Q: Which enzyme catalyzes the most important anaplerotic reaction of the citric acid cycle, and what is the reaction?

Pyruvate carboxylase; $$\text{Pyruvate} + \text{CO}_2 + \text{ATP} \rightarrow \text{Oxaloacetate} + \text{ADP} + \text{Pi}$$

Question 1

Which enzyme in the TCA cycle is responsible for the first oxidative decarboxylation, and what is the main product of this reaction?

Accepted Answer

Isocitrate dehydrogenase; produces α-ketoglutarate and CO$$_2$$

Question 2

Which of the following best describes the fate of the two carbons from acetyl-CoA in the first round of the citric acid cycle?

Accepted Answer

They remain in the cycle and are not released as CO$$_2 in $$the first round

Question 3

Which shuttle system is responsible for transferring cytosolic NADH into the mitochondria for oxidative phosphorylation?

Accepted Answer

Aspartate-malate shuttle

Question 4

A patient with fumarase deficiency develops lactic acidosis. What is the most likely biochemical explanation for this symptom?

Accepted Answer

Accumulation of fumarate leads to increased glycolysis and lactate production

Question 5

Which of the following is a key regulatory mechanism of the TCA cycle involving feedback inhibition?

Accepted Answer

Inhibition of citrate synthase by ATP

Question 6

Which of the following statements about reactive oxygen species (ROS) in mitochondria is CORRECT?

Accepted Answer

Superoxide dismutase (SOD) converts superoxide to hydrogen peroxide

Question 7

Which enzyme catalyzes the most important anaplerotic reaction of the citric acid cycle, and what is the reaction?

Accepted Answer

Pyruvate carboxylase; $$	ext{Pyruvate} + 	ext{CO}_2 + 	ext{ATP} ightarrow 	ext{Oxaloacetate} + 	ext{ADP} + 	ext{Pi}$$

Question 8

Why is the activation of pyruvate carboxylase by acetyl-CoA considered a good regulatory strategy?

Accepted Answer

It ensures oxaloacetate is produced only when acetyl-CoA is abundant

Question 9

Which of the following compounds can be used for net synthesis of glucose in mammals?

Accepted Answer

Glyceraldehyde-3-phosphate

Question 10

Which TCA cycle enzyme is mutated in some cancers, leading to the accumulation of the oncometabolite 2-hydroxyglutarate?

Accepted Answer

Isocitrate dehydrogenase (IDH1/2)

Question 11

What is the main function of the citrate transport system in cellular metabolism?

Accepted Answer

To export citrate from the mitochondria to the cytosol for fatty acid biosynthesis

Question 12

Which of the following best explains why individuals with α-ketoglutarate dehydrogenase deficiency have a high lactate/pyruvate ratio?

Accepted Answer

Impaired TCA cycle flux leads to increased conversion of pyruvate to lactate

Question 13

Which of the following is a direct product of the TCA cycle that is used in the electron transport chain to generate ATP?

Accepted Answer

NADH

Question 14

Which of the following statements about the energetic yield of the TCA cycle is correct?

Accepted Answer

Each NADH yields 2.5 ATP

Question 15

Which of the following is a consequence of decreased aconitase activity during hyperoxia in lung tissue?

Accepted Answer

Increased concentration of citrate

Question 16

Which enzyme is responsible for converting superoxide (O$$_2^-) to $$hydrogen peroxide (H$$_2O$_2) in $$mitochondria?

Accepted Answer

Superoxide dismutase (SOD)

Question 17

Which of the following best describes the role of IDH2 in the mitochondria?

Accepted Answer

It generates mitochondrial NADPH to protect against ROS

Question 18

Which of the following is a plausible drug target for treating Mycobacterium tuberculosis infection based on its metabolic adaptation in the phagosome?

Accepted Answer

Isocitrate lyase

Question 19

Which of the following best explains why decreased mitochondrial respiration during hyperoxia is accompanied by elevated rates of glycolysis and the pentose phosphate pathway?

Accepted Answer

Cells compensate for reduced ATP production by increasing glycolysis and generate NADPH for antioxidant defense via the pentose phosphate pathway

Citric Acid Cycle: Pathways, Regulation, and Clinical Connections

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