyl-CoA are incorporated into oxaloacetate and released as CO$$_2 in $$subsequent turns

Question 13

A researcher isolates a mutant bacterium that lacks the gene for isocitrate lyase. Which metabolic pathway will be directly impaired in this organism?

Accepted Answer

Glyoxylate cycle

Question 14

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

Accepted Answer

It acts as a swinging arm to transfer acyl groups between E1 and E2 subunits

Question 15

Which of the following is a major regulatory factor that inhibits the pyruvate dehydrogenase complex?

Accepted Answer

High levels of NADH

Question 16

Which of the following best describes the evolutionary significance of the citric acid cycle?

Accepted Answer

Genes encoding citric acid cycle enzymes are found in all three domains of life, indicating an ancient and conserved pathway

Question 17

A patient with a deficiency in malate dehydrogenase would most likely experience which of the following metabolic consequences?

Accepted Answer

Accumulation of malate and decreased regeneration of oxaloacetate

Question 18

Which of the following statements about the structure of the eukaryotic pyruvate dehydrogenase complex is correct?

Accepted Answer

It is a large multienzyme complex with an overall mass of about 10,000 kDa

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

Question 20

Which of the following is a direct consequence of a complete lack of any one of the citric acid cycle enzymes in humans?

Accepted Answer

Lethality due to inability to generate essential metabolic intermediates and energy

The Citric Acid Cycle: Structure, Function, and Regulation

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