Chapter 19—The Tricarboxylic Acid Cycle
MULTIPLE CHOICE
1. In the TCA cycle, carbon enters the cycle as ____ and exits as ____ with metabolic energy captured as
____, ____ and ____.
a.
malonate; water; NADH; ATP; NADPH
b.
acetyl-CoA; CO2; NADH; ATP; NADPH
c.
succinyl-CoA; CO2; ATP; NADH; NADPH
d.
acetyl-CoA; CO2; ATP; NADH; [FADH2]
e.
malonyl-CoA; water; NADH; [FADH2]; ATP
2. In eukaryotic cells, glycolysis occurs in the ____, and the TCA cycle reactions take place in ____.
a.
mitochondria; mitochondria
b.
cytoplasm; mitochondria
c.
cytoplasm; cytoplasm
d.
mitochondria; ribosomes
e.
cytoplasm; ribosomes
3. The ____ of pyruvate to acetyl-CoA is catalyzed by ____.
a.
dehydration; pyruvate dehydration complex
b.
decarboxylation; pyruvate dehydrogenase complex
c.
decarboxylation; pyruvate decarboxylase
d.
transacylation; pyruvate transacylase
e.
none of the above.
4. The pyruvate dehydrogenase complex contains three multimeric enzymes (ETA, EPDH, and EDLD). All
are properties of EPDH EXCEPT:
a.
It uses thiamin pyrophosphate as a catalytic coenzyme.
b.
It oxidatively decarboxylates pyruvate.
c.
It binds NAD+ in its active site.
d.
It transfers an acetyl group to lipoamide of ETA.
e.
It forms a hydroxyethyl-TPP intermediate.
5. The following statements are correct about the reaction catalyzed by the pyruvate dehydrogenase
complex EXCEPT:
a.
it is the link between glycolysis and the TCA cycle
b.
The activity of the complex is regulated by the phosphorylation of EPDH.
c.
NAD+ is the direct oxidant of reduced lipoamide.
d.
An FAD is covalently linked to the EDLD, the lipoamide dehydrogenase component.
e.
Although EPDH is called “pyruvate dehydrogenase” it is not a dehydrogenase.
6. Order the coenzymes according to their involvement in the pyruvate dehydrogenase complex.
A.
NAD+
B.
CoA-SH
C.
TPP
D.
Lipoate (lipoamide)
E.
[FAD]
a.
A, B, C, D, E
b.
C, B, A, E, D
c.
C, D, B, E, A
d.
B, D, E, A, C
e.
C, E, D, B, A
7. All are correct statements about the reaction catalyzed by citrate synthase EXCEPT:
a.
Citrate synthase is allosterically activated by ATP.
b.
The complete chemical equation is:
Oxaloacetate + acetyl-CoA + H2O → 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.
e.
The enzyme uses general base catalysis to generate the reactive species.
8. Citrate synthase has a ____ G and is essentially ____ due to the ____ of the CoA thioester.
a.
large positive; reversible; oxidation
b.
nearly zero; reversible; reduction
c.
nearly zero; irreversible; reduction
d.
large negative; irreversible; hydrolysis
e.
large negative; reversible; hydrolysis
9. Citrate synthase is a ____ with binding of ____ inducing a conformational change ____ the binding of
acetyl-CoA, and ____ is an allosteric inhibitor.
a.
dimer; OAA; increasing; NADH
b.
dimer; OAA; decreasing; NADH
c.
tetramer; OAA; decreasing; NAD+
d.
monomer; pyruvate; decreasing; ATP
e.
monomer; pyruvate; increasing; ATP
10. Which is NOT true of citrate synthase?
a.
it catalyzes the first reaction in the TCA cycle
b.
it is not regulated
c.
OAA and acetyl-CoA bind to the active sites
d.
the citrate synthase reaction liberates a relatively large amount of energy
e.
all of the above are true
11. Citrate must be isomerized to isocitrate because the ____ group of citrate is not oxidizable, but the
____ group of isocitrate can be oxidized.
a.
ketone; primary alcohol
b.
tertiary alcohol; secondary alcohol
c.
ketone; secondary alcohol
d.
aldehyde; primary alcohol
e.
secondary alcohol; ketone
12. ____ catalyzes citrate isomerization to isocitrate by abstracting ____ from citrate to yield the enzyme-
bound intermediate ____, followed by ____ to produce isocitrate.
a.
Citrate isomerase; CO2; trans–aconitate; carboxylation
b.
Citrate isomerase; water; trans–aconitate; rehydration
c.
Aconitase; water; cis-aconitate; rehydration
d.
Aconitase; CO2; cis-aconitate; carboxylation
e.
None are true
13. All are true for aconitase EXCEPT:
a.
iron atom acts as a Lewis acid.
b.
equilibrium favors citrate.
c.
contains an iron-sulfur cluster.
d.
one Fe3+ coordinates with C-3 carbonyl and hydroxyl group of citrate.
e.
all are true.
14. Fluoroacetate is a potent inhibitor of the TCA cycle. Which step of the TCA cycle is inhibited as a
result of fluoroacetate entering the TCA cycle?
a.
citrate synthase
b.
aconitase
c.
isocitrate dehydrogenase
d.
