Deck 18: The Tricarboxylic Acid Cycle

Question 1

Which of the following is an allosteric activator of isocitrate dehydrogenase?

A)ATP
B)NADH
C)[FADH₂]
D)ADP

Answer 1

D

Question 2

Which alcohol group of isocitrate is oxidizable?

A)primary
B)secondary
C)tertiary
D)quaternary

Answer 2

B

Question 3

Which carbon molecule enters the TCA cycle and exits as CO₂,with metabolic energy captured as ATP,NADH,and [FADH₂]?

A)malonate
B)succinyl-CoA
C)acetyl-CoA
D)malonyl-CoA

Answer 3

C

Question 4

Which molecule is an allosteric inhibitor of citrate synthase?

A)NADH
B)OAA
C)[FADH₂]
D)ATP

Question 5

Which of the following is NOT part of the 2-step reaction catalyzed by isocitrate dehydrogenase?

A)?-decarboxylation expelling the a-keto carboxyl as CO?
B)oxidation of the C-2 alcohol of isocitrate to form oxalosuccinate
C)oxidation of the C-2 alcohol to form oxaloacetate
D)?-decarboxylation expelling the central carboxyl group as CO?

Question 6

Which enzyme catalyzes the decarboxylation of pyruvate to acetyl-CoA?

A)pyruvate dehydrogenase complex
B)pyruvate kinase
C)pyruvate decarboxylase
D)pyruvate transacylase

Question 7

Which coenzyme is involved in the reactions carried out by the pyruvate dehydrogenase complex?

A)NADH
B)acetyl-CoASH
C)TPP
D)[FADH₂]

Question 8

Which enzyme is analogous to the multi-enzyme complex a-ketoglutarate?

A)glyceraldehyde-3-phosphate dehydrogenase
B)isocitrate dehydrogenase
C)pyruvate dehydrogenase
D)lactate dehydrogenase

Question 9

Which of the following is NOT a characteristic of isocitrate dehydrogenase?

A)ADP raises the Km for isocitrate by a factor of 10.
B)It is virtually inactive in the absence of ADP.
C)It is sufficiently exergonic to pull the aconitase reaction forward.
D)It is allosterically inhibited by NADH and ATP.

Question 10

Which of the following is NOT a coenzyme for the a-ketoglutarate dehydrogenase complex?

A)TPP
B)lipoamide
C)NAD⁺
D)biotin

Question 11

Which enzymes of the TCA cycle catalyze oxidative decarboxylation reactions?

A)malate dehydrogenase and citrate synthase
B)fumarase and succinate dehydrogenase
C)a-ketoglutarate dehydrogenase and succinate dehydrogenase
D)isocitrate dehydrogenase and a-ketoglutarate dehydrogenase

Question 12

Which of the following is NOT correct about the reaction catalyzed by citrate synthase?

A)Citrate synthase is allosterically activated by ATP.
B)The complete chemical equation is:
oxaloacetate + acetyl-CoA + H?O citrate + CoA
C)Citryl-CoA is formed as an intermediate.
D)The mechanism involves attack of the carbanion of acetyl-CoA on the carbonyl carbon of oxaloacetate.

Question 13

Which of the following is NOT correct for aconitase?

A)Iron atom acts as a Lewis acid.
B)Equilibrium favours isocitrate.
C)Aconitase contains an iron-sulfur cluster.
D)One Fe³⁺ coordinates with C-3 carbonyl and hydroxyl group of citrate.

Question 14

Where do glycolysis and the TCA cycle,respectively,occur in eukaryotic cells?

A)cytoplasm;mitochondria
B)mitochondria;mitochondria
C)cytoplasm;cytoplasm
D)mitochondria;ribosomes

Question 15

Which of the following statements is NOT correct about the reaction catalyzed by the pyruvate dehydrogenase complex?

A)It is the link between glycolysis and the TCA cycle.
B)The activity of the complex is regulated by the phosphorylation of the pyruvate dehydrogenase complex.
C)NAD⁺ is the direct oxidant of reduced lipoamide.
D)An FAD is covalently linked to the EDLD,the lipoamide dehydrogenase component.

