Chapter 16: The Citric Acid Cycle

A

Which of the following 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.

A

Which of the below 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⁺

E

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⁺

A

Which of the following statements 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 atfull speed under all conditions.

D

Glucose labeled with C 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.

A

Which of the following 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.

A

Acetyl-CoA labeled with C 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.

B

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

A) Citrate

B) Fumarate

C) Isocitrate

D) Pyruvate

E) Succinate

A

Which of the following is not an intermediate of the citric acid cycle?

A) Acetyl-coA

B) Citrate

C) Oxaloacetate

D) Succinyl-coA

E) α-Ketoglutarate

D

In mammals, each of the following occurs during the citric acid cycle except:

A) formation of α-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.

B

Oxaloacetate uniformly labeled with C (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

B

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.

E

Which one of the following is not associated with the oxidation of substrates by the citric acid cycle?

A) All of the below are involved.

B) CO₂ production

C) Flavin reduction

D) Lipoic acid present in some of the enzyme systems

E) Pyridine nucleotide oxidation

E

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.

A

The oxidative decarboxylation of α-ketoglutarate proceeds by means of multistep reactions in which

all but one of the following cofactors are required. Which one is not required?

A) ATP

B) Coenzyme A

C) Lipoic acid

D) NAD⁺

E) Thiamine pyrophosphate

E

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) α-ketoglutarate to succinyl-CoA.

E

Which one of the following enzymatic activities would be decreased by thiamine deficiency?

A) Fumarase

B) Isocitrate dehydrogenase

C) Malate dehydrogenase

D) Succinate dehydrogenase

E) α-Ketoglutarate dehydrogenase complex

E

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.

B

The standard reduction potentials (E'°) for the following half reactions are given.

Fumarate + 2H⁺ + 2e⁻ → succinate E'° = +0.031 V

FAD + 2H⁺ + 2e⁻ → FADH₂ E'° = –0.219 V

If succinate, fumarate, FAD, and FADH₂, all at 1 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.

D

For the following reaction, ΔG'° = 29.7 kJ/mol.

L-Malate + NAD⁺ → 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 ΔG'° 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

D

All of the oxidative steps of the citric acid cycle are linked to the reduction of NAD⁺ except the reaction catalyzed by:

A) isocitrate dehydrogenase.

B) malate dehydrogenase.

C) pyruvate dehydrogenase

D) succinate dehydrogenase.

E) the α-ketoglutarate dehydrogenase complex.

C

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

A) ATP

B) Biotin

C) FAD

D) NAD⁺

E) NADP⁺

E

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.

A

Which of the following intermediates of the citric acid cycle is prochiral?

A) Citrate

B) Isocitrate

C) Malate

D) Oxaloacetate

E) Succinate

D

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

D

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.

E

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.

A

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.

E

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

A) complete oxidation of acetyl-CoA to CO₂ plus reduced coenzymes.

B) net conversion of lipid to carbohydrate.

C) net synthesis of four-carbon dicarboxylic acids from acetyl-CoA.

D) net synthesis of long-chain fatty acids from citric acid cycle intermediates.

E) both B and C are correct

A

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).