-ketoglutarate dehydrogenase
e.
succinyl-CoA synthetase
15. Which enzymes of the TCA cycle catalyze oxidative decarboxylation reactions?
a.
malate dehydrogenase and citrate synthase
b.
fumarase and succinate dehydrogenase
c.
-ketoglutarate dehydrogenase and succinate dehydrogenase
d.
isocitrate dehydrogenase and –ketoglutarate dehydrogenase
e.
aconitase and succinate dehydrogenase
16. The two step reaction catalyzed by isocitrate dehydrogenase involves:
A.
-decarboxylation expelling the -keto carboxyl as CO2.
B.
oxidation of the C-2 alcohol of isocitrate to form oxalosuccinate.
C.
oxidation of the C-2 alcohol to form oxaloacetate.
D.
-elimination expelling the central carboxyl group as CO2.
a.
A and B
b.
B and C
c.
C and D
d.
A and C
e.
B and D
17. Allosteric inhibitors of isocitrate dehydrogenase include ____ and ____, whereas ____ acts as an
allosteric activator, ____ the Km for isocitrate.
a.
ATP; NADH; ADP; lowering
b.
ATP; ADP; AMP; lowering
c.
NADH; NADPH; AMP; increasing
d.
NADH; ATP; ADP; increasing
e.
NADH; ATP; AMP; lowering
18. Isocitrate dehydrogenase has all of the characteristics EXCEPT:
a.
ADP raises the Km for isocitrate by a factor of 10.
b.
virtually inactive in the absence of ADP.
c.
sufficiently exergonic to pull the aconitase reaction forward.
d.
allosterically inhibited by NADH and ATP.
e.
an oxidative-decarboxylation reaction.
19. –Ketoglutarate dehydrogenase is a multi-enzyme complex analogous to:
a.
pyruvate kinase.
b.
glyceraldehyde-3-phosphate dehydrogenase.
c.
isocitrate dehydrogenase.
d.
pyruvate dehydrogenase.
e.
lactate dehydrogenase.
20. The coenzymes listed below are associated with -ketoglutarate dehydrogenase complex EXCEPT:
a.
[FAD].
b.
TPP.
c.
lipoamide.
d.
NAD+.
e.
biotin.
21. The only reaction of the citric acid cycle that provides substrate-level phosphorylation is catalyzed by:
a.
malate dehydrogenase.
b.
citrate synthase.
c.
isocitrate dehydrogenase.
d.
succinyl-CoA synthetase.
e.
nucleotide triphosphate kinase.
22. All are characteristics of succinyl-CoA synthetase EXCEPT:
a.
succinyl-CoA can be used to drive phosphorylation of GDP or ADP.
b.
the enzyme is named for the reverse reaction.
c.
it provides an example of substrate-level phosphorylation.
d.
succinyl-phosphate is an intermediate in the reaction catalyzed by succinyl-CoA
synthetase.
e.
all of the above are true.
23. The correct sequence of steps between succinyl-CoA and ATP in mammals to “preserve” the energy of
the thioester bond are:
A.
succinyl-phosphate.
B.
phosphohistidine.
C.
GTP.
D.
Acetyl-CoA.
a.
A, B, C
b.
B, C, A
c.
A, C, D
d.
C, B, A
e.
D, A, B
24. Characteristics of succinate dehydrogenase include all EXCEPT:
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.
e.
it carries out either 1-electron or 2-electron transfers to/from FAD.
25. The succinate dehydrogenase mechanism involves the ____ of succinate, which is exergonic and can
be used to provide for the ____.
a.
oxidation; reduction of NAD+
b.
reduction; oxidation of NAD+
c.
oxidation; reduction of [FAD]
d.
reduction; oxidation of [FAD]
e.
none are true
26. The succinate dehydrogenase catalyzed reaction involves dehydrogenation ____ to a carbonyl group
and is ____ to yield a ____ double bond.
a.
,; electrophilic; trans
b.
,; electrophilic; cis
c.
,; stereospecific; trans
d.
,; stereospecific; cis
e.
none are true
27. The correct sequence of electron transport in the succinate dehydrogenase reaction mechanism is:
A.
Coenzyme Q (UQ).
B.
[FAD].
C.
iron-sulfur clusters.
D.
electron transport chain.
a.
A, B, C, D
b.
B, C, D, A
c.
B, C, A, D
d.
C, B, D, A
e.
C, D, A, B
28. Fumarase catalyzes a reaction for which each of the following is true EXCEPT:
a.
fumarate is hydrated
b.
the reaction is stereospecific for a trans product
c.
the reaction involved is similar to that carried out by aconitase
d.
L-malate is the produced
e.
all are true
29. Characteristics of the oxidation of malate to oxaloacetate (OAA) include all EXCEPT:
a.
catalyzed by malate dehydrogenase.
b.
uses NAD+ as an electron acceptor.
c.
is very exergonic.
d.
results in higher levels of malate than oxaloacetate.
e.
is structurally and functionally similar to lactate dehydrogenase.
30. The oxidation of malate to oxaloacetate is not thermodynamically favored under standard conditions.
It occurs because:
a.
it involves substrate-