Q: Which of the following equations correctly represents the reaction catalyzed by the pyruvate dehydrogenase complex?

$$\text{Pyruvate} + \text{CoA-SH} + \text{NAD}^+ \rightarrow \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+$$

Question 1

Which of the following best explains why the flow of carbon into the citric acid cycle must be regulated?

Accepted Answer

To match the rate of oxidative metabolism to the cell's energy requirements.

Question 2

A cell with a high ATP/ADP ratio is most likely to exhibit which of the following metabolic responses?

Accepted Answer

Decreased oxidative metabolism and reduced fuel oxidation.

Question 3

Which of the following equations correctly represents the reaction catalyzed by the pyruvate dehydrogenase complex?

Accepted Answer

$$	ext{Pyruvate} + 	ext{CoA-SH} + 	ext{NAD}^+ ightarrow 	ext{Acetyl-CoA} + 	ext{CO}_2 + 	ext{NADH} + 	ext{H}^+$$

Question 4

Which of the following best describes the effect of phosphorylation on pyruvate dehydrogenase (PDH) activity?

Accepted Answer

Phosphorylation inactivates PDH, decreasing acetyl-CoA production.

Question 5

Which enzyme is responsible for removing the phosphate group from phosphorylated PDH, thereby reactivating it?

Accepted Answer

Phosphoprotein phosphatase

Question 6

High concentrations of which of the following molecules would most likely inhibit the activity of both pyruvate dehydrogenase and citrate synthase?

Accepted Answer

ATP

Question 7

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

Accepted Answer

ADP

Question 8

Suppose a cell experiences a sudden increase in NADH concentration. What is the most likely immediate effect on the citric acid cycle?

Accepted Answer

Decreased activity of several regulatory enzymes, slowing the cycle.

Question 9

Which of the following best explains why succinyl-CoA inhibits both citrate synthase and α-ketoglutarate dehydrogenase?

Accepted Answer

It is a product of the TCA cycle, and its accumulation prevents unnecessary substrate oxidation.

Question 10

A patient is found to have a mutation that causes constitutively active pyruvate dehydrogenase kinase. Which of the following metabolic consequences is most likely?

Accepted Answer

Decreased entry of glucose-derived carbon into the TCA cycle.

Question 11

Which of the following best describes the coordinated regulation of glycolysis, pyruvate oxidation, and the citric acid cycle under high-energy conditions?

Accepted Answer

Glycolysis, pyruvate oxidation, and the TCA cycle are all inhibited to prevent unnecessary ATP production.

Question 12

Which of the following is NOT an inhibitor of citrate synthase?

Accepted Answer

ADP

Question 13

Which of the following best predicts the metabolic state of a cell with low ATP, high ADP, and high NAD⁺?

Accepted Answer

The cell is in a low-energy state and oxidative metabolism is increased.

Question 14

Which of the following is a correct summary of the regulatory effects of ATP and NADH on the citric acid cycle?

Accepted Answer

Both ATP and NADH inhibit the cycle, signaling energy abundance.

Question 15

Which of the following scenarios would most likely result in increased activity of pyruvate dehydrogenase?

Accepted Answer

High ADP and increased Ca²⁺ concentration.

Question 16

Which of the following best explains why NADH acts as both a product and a regulatory signal in the TCA cycle?

Accepted Answer

NADH accumulation signals that production of reducing equivalents is exceeding their utilization, slowing further oxidation.

Question 17

Which of the following is the correct effect of Ca²⁺ on pyruvate dehydrogenase activity?

Accepted Answer

Ca²⁺ activates PDH by stimulating phosphoprotein phosphatase.

Question 18

Which of the following is a direct consequence of inhibiting pyruvate dehydrogenase?

Accepted Answer

Decreased entry of glucose-derived carbon into the TCA cycle.

Question 19

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

Accepted Answer

It controls the entry of glucose-derived carbon into the TCA cycle by converting pyruvate to acetyl-CoA.

Regulation of Pyruvate Dehydrogenase and the Citric Acid Cycle

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