Deck 16: The Citric Acid Cycle

Question 1

Which enzymatic activity would be decreased by thiamine deficiency?

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

Answer 1

$\alpha$ -ketoglutarate dehydrogenase complex

Question 2

Which statement about the oxidative decarboxylation of pyruvate in aerobic conditions in animal cells is CORRECT?

A) One of the products of the reactions of the pyruvate dehydrogenase complex is a thioester of acetate.
B) The methyl (-CH₃) group is eliminated as CO₂.
C) The process occurs in the cytosolic compartment of the cell.
D) The pyruvate dehydrogenase complex uses all of the following as cofactors: NAD⁺, lipoic acid, pyridoxal phosphate (PLP), and FAD.
E) The reaction is so important to energy production that pyruvate dehydrogenase operates at full speed under all conditions.

Answer 2

A

Question 3

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

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.

Answer 3

A

Question 4

The reaction of the citric acid cycle that produces an ATP equivalent (in the form of GTP) by substrate level phosphorylation is the conversion of:

A) citrate to isocitrate.
B) fumarate to malate.
C) malate to oxaloacetate.
D) succinate to fumarate.
E) succinyl-CoA to succinate.

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:

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

Which statement is NOT true of the citric acid cycle?

A) All enzymes of the cycle are located in the cytoplasm, except succinate dehydrogenase, which is bound to the inner mitochondrial membrane.
B) In the presence of malonate, one would expect succinate to accumulate.
C) Oxaloacetate is used as a substrate but is not consumed in the cycle.
D) Succinate dehydrogenase channels electrons directly into the electron transfer chain.
E) The condensing enzyme is subject to allosteric regulation by ATP and NADH.

Question 7

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?

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.

Question 8

The oxidative decarboxylation of $\alpha$ -ketoglutarate proceeds by means of multistep reactions. Which cofactor is NOT required?

A) ATP
B) coenzyme A
C) lipoic acid
D) NAD⁺
E) thiamine pyrophosphate

Question 9

The reaction of the citric acid cycle that is most similar to the pyruvate dehydrogenase complex-catalyzed conversion of pyruvate to acetyl-CoA is the conversion of:

A) citrate to isocitrate.
B) fumarate to malate.
C) malate to oxaloacetate.
D) succinyl-CoA to succinate.
E) $\alpha$ -ketoglutarate to succinyl-CoA.

Question 10

The standard reduction potentials (E'°) for the following half reactions are given. Fumarate + 2H⁺ + 2e^- $\rightarrow$ succinate
E'° = +0.031 V
FAD + 2H⁺ + 2e^- $\rightarrow$ FADH₂
E'° = -0.219 V
If succinate, fumarate, FAD, and FADH₂, all at l M concentrations, were mixed together in the presence of succinate dehydrogenase, which of the following would happen initially?

A) Fumarate and succinate would become oxidized; FAD and FADH₂ would become reduced.
B) Fumarate would become reduced; FADH₂ would become oxidized.
C) No reaction would occur because all reactants and products are already at their standard concentrations.
D) Succinate would become oxidized; FAD would become reduced.
E) Succinate would become oxidized; FADH₂ would be unchanged because it is a cofactor, not a substrate.

Question 11

The two moles of CO₂ produced in the first turn of the citric acid cycle have their origin in the:

A) carboxyl and methylene carbons of oxaloacetate
B) carboxyl group of acetate and a carboxyl group of oxaloacetate.
C) carboxyl group of acetate and the keto group of oxaloacetate.
D) two carbon atoms of acetate.
E) two carboxyl groups derived from oxaloacetate.

Question 12

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?

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

Question 13

For the following reaction, $\Delta$ G'° = 29.7 kJ/mol. L-Malate + NAD⁺ $\rightarrow$ oxaloacetate + NADH + H⁺
The reaction as written:

A) can never occur in a cell.
B) can only occur in a cell if it is coupled to another reaction for which $\Delta$ G'° is positive.
C) can only occur in a cell in which NADH is converted to NAD⁺ by electron transport.
D) may occur in cells at certain concentrations of substrate and product.
E) would always proceed at a very slow rate

Question 14

Which factor is NOT associated with the oxidation of substrates by the citric acid cycle?

