Question 1

Describe the enzymes, cofactors, intermediates, and products the pyruvate dehydrogenase complex.

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Question 2

What is the function of FAD in the pyruvate dehydrogenase complex? How is it regenerated?

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Question 3

At what point in the citric acid cycle do the methyl carbon from acetyl-CoA and the carbonyl carbon from oxaloacetate become chemically equivalent?

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Question 4

In which reaction of the citric acid cycle does substrate-level phosphorylation occur?

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Question 5

Suppose you found an overly high level of pyruvate in a patient's blood and urine. One possible cause is a genetic defect in the enzyme pyruvate dehydrogenase, but another plausible cause is a specific vitamin deficiency. Explain what vitamin might be deficient in the diet, and why that would account for high levels of pyruvate to be excreted in the urine. How would you determine which explanation is correct?

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Question 6

Preparation of an extract of muscle results in a dramatic decrease in the concentration of citric acid cycle intermediates compared to their concentrations in the tissue. However, in 1935, Szent-Gyorgi showed that the production of CO₂ by the extract increased when succinate was added. In fact, for every mole of succinate added, many extra moles of CO₂ were produced. Explain this effect in terms of the known catabolic pathways.

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Question 7

Show the reactions by which $\alpha$-ketoglutarate is converted to malate in the citric acid cycle.

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Question 8

Match the cofactors below with their roles in the pyruvate dehydrogenase complex reaction. Cofactors: A. Coenzyme A (CoA-SH) B. NAD⁺ C. Thiamine pyrophosphate (TPP) D. FAD E. Lipoic acid in oxidized form Roles: _______ Attacks and attaches to the central carbon in pyruvate _______ Oxidizes FADH₂ _______ Accepts the acetyl group from reduced lipoic acid _______ Oxidizes the reduced form of lipoic acid _______ Initial electron acceptor in oxidation of pyruvate.

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Question 9

Show the structures of the reactants and products for two of the four redox reactions in the citric acid cycle. Indicate where any cofactors participate, and label the reactants, products, and cofactors as oxidants or reductants in the reaction.

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Question 10

In the citric acid cycle, a five-carbon compound is decarboxylated to yield an activated four-carbon compound. Show the substrate and product in this step, and indicate where any cofactor(s) participate(s).

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Question 11

The human disease beriberi is caused by a deficiency of thiamine in the diet. People with severe beriberi have higher than normal levels of pyruvate in their blood and urine. Explain this observation in terms of specific enzymatic reaction(s).

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Question 12

Which part of the citric acid cycle from isocitrate to fumarate only are the high-energy phosphate compounds or reduced electron carriers produced or consumed, and name the enzyme that catalyzes each step.

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Question 13

Show the steps of the citric acid cycle from succinyl-CoA to oxaloacetate only. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

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Question 14

Show the three reactions in the citric acid cycle in which NADH is produced, including the structures. None of these reactions involves molecular oxygen (O₂), but all three reactions are strongly inhibited by anaerobic conditions; explain why.

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Question 15

The citric acid cycle is frequently described as the major pathway of aerobic catabolism, which means that it is an oxygen-dependent degradative process. However, none of the reactions of the cycle directly involves oxygen as a reactant. Why is the pathway oxygen-dependent?

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Question 16

Explain why fluorocitrate, a potent inhibitor of the enzyme aconitase, is a deadly poison.

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Question 17

There are few, if any, humans with defects in the enzymes of the citric acid cycle. Explain this observation in terms of the role of the citric acid cycle.

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Question 18

CO₂ is produced in two reactions in the citric acid cycle. For each of these reactions, name and show the structures of reactant and product, name the enzyme, and show how any cofactors participate.

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Question 19

Show the steps of the citric acid cycle in which a six-carbon compound is converted into the first four-carbon intermediate in the path. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

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Question 20

Two of the steps in the oxidative decarboxylation of pyruvate to acetyl-CoA do not involve the three carbons of pyruvate, yet are essential to the operation of the pyruvate dehydrogenase complex. Explain.

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Quiz 16: The Citric Acid Cycle

Describe the enzymes, cofactors, intermediates, and products the pyruvate dehydrogenase complex.

What is the function of FAD in the pyruvate dehydrogenase complex? How is it regenerated?

At what point in the citric acid cycle do the methyl carbon from acetyl-CoA and the carbonyl carbon from oxaloacetate become chemically equivalent?

In which reaction of the citric acid cycle does substrate-level phosphorylation occur?

Show the reactions by which $\alpha$ -ketoglutarate is converted to malate in the citric acid cycle.

Show the structures of the reactants and products for two of the four redox reactions in the citric acid cycle. Indicate where any cofactors participate, and label the reactants, products, and cofactors as oxidants or reductants in the reaction.

