Question 1

Which enzyme complex is responsible for converting pyruvate into acetyl-CoA and CO$$_2 in $$the mitochondria?

Accepted Answer

Pyruvate dehydrogenase complex

Question 2

What is the net reaction of the citric acid cycle per acetyl-CoA molecule oxidized?

Accepted Answer

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

Question 3

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

Accepted Answer

Thiamine pyrophosphate (TPP)

Question 4

In the pyruvate dehydrogenase complex, which subunit is responsible for transferring the acetyl group to CoA?

Accepted Answer

E$$_2 ($$Dihydrolipoyl transacetylase)

Question 5

Which molecule acts as an allosteric inhibitor of the pyruvate dehydrogenase complex?

Accepted Answer

NADH

Question 6

What is the role of lipoamide in the pyruvate dehydrogenase complex?

Accepted Answer

It acts as a carrier for the acetyl group between active sites.

Question 7

Which of the following best describes the regulation of pyruvate dehydrogenase in mammals?

Accepted Answer

It is regulated by phosphorylation and allosteric effectors.

Question 8

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 9

Which step in the pyruvate dehydrogenase reaction involves the reduction of FAD?

Accepted Answer

Oxidation of dihydrolipoamide

Question 10

Which of the following is NOT a product of the pyruvate dehydrogenase complex reaction?

Accepted Answer

FADH$$_2$$

Question 11

If the mitochondrial matrix has high concentrations of NADH and acetyl-CoA, what is the expected effect on pyruvate dehydrogenase activity?

Accepted Answer

It will be inhibited.

Question 12

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

Accepted Answer

FAD oxidizes dihydrolipoamide and is then reoxidized by NAD$$^+.$$

Question 13

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

Accepted Answer

Oxidation of dihydrolipoamide by FAD

Question 14

Which of the following statements about the transport of pyruvate into mitochondria is correct?

Accepted Answer

Pyruvate is transported specifically by a translocase across the inner mitochondrial membrane.

Question 15

Which of the following best explains why the citric acid cycle is considered a central metabolic pathway?

Accepted Answer

It provides intermediates for biosynthesis of amino acids, nucleotides, and other molecules.

Question 16

A mutation that prevents phosphorylation of pyruvate dehydrogenase in mammals would most likely result in:

Accepted Answer

Constant activation of the enzyme

Question 17

Which of the following is a common fate of pyruvate under aerobic conditions in eukaryotic cells?

Accepted Answer

Conversion to acetyl-CoA

Question 18

Which of the following is the correct order of electron transfer in the pyruvate dehydrogenase complex?

Accepted Answer

Lipoamide \rightarrow FAD \rightarrow NAD$$^+$$

Question 19

Which of the following is a direct consequence of pyruvate dehydrogenase deficiency in humans?

Accepted Answer

Increased lactate production

Question 20

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

Accepted Answer

It is a large multienzyme complex with multiple copies of three different enzymes.

The Aerobic Fate of Pyruvate and the Citric Acid Cycle

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