A) CO₂ production
B) flavin reduction
C) the presence of lipoic acid in some of the enzyme systems
D) pyridine nucleotide oxidation
E) All of these factors are associated with the oxidation of substrates by the citric acid cycle.

Question 15

Which compound is NOT an intermediate of the citric acid cycle?

A) acetyl-CoA
B) citrate
C) oxaloacetate
D) succinyl-CoA
E) $\alpha$ -ketoglutarate

Question 16

In mammals, what process does NOT occur during the citric acid cycle?

A) formation of $\alpha$ -ketoglutarate
B) generation of NADH and FADH₂
C) metabolism of acetate to carbon dioxide and water
D) net synthesis of oxaloacetate from acetyl-CoA
E) oxidation of acetyl-CoA

Question 17

Conversion of 1 mol of acetyl-CoA to 2 mol of CO₂ and CoA via the citric acid cycle results in the net production of:

A) 1 mol of citrate.
B) 1 mol of FADH₂.
C) 1 mol of NADH.
D) 1 mol of oxaloacetate.
E) 7 mol of ATP.

Question 18

Which combination of cofactors is involved in the conversion of pyruvate to acetyl-CoA?

A) biotin, FAD, and TPP
B) biotin, NAD⁺, and FAD
C) NAD⁺, biotin, and TPP
D) pyridoxal phosphate, FAD, and lipoic acid
E) TPP, lipoic acid, and NAD⁺

Question 19

Malonate is a competitive inhibitor of succinate dehydrogenase. If malonate is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration?

A) citrate
B) fumarate
C) isocitrate
D) pyruvate
E) succinate

Question 20

Which compound is NOT required for the oxidative decarboxylation of pyruvate to form acetyl-CoA?

A) ATP
B) CoA-SH
C) FAD
D) lipoic acid
E) NAD⁺

Question 21

During the reaction of pyruvate carboxylase, CO₂ is NOT covalently attached to:

A) phosphate.
B) biotin.
C) pyruvate.
D) lysine.
E) All of the answers are correct.

Question 22

Citrate synthase and the NAD⁺-specific isocitrate dehydrogenase are two key regulatory enzymes of the citric acid cycle. These enzymes are inhibited by:

A) acetyl-CoA and fructose 6-phosphate.
B) AMP and/or NAD⁺.
C) AMP and/or NADH.
D) ATP and/or NAD⁺.
E) ATP and/or NADH.

Question 23

A function of the glyoxylate cycle, in conjunction with the citric acid cycle, is to accomplish:

A) the complete oxidation of acetyl-CoA to CO₂ plus reduced coenzymes.
B) the net conversion of lipid to carbohydrate.
C) the net synthesis of four-carbon dicarboxylic acids from acetyl-CoA.
D) the net synthesis of long-chain fatty acids from citric acid cycle intermediates.
E) both the net conversion of lipid to carbohydrate and the net synthesis of four-carbon dicarboxylic acids from acetyl-CoA.

Question 24

In the citric acid cycle, a flavin coenzyme is required for:

A) condensation of acetyl-CoA and oxaloacetate.
B) oxidation of fumarate.
C) oxidation of isocitrate.
D) oxidation of malate.
E) oxidation of succinate.

Question 25

During seed germination, the glyoxylate pathway is important to plants because it enables them to:

A) carry out the net synthesis of glucose from acetyl-CoA.
B) form acetyl-CoA from malate.
C) get rid of isocitrate formed from the aconitase reaction.
D) obtain glyoxylate for cholesterol biosynthesis.
E) obtain glyoxylate for pyrimidine synthesis.

Question 26

Which compound is NOT required for the oxidative decarboxylation of $\alpha$ -ketoglutarate?

A) FAD
B) lipoic acid
C) NAD⁺
D) ATP
E) CoA-SH

Question 27

The glyoxylate cycle is:

A) a means of using acetate for both energy and biosynthetic precursors.
B) an alternative path of glucose metabolism in cells that do not have enough O₂.
C) defective in people with phenylketonuria.
D) is not active in a mammalian liver.
E) the most direct way of providing the precursors for synthesis of nucleic acids (e.g., ribose).