Question 16

Which enzyme catalyzes citrate isomerization to isocitrate by abstracting water from citrate?

A)citrate isomerase
B)citrate synthase
C)aconitase
D)isocitrate dehydrogenase

Question 17

Which is NOT a feature of citrate synthase?

A)It catalyzes the first reaction in the TCA cycle.
B)OAA and acetyl-CoA bind to the active sites.
C)The citrate synthase reaction liberates a relatively large amount of energy.
D)It is not regulated.

Question 18

What is the DG°' value for citrate synthase?

A)It is largely positive.
B)It is nearly zero.
C)It is not measurable.
D)It is largely negative.

Question 19

The pyruvate dehydrogenase complex contains 3 multimeric enzymes.Which of the following is NOT a property of the complex?

A)It uses thiamine pyrophosphate as a catalytic coenzyme.
B)It oxidatively decarboxylates pyruvate.
C)It binds NAD⁺ in its active site.
D)It forms a hydroxyethyl-TPP intermediate.

Question 20

Which TCA cycle enzyme is inhibited by fluorocitrate,the product of citrate synthase's action on fluoroacetate?

A)aconitase
B)isocitrate dehydrogenase
C)citrate isomerase
D)succinate dehydrogenase

Question 21

Why is it crucial that regulation occur at pyruvate dehydrogenase?

A)Pyruvate kinase is reversible.
B)Lactate dehydrogenase is the only other enzyme to use pyruvate.
C)The product acetyl-CoA is committed to oxidation in the citric acid cycle or fatty acid biosynthesis.
D)Alanine aminotransferase would use the pyruvate.

Question 22

The oxidation of malate to oxaloacetate is not thermodynamically favoured under standard conditions.Why does it occur?

A)It involves substrate-level phosphorylation.
B)Oxaloacetate is used in the next reaction,which has a negative DG.
C)It is coupled with ATP hydrolysis.
D)It is coupled with a strong reduction.

Question 23

Which of the following represents a reaction that could be used to refill the CAC?

A)pyruvate → OAA
B)OAA → PEP
C)citrate → OAA + acetyl-CoA
D)Asp → a-ketoglutarate

Question 24

Which of the following is NOT correct for pyruvate carboxylase?

A)It carries out an anaplerotic reaction.
B)It resides in mitochondria of plants,but not animals.
C)It contains covalently bound biotin.
D)It has an absolute allosteric requirement for acetyl-CoA.

Question 25

Which of the following is NOT a characteristic of succinyl-CoA synthetase?

A)Succinyl-CoA can be used to drive phosphorylation of GDP or ADP.
B)The enzyme is named for the forward reaction.
C)It provides an example of substrate-level phosphorylation.
D)Succinyl-phosphate is an intermediate in the reaction catalyzed by succinyl-CoA synthetase.

Question 26

Which enzyme carries out a reaction of the citric acid cycle with the largest negative DG value?

A)citrate synthase
B)succinyl-CoA synthetase
C)succinyl-CoA dehydrogenase
D)malate dehydrogenase

Question 27

Energy is derived from the succinate dehydrogenase mechanism.What process is the energy used for?

A)reduction of NAD⁺
B)oxidation of NAD⁺
C)reduction of [FAD]
D)oxidation of [FAD]

Question 28

How many NADH molecules are produced in the TCA cycle per molecule of acetyl-CoA oxidized?

A)1
B)2
C)3
D)4

Question 29

Which of the following is NOT a characteristic of succinate dehydrogenase?

A)It is also known as succinate-coenzyme Q reductase.
B)It has covalently bound FAD.
C)It is a membrane-bound enzyme.
D)It removes hydrogens from C-O bonds.

Question 30

Which molecule is generated between succinyl-CoA and ATP in mammals to "preserve" the energy of the thioester bond?

A)a-ketoglutarate
B)phosphohistidine
C)citrate
D)acetyl-CoA

Question 31

Which feature of electron transport is NOT involved in the succinate dehydrogenase reaction mechanism?

