Question 1

Which statement is FALSE regarding regulation of the pyruvate dehydrogenase complex?

Accepted Answer

A)  In E. coli, a high ratio of [NAD+]/[NADH] activates the complex. 
B)  In E. coli, high levels of ATP enhance the activity of pyruvate dehydrogenase kinase. 
C)  In plants, NH4+ inhibits the complex by enhancing the activity of pyruvate dehydrogenase kinase. 
D)  In mammals, both fatty acids and acetyl-CoA inhibit the complex. 
E)  In skeletal muscle, both NAD+ and Ca2+ activate the complex. 
A)  In E. coli, a high ratio of [NAD+]/[NADH] activates the complex. 
B)  In E. coli, high levels of ATP enhance the activity of pyruvate dehydrogenase kinase. 
C)  In plants, NH4+ inhibits the complex by enhancing the activity of pyruvate dehydrogenase kinase. 
D)  In mammals, both fatty acids and acetyl-CoA inhibit the complex. 
E)  In skeletal muscle, both NAD+ and Ca2+ activate the complex.

Question 2

Which high-energy bond is associated with the succinyl-CoA synthetase reaction?

Accepted Answer

A)  acyl phosphate 
B)  thioester 
C)  phosphohistidine 
D)  mixed anhydride 
E)  All of the answers are correct. 
A)  acyl phosphate 
B)  thioester 
C)  phosphohistidine 
D)  mixed anhydride 
E)  All of the answers are correct.

Question 3

The glyoxylate cycle in germinating seeds is a pathway that has some similarity to the citric acid cycle. It uses the carbon-based products from the catabolism of fatty acids as the precursors for gluconeogenesis. How can the glyoxylate cycle accomplish this?

Accepted Answer

A)  It produces an excess of citrate, which is exported out of the mitochondria. 
B)  Its reactions take place partly in the matrix of the mitochondria and partly in the cytosol. 
C)  It uses only the catabolic products of even-numbered, saturated fatty acids. 
D)  Its reactions bypass the decarboxylation steps of the citric acid cycle. 
E)  It prevents the formation of malate, allowing carbons from acetate to form oxaloacetate. 
A)  It produces an excess of citrate, which is exported out of the mitochondria. 
B)  Its reactions take place partly in the matrix of the mitochondria and partly in the cytosol. 
C)  It uses only the catabolic products of even-numbered, saturated fatty acids. 
D)  Its reactions bypass the decarboxylation steps of the citric acid cycle. 
E)  It prevents the formation of malate, allowing carbons from acetate to form oxaloacetate.

Question 4

Which vitamin is NOT a precursor for a cofactor in the pyruvate dehydrogenase complex?

Accepted Answer

A)  thiamine for TPP 
B)  riboflavin for FAD 
C)  niacin for lipoate 
D)  nictotinamide for NAD+ 
E)  pantothenate for CoA 
A)  thiamine for TPP 
B)  riboflavin for FAD 
C)  niacin for lipoate 
D)  nictotinamide for NAD+ 
E)  pantothenate for CoA

Question 5

Acetyl-CoA labeled with 14C in both of its acetate carbon atoms is incubated with unlabeled oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle. After one turn of the cycle, oxaloacetate would have 14C in:

Accepted Answer

A)  all four carbon atoms. 
B)  no pattern that is predictable from the information provided. 
C)  none of its carbon atoms. 
D)  the keto carbon and one of the carboxyl carbons. 
E)  the two carboxyl carbons. 
A)  all four carbon atoms. 
B)  no pattern that is predictable from the information provided. 
C)  none of its carbon atoms. 
D)  the keto carbon and one of the carboxyl carbons. 
E)  the two carboxyl carbons.

Question 6

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.

Accepted Answer

The first reaction in the citric acid cy

Question 7

Oxaloacetate uniformly labeled with 14C (i.e., with equal amounts of 14C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate?

Accepted Answer

A)  All 
B)  1/2 
C)  1/3 
D)  1/4 
E)  3/4 
A)  All 
B)  1/2 
C)  1/3 
D)  1/4 
E)  3/4

Question 8

Which enzymatic activity would be decreased by thiamine deficiency?

Accepted Answer

A)  fumarase 
B)  isocitrate dehydrogenase 
C)  malate dehydrogenase 
D)  succinate dehydrogenase 
E)  $\alpha$-ketoglutarate dehydrogenase complex 
A)  fumarase 
B)  isocitrate dehydrogenase 
C)  malate dehydrogenase 
D)  succinate dehydrogenase 
E)  $\alpha$-ketoglutarate dehydrogenase complex

Question 9

Which statement is NOT true of the reaction catalyzed by the pyruvate dehydrogenase complex?