Question 28

The conversion of 1 mol of pyruvate to 3 mol of CO₂ via pyruvate dehydrogenase and the citric acid cycle also yields _____ mol of NADH, _____ mol of FADH₂, and _____ mol of ATP (or GTP).

A) 2; 2; 2
B) 3; 1; 1
C) 3; 2; 0
D) 4; 1; 1
E) 4; 2; 1

Question 29

The pyruvate dehydrogenase complex and the $\alpha$ -ketoglutarate dehydrogenase complex are homologous enzyme assemblies that require several different coenzymes to carry out their reactions. Which compound does NOT participate directly in the function of these complexes?

A) flavin mononucleotide
B) coenzyme A
C) lipoic acid
D) thiamine pyrophosphate
E) nicotinamide adenine dinucleotide

Question 30

Covalent modification is one way the activity of specific enzymes can be regulated. Which enzyme is NOT regulated in this way?

A) phosphofructokinase-2
B) $\alpha$ -ketoglutarate dehydrogenase
C) pyruvate dehydrogenase
D) phosphorylase kinase
E) All of these enzymes are regulated by covalent modification.

Question 31

Intermediates in the citric acid cycle are used as precursors in the biosynthesis of:

A) amino acids.
B) nucleotides.
C) fatty acids.
D) sterols.
E) All of the answers are correct.

Question 32

Which statement about NAD⁺ and NADH is TRUE?

A) NADH inhibits the pyruvate dehydrogenase complex.
B) NAD⁺ is oxidized to NADH by the $\alpha$ -ketoglutarate dehydrogenase complex.
C) NAD⁺ is a coenzyme for two of the enzymes in the pyruvate dehydrogenase complex.
D) NAD⁺ is oxidized to NADH by isocitrate dehydrogenase.
E) NAD⁺ is oxidized to NADH by both isocitrate dehydrogenase and the $\alpha$ -ketoglutarate dehydrogenase complex.

Question 33

Which intermediate of the citric acid cycle is prochiral?

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

Question 34

Which process is NOT a metabolic fate for pyruvate in liver tissue?

A) reduction to lactate
B) oxidative decarboxylation to acetyl-CoA
C) transamination to alanine
D) phosphorylation to phosphoenolpyruvate
E) carboxylation to oxaloacetate

Question 35

Anaplerotic reactions:

A) produce oxaloacetate and malate to maintain constant levels of citric acid cycle intermediates.
B) produce biotin needed by pyruvate carboxylase.
C) recycle pantothenate used to make CoA.
D) produce pyruvate and citrate to maintain constant levels of citric acid cycle intermediates.
E) All of the answers are correct.

Question 36

Entry of acetyl-CoA into the citric acid cycle is DECREASED when:

A) [AMP] is high.
B) NADH is rapidly oxidized through the respiratory chain.
C) the ratio of [ATP]/[ADP is low
D) the ratio of [ATP]/[ADP] is high.
E) the ratio of [NAD⁺]/[NADH] is high.

Question 37

Which enzyme does NOT have water as a substrate in the reaction it catalyzes?

A) succinyl-CoA sythetase
B) aconitase
C) fumarase
D) citrate synthase
E) succinyl-CoA sythetase and aconitase

Question 38

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

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

Question 39

Which statement is FALSE regarding the reaction catalyzed by the $\alpha$ -ketoglutarate dehydrogenase complex?

A) A long lipollysyl arm transfers intermediates from one active site to another within the complex.
B) This reaction would be expected to be abnormally low in a person suffering from beriberi, a dietary deficiency of thiamine.
C) Two different coenzymes containing sulfhydryl groups participate in the overall reaction.
D) Both this complex and the pyruvate dehydrogenase complex contain a common subunit.
E) The proximity of the catalytic enzymes allows for side-reactions to take place in the cluster.

Question 40

Which cofactor is required for the conversion of succinate to fumarate in the citric acid cycle?

A) ATP
B) biotin
C) FAD
D) NAD⁺
E) NADP⁺

Question 41

Which compound is NOT required for the oxidative decarboxylation of the carbon skeleton of isoleucine?

A) FAD
B) thiamine pyrophosphate
C) NAD⁺
D) CoA-SH
E) All are these compounds are required.

Question 42

Thyroxine is a competitive inhibitor of malate dehydrogenase. If thyroxine is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration FIRST?