A)coenzyme Q (UQ)
B)NADH
C)[FAD]
D)iron-sulfur clusters

Question 32

Which of the following is NOT a feature of the fumarase catalyzed reaction?

A)Fumarate is hydrated.
B)The reaction is stereospecific for a cis product.
C)The reaction involved is similar to that carried out by aconitase.
D)L-Malate is the product.

Question 33

What type of double bond does the succinate dehydrogenase-catalyzed reaction generate?

A)saturated
B)electrophilic cis
C)stereospecific
D)trans

Question 34

Which of the following is NOT a characteristic of the oxidation of malate to oxaloacetate (OAA)?

A)It is catalyzed by malate dehydrogenase.
B)It uses NAD⁺ as an electron acceptor.
C)It is very exergonic.
D)It results in higher levels of malate than oxaloacetate.

Question 35

Which enzyme phosphorylates mammalian pyruvate dehydrogenase and is allosterically activated by high levels of NADH and acetyl CoA?

A)pyruvate kinase
B)pyruvate kinase
C)pyruvate carboxylase
D)pyruvate dehydrogenase kinase

Question 36

Which enzyme is allosterically activated when acetyl-CoA levels exceed oxaloacetate (OAA)supply?

A)citrate synthase
B)malate dehydrogenase
C)citrate synthase
D)pyruvate carboxylase

Question 37

Which of these enzymes catalyzes the only reaction of the citric acid cycle that provides substrate-level phosphorylation?

A)malate dehydrogenase
B)succinyl-CoA synthetase
C)citrate synthase
D)isocitrate dehydrogenase

Question 38

Which of the following is NOT a principal allosteric regulator of TCA cycle activity?

A)[FADH₂]
B)NADH
C)acetyl-CoA
D)ATP

Question 39

What are the anaplerotic reactions associated with the TCA cycle a result of?

A)the use of many of the TCA cycle intermediates in biosynthesis
B)the oxidative nature of the TCA cycle
C)the production of GTP and reduced coenzymes
D)the irreversible nature of some of the TCA cycle reactions

Question 40

Which of these enzymes catalyzes the reaction CO₂ + PEP + GDP ↔ OAA + GTP?

A)PEP carboxylase
B)PEP carboxykinase
C)malic enzyme
D)pyruvate carboxylase

Question 41

Which of the following is an essential feature of the glyoxylate cycle that is NOT associated with the TCA cycle?

A)It utilizes condensation of acetyl-CoA with succinate.
B)It can produce a net synthesis of 4-carbon units,which are TCA cycle intermediates,from acetyl-CoAs.
C)It occurs in chloroplasts.
D)Mammals use the glyoxylate cycle for gluconeogenesis.

Question 42

Which of the following is NOT correct for the glyoxylate pathway?

A)The glyoxylate pathway enzymes are found in the glyoxysomes.
B)Although glyoxysomes are found in germinating seeds,they disappear after a plant begins photosynthesis.
C)Glyoxysomes contain all the enzymes for the glyoxylate cycle.
D)The glyoxylate pathway bypasses the oxidative-decarboxylation steps of the TCA cycle.

Question 43

Which of the following allosterically stimulate mammalian isocitrate dehydrogenase?

A)high NAD⁺/NADH and high ADP/ATP ratios
B)low ADP/ATP and low NAD⁺/NADH ratios
C)high NAD⁺/NADH and low ADP/ATP ratios
D)low NAD⁺/NADH and high ADP/ATP ratios

Question 44

Which of the following is an inhibitor of citrate synthase?

A)acetyl-CoA
B)[FADH₂]
C)succinyl-CoA
D)ADP

Question 45

Which molecules are generated when isocitrate lyase cleaves isocitrate?

A)glyoxylate and fumarate
B)succinate and acetyl-CoA
C)malate and acetyl-CoA
D)succinate and glyoxylate

Question 46

Which molecules may be increased when isocitrate dehydrogenase is inhibited?

A)pyruvate and oxaloacetate
B)glyoxylate and cytosolic acetyl-CoA
C)succinate and fumarate
D)lactate and alanine