Accepted Answer

A)  Biotin participates in the decarboxylation. 
B)  Both NAD+ and a flavin nucleotide act as electron carriers. 
C)  The reaction occurs in the mitochondrial matrix. 
D)  The substrate is held by the lipoyl-lysine "swinging arm." 
E)  Two different cofactors containing -SH groups participate. 
A)  Biotin participates in the decarboxylation. 
B)  Both NAD+ and a flavin nucleotide act as electron carriers. 
C)  The reaction occurs in the mitochondrial matrix. 
D)  The substrate is held by the lipoyl-lysine "swinging arm." 
E)  Two different cofactors containing -SH groups participate.

Question 10

Which reaction of the citric acid cycle requires a water molecule?

Accepted Answer

A)  formation of citrate 
B)  succinyl-CoA to succinate 
C)  fumarate to malate 
D)  All of the answers are correct. 
E)  None of the answers is correct. 
A)  formation of citrate 
B)  succinyl-CoA to succinate 
C)  fumarate to malate 
D)  All of the answers are correct. 
E)  None of the answers is correct.

Question 11

Which step is NOT catalyzed by the pyruvate dehydrogenase complex?

Accepted Answer

A)  E1 catalyzes the decarboxylation of pyruvate. 
B)  E1 catalyzes the oxidation of a hydroxyethyl group to an acetyl group. 
C)  E2 catalyzes the transfer of an acetyl group to CoA-SH 
D)  E3 catalyzes the reduction of oxidized lipoate. 
E)  E3 catalyzes the transfer of electrons from FADH2 to NAD+. 
A)  E1 catalyzes the decarboxylation of pyruvate. 
B)  E1 catalyzes the oxidation of a hydroxyethyl group to an acetyl group. 
C)  E2 catalyzes the transfer of an acetyl group to CoA-SH 
D)  E3 catalyzes the reduction of oxidized lipoate. 
E)  E3 catalyzes the transfer of electrons from FADH2 to NAD+.

Question 12

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

Accepted Answer

The pyruvate dehydrogenase complex is a

Question 13

Arsenic inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of dihydrolipoyl transacetylase. Which enzyme is also MOST likely to be inhibited by arsenic?

Accepted Answer

A)  GAP dehydrogenase 
B)  isocitrate dehydrogenase 
C)  succinyl-CoA synthetase 
D)  malate dehydrogenase 
E)  $\alpha$-ketoglutarate dehydrogenase 
A)  GAP dehydrogenase 
B)  isocitrate dehydrogenase 
C)  succinyl-CoA synthetase 
D)  malate dehydrogenase 
E)  $\alpha$-ketoglutarate dehydrogenase

Question 14

The oxidative step of the citric acid cycle that is NOT linked to the reduction of NAD+ is the reaction catalyzed by:

Accepted Answer

A)  isocitrate dehydrogenase. 
B)  malate dehydrogenase. 
C)  pyruvate dehydrogenase 
D)  succinate dehydrogenase. 
E)  the $\alpha$-ketoglutarate dehydrogenase complex. 
A)  isocitrate dehydrogenase. 
B)  malate dehydrogenase. 
C)  pyruvate dehydrogenase 
D)  succinate dehydrogenase. 
E)  the $\alpha$-ketoglutarate dehydrogenase complex.

Question 15

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.

Accepted Answer

The two steps in the oxidative decarboxy

Question 16

Why is flux through the citric acid low in the resting state?

Accepted Answer

A)  Flux through the cycle is low due to a drop in serum adrenaline levels. 
B)  Flux through the cycle is low due to allosteric inhibition of pyruvate carboxylase. 
C)  Flux through the cycle is low due to complete oxidation of NADH because oxygen is readily available. 
D)  Flux through the cycle is low due to low local concentrations of NAD+ because of a diminished rate of oxidative phosphorylation. 
E)  Flux through the cycle is low due to product inhibition of the pyruvate dehydrogenase complex by ATP. 
A)  Flux through the cycle is low due to a drop in serum adrenaline levels. 
B)  Flux through the cycle is low due to allosteric inhibition of pyruvate carboxylase. 
C)  Flux through the cycle is low due to complete oxidation of NADH because oxygen is readily available. 
D)  Flux through the cycle is low due to low local concentrations of NAD+ because of a diminished rate of oxidative phosphorylation. 
E)  Flux through the cycle is low due to product inhibition of the pyruvate dehydrogenase complex by ATP.

Question 17

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?