A) citrate
B) fumarate
C) oxaloacetate
D) pyruvate
E) malate

Question 43

Which citric acid cycle intermediate is chiral?

A) citrate
B) $\alpha$ -ketoglutarate
C) fumarate
D) succinate
E) malate

Question 44

Which statement is FALSE with respect to the pyruvate dehydrogenase complex?

A) It is located in the mitochondrial matrix.
B) Its activity is increased in the presence of Ca²⁺ ions in skeletal muscle.
C) Its activity is increased in the presence of coenzyme A and ADP.
D) The proximity of the catalytic enzymes in the cluster slows down the overall rate.
E) All of the statements are false.

Question 45

Shown below is a list of pairs of compounds. In which pair is the second compound produced by a reduction of the first compound?

A) fumarate and succinate
B) oxaloacetate and citrate
C) fumarate and malate
D) malate and oxaloacetate
E) isocitrate and $\alpha$ -ketoglutarate

Question 46

Which cofactor is required for the conversion of fumarate to succinate?

A) FAD
B) GDP
C) CoA
D) FADH₂
E) both GDP and CoA

Question 47

Why is the citric acid cycle considered to be part of aerobic metabolism, even though oxygen is not a substrate in any reaction?

A) because it takes place in the mitochondrion
B) because it contains oxidation reactions
C) because it produces reduced electron carriers, which are reoxidized by transferring their electrons ultimately to oxygen
D) because NADH and FADH₂ produce a lot of ATP when reoxidized
E) because it produces carbon dioxide

Question 48

Which statement about the oxidative decarboxylation of pyruvate in animal cells is CORRECT?

A) One of the products of this reaction is a molecule containing a thioether bond.
B) It only occurs under aerobic conditions.
C) The process occurs in the glyoxysome compartment of the cell.
D) It is often considered the first step in the glyoxylate cycle, as it generates acetyl-CoA, which is required by this pathway.
E) It is the most important anaplerotic reaction in hepatocytes.

Question 49

How much carbon dioxide is produced from the complete aerobic catabolism of one molecule of 2-phosphoglycerate via catabolic pathways?

A) 2 CO₂
B) 3 CO₂
C) 4 CO₂
D) 5 CO₂
E) 6 CO₂

Question 50

When is the consumption of acetyl-CoA by the citric acid cycle is decreased?

A) when the concentration of NAD⁺ is high
B) when the concentration of oxaloacetate is high
C) when NADH is rapidly reoxidized via the electron transport chain
D) when the mitochondrial ATP synthase is inactive
E) when the concentrations of NAD⁺ and oxaloacetate are both high

Question 51

Which reaction in the citric acid cycle will NOT proceed in the absence of inorganic phosphate ions?

A) the reaction catalyzed by $\alpha$ -ketoglutarate dehydrogenase
B) the reaction catalyzed by succinate dehydrogenase
C) the reaction catalyzed by succinyl-CoA synthetase
D) the reaction catalyzed by aconitase
E) the reaction catalyzed by fumarase

Question 52

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?

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

Question 53

Which statement is FALSE regarding the citric acid cycle?

A) Succinate dehydrogenase is a flavoprotein associated with the electron-transport chain.
B) The rate of the citric acid cycle increases when the NAD⁺/NADH ratio is high.
C) In eukaryotes, the citric acid cycle occurs in the mitochondrial matrix.
D) Acetyl-CoA inhibits the $\alpha$ -ketoglutarate dehydrogenase complex.
E) All of the statements are false.

Question 54

Which cofactors associated with the pyruvate dehydrogenase complex are classified as prosthetic groups? 1) TPP
2) lipoate
3) CoA-SH
4) NAD⁺
5) FAD

A) 1 and 5
B) 1, 2, and 4
C) 1, 2, and 5
D) 3 and 4
E) 4 and 5

Question 55

Which enzymes catalyze oxidation reactions in the citric acid cycle? 1) succinate dehydrogenase
2) pyruvate dehydrogenase
3) citrate dehydrogenase
4) $\alpha$ -ketoglutarate dehydrogenase

A) 1 and 4
B) 1, 2, and 4
C) 1, 3, and 4
D) 2 and 3
E) All of the listed enzymes catalyze oxidation reactions in the citric acid cycle.