In the citric acid cycle, a five-carbon compound is decarboxylated to yield an activated four-carbon compound. Show the substrate and product in this step, and indicate where any cofactor(s) participate(s).

The human disease beriberi is caused by a deficiency of thiamine in the diet. People with severe beriberi have higher than normal levels of pyruvate in their blood and urine. Explain this observation in terms of specific enzymatic reaction(s).

Which part of the citric acid cycle from isocitrate to fumarate only are the high-energy phosphate compounds or reduced electron carriers produced or consumed, and name the enzyme that catalyzes each step.

Show the steps of the citric acid cycle from succinyl-CoA to oxaloacetate only. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

Show the three reactions in the citric acid cycle in which NADH is produced, including the structures. None of these reactions involves molecular oxygen (O₂), but all three reactions are strongly inhibited by anaerobic conditions; explain why.

The citric acid cycle is frequently described as the major pathway of aerobic catabolism, which means that it is an oxygen-dependent degradative process. However, none of the reactions of the cycle directly involves oxygen as a reactant. Why is the pathway oxygen-dependent?

Explain why fluorocitrate, a potent inhibitor of the enzyme aconitase, is a deadly poison.

There are few, if any, humans with defects in the enzymes of the citric acid cycle. Explain this observation in terms of the role of the citric acid cycle.

CO₂ is produced in two reactions in the citric acid cycle. For each of these reactions, name and show the structures of reactant and product, name the enzyme, and show how any cofactors participate.

Show the steps of the citric acid cycle in which a six-carbon compound is converted into the first four-carbon intermediate in the path. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

Two of the steps in the oxidative decarboxylation of pyruvate to acetyl-CoA do not involve the three carbons of pyruvate, yet are essential to the operation of the pyruvate dehydrogenase complex. Explain.

Quiz 16: The Citric Acid Cycle

Describe the enzymes, cofactors, intermediates, and products the pyruvate dehydrogenase complex.

What is the function of FAD in the pyruvate dehydrogenase complex? How is it regenerated?

At what point in the citric acid cycle do the methyl carbon from acetyl-CoA and the carbonyl carbon from oxaloacetate become chemically equivalent?

In which reaction of the citric acid cycle does substrate-level phosphorylation occur?

Show the reactions by which α\alphaα -ketoglutarate is converted to malate in the citric acid cycle.

Show the structures of the reactants and products for two of the four redox reactions in the citric acid cycle. Indicate where any cofactors participate, and label the reactants, products, and cofactors as oxidants or reductants in the reaction.

In the citric acid cycle, a five-carbon compound is decarboxylated to yield an activated four-carbon compound. Show the substrate and product in this step, and indicate where any cofactor(s) participate(s).

The human disease beriberi is caused by a deficiency of thiamine in the diet. People with severe beriberi have higher than normal levels of pyruvate in their blood and urine. Explain this observation in terms of specific enzymatic reaction(s).

Which part of the citric acid cycle from isocitrate to fumarate only are the high-energy phosphate compounds or reduced electron carriers produced or consumed, and name the enzyme that catalyzes each step.

Show the steps of the citric acid cycle from succinyl-CoA to oxaloacetate only. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

Show the three reactions in the citric acid cycle in which NADH is produced, including the structures. None of these reactions involves molecular oxygen (O2), but all three reactions are strongly inhibited by anaerobic conditions; explain why.

The citric acid cycle is frequently described as the major pathway of aerobic catabolism, which means that it is an oxygen-dependent degradative process. However, none of the reactions of the cycle directly involves oxygen as a reactant. Why is the pathway oxygen-dependent?

Explain why fluorocitrate, a potent inhibitor of the enzyme aconitase, is a deadly poison.

There are few, if any, humans with defects in the enzymes of the citric acid cycle. Explain this observation in terms of the role of the citric acid cycle.

CO2 is produced in two reactions in the citric acid cycle. For each of these reactions, name and show the structures of reactant and product, name the enzyme, and show how any cofactors participate.

Show the steps of the citric acid cycle in which a six-carbon compound is converted into the first four-carbon intermediate in the path. For each step, show structures of substrate and product, name the enzyme responsible, and show where cofactors participate.

Two of the steps in the oxidative decarboxylation of pyruvate to acetyl-CoA do not involve the three carbons of pyruvate, yet are essential to the operation of the pyruvate dehydrogenase complex. Explain.

Show the reactions by which $\alpha$ -ketoglutarate is converted to malate in the citric acid cycle.

Show the three reactions in the citric acid cycle in which NADH is produced, including the structures. None of these reactions involves molecular oxygen (O₂), but all three reactions are strongly inhibited by anaerobic conditions; explain why.

CO₂ is produced in two reactions in the citric acid cycle. For each of these reactions, name and show the structures of reactant and product, name the enzyme, and show how any cofactors participate.