Accepted Answer

The vitamin that might be deficient in t

Question 18

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.

Accepted Answer

The citric acid cycle, also known as the

Question 19

Which intermediate of the citric acid cycle is prochiral?

Accepted Answer

A)  citrate 
B)  isocitrate 
C)  malate 
D)  oxaloacetate 
E)  succinate 
A)  citrate 
B)  isocitrate 
C)  malate 
D)  oxaloacetate 
E)  succinate

Question 20

Glucose labeled with 14C in C-3 and C-4 is completely converted to acetyl-CoA via glycolysis and the pyruvate dehydrogenase complex. What percentage of the acetyl-CoA molecules formed will be labeled with 14C, and in which position of the acetyl moiety will the 14C label be found?

Accepted Answer

A)  100% of the acetyl-CoA will be labeled at C-1 (carboxyl). 
B)  100% of the acetyl-CoA will be labeled at C-2. 
C)  50% of the acetyl-CoA will be labeled, all at C-2 (methyl). 
D)  No label will be found in the acetyl-CoA molecules. 
E)  Not enough information is given to answer this question. 
A)  100% of the acetyl-CoA will be labeled at C-1 (carboxyl). 
B)  100% of the acetyl-CoA will be labeled at C-2. 
C)  50% of the acetyl-CoA will be labeled, all at C-2 (methyl). 
D)  No label will be found in the acetyl-CoA molecules. 
E)  Not enough information is given to answer this question.

Exam 16: The Citric Acid Cycle

Which statement is FALSE regarding regulation of the pyruvate dehydrogenase complex?

Which high-energy bond is associated with the succinyl-CoA synthetase reaction?

The glyoxylate cycle in germinating seeds is a pathway that has some similarity to the citric acid cycle. It uses the carbon-based products from the catabolism of fatty acids as the precursors for gluconeogenesis. How can the glyoxylate cycle accomplish this?

Which vitamin is NOT a precursor for a cofactor in the pyruvate dehydrogenase complex?

Acetyl-CoA labeled with 14C in both of its acetate carbon atoms is incubated with unlabeled oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle. After one turn of the cycle, oxaloacetate would have 14C in:

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.

Oxaloacetate uniformly labeled with 14C (i.e., with equal amounts of 14C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate?

Which enzymatic activity would be decreased by thiamine deficiency?

Which statement is NOT true of the reaction catalyzed by the pyruvate dehydrogenase complex?

Which reaction of the citric acid cycle requires a water molecule?

Which step is NOT catalyzed by the pyruvate dehydrogenase complex?

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

Arsenic inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of dihydrolipoyl transacetylase. Which enzyme is also MOST likely to be inhibited by arsenic?

The oxidative step of the citric acid cycle that is NOT linked to the reduction of NAD+ is the reaction catalyzed by:

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.

Why is flux through the citric acid low in the resting state?

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.

Which intermediate of the citric acid cycle is prochiral?

Glucose labeled with 14C in C-3 and C-4 is completely converted to acetyl-CoA via glycolysis and the pyruvate dehydrogenase complex. What percentage of the acetyl-CoA molecules formed will be labeled with 14C, and in which position of the acetyl moiety will the 14C label be found?

The Foundations of Biochemistry

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Amino Acids, Peptides, and Proteins

The Three-Dimensional Structure of Proteins

Protein Function

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Nucleotides and Nucleic Acids

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Bioenergetics and Biochemical Reaction Types

Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway

Principles of Metabolic Regulation

Fatty Acid Catabolism

Amino Acid Oxidation and the Production of Urea

Oxidative Phosphorylation and Photophosphorylation

Carbohydrate Biosynthesis in Plants and Bacteria

Lipid Biosynthesis

Biosynthesis of Amino Acids, Nucleotides, and Related Molecules

Hormonal Regulation and Integration of Mammalian Metabolism

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Acetyl-CoA labeled with ¹⁴C in both of its acetate carbon atoms is incubated with unlabeled oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle. After one turn of the cycle, oxaloacetate would have ¹⁴C in:

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.

Oxaloacetate uniformly labeled with ¹⁴C (i.e., with equal amounts of ¹⁴C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate?

The oxidative step of the citric acid cycle that is NOT linked to the reduction of NAD⁺ is the reaction catalyzed by:

Glucose labeled with ¹⁴C in C-3 and C-4 is completely converted to acetyl-CoA via glycolysis and the pyruvate dehydrogenase complex. What percentage of the acetyl-CoA molecules formed will be labeled with ¹⁴C, and in which position of the acetyl moiety will the ¹⁴C label be found?