Question 56

Which enzyme catalyzes a reversible reaction under normal cellular conditions?

A) $\alpha$ -ketoglutarate dehydrogenase
B) isocitrate dehydrogenase
C) succinate dehydrogenase
D) pyruvate dehydrogenase
E) None of the answers is correct.

Question 57

Which molecule is an $\alpha$ -keto acid?

A) oxaloacetate
B) citrate
C) $\alpha$ -ketoglutarate
D) oxaloacetate and $\alpha$ -ketoglutarate
E) citrate and $\alpha$ -ketoglutarate

Question 58

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

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

Question 59

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

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 60

Which reaction of the citric acid cycle produces a net of one water molecule?

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

Question 61

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?

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 62

Which three of the choices listed are possible sources for acetyl-CoA that enters the citric acid cycle?

Question 63

Which enzyme class is NOT found in the citric acid cycle?

A) oxidoreductase
B) hydrolase
C) ligase
D) lyase
E) isomerase

Question 64

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

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 65

Which statement is FALSE regarding the glyoxylate cycle?

A) The glyoxylate cycle utilizes three of the eight enzymes associated with the citric acid cycle.
B) In the glyoxylate cycle, the two decarboxylation reactions of the citric acid cycle are bypassed, allowing for the net synthesis of two succinate molecules.
C) The glyoxylate cycle is an anabolic pathway occurring in plants and certain microorganisms.
D) Because mammals lack the glyoxylate cycle, they cannot convert acetate from the oxidation of fatty acids to glucose.
E) The glyoxylate cycle allows plants to use acetate as a source of carbon and as a source of energy.

Question 66

Name a metabolite that is required for pyruvate carboxylase activity (NOT part of the reaction), and explain why this makes metabolic sense.

Question 67

Although ADP is not directly involved in the isocitrate dehydrogenase reaction, the activity of this enzyme is dependent on the concentration of ADP in the matrix. Briefly explain how.

Question 68

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

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

Question 69

What process occurs in the citric acid cycle?

A) formation of a mixed anhydride (acyl phosphate)
B) net synthesis of oxaloacetate from two acetyl-CoA molecules
C) hydrolysis of two thioester bonds
D) All of the answers are correct.
E) None of the answers is correct.

Question 70

Which enzyme in the citric acid cycle has a standard free energy that is large and positive?

A) malate dehydrogenase
B) citrate synthase
C) succinyl-CoA synthetase
D) fumarase
E) No enzyme in the CAC has a positive $\Delta$ G'°.

Question 71

Which type of reaction is found in the citric acid cycle?

A) isomerization
B) hydration
C) condensation
D) redox
E) All of the answers are correct.

Question 72

How many ATP equivalents are made from the aerobic conversion of pyruvate to oxaloacetate?

A) 9 ATP
B) 10 ATP
C) 11 ATP
D) 11.5 ATP
E) 12.5 ATP

Question 73

The citric acid cycle has an important anabolic function by:

A) allowing for the conversion of certain carbon atoms from even-chain fatty acids into glucose.
B) producing lactate for gluconeogenesis.
C) providing metabolites for the synthesis of cholesterol, amino acids and glucose.
D) completing the oxidation of acetyl-CoA and storing electrons in the form of NADH.
E) All the answers are correct.

Question 74

Succinate can be synthesized from pyruvate via two distinct pathways that have no enzyme in common. What are the energy conditions of each pathway?

Question 75

Briefly describe the relationship of the pyruvate dehydrogenase complex reaction to glycolysis and the citric acid cycle.

Question 76

Degradation of leucine yields acetyl-CoA. Can degradation of this amino acid replenish the pool of citric acid cycle intermediates? Explain your reasoning.

Question 77

The citric acid cycle begins with the condensation of acetyl-CoA with oxaloacetate. Describe three possible sources for the acetyl-CoA.

Question 78

The disease beriberi is caused by a deficiency of thiamine in the diet. Why is this disease often associated with metabolic acidosis and neurological disorders?

Question 79

Which statement is FALSE regarding regulation of the pyruvate dehydrogenase 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 80

Which step is NOT catalyzed by the pyruvate dehydrogenase complex?

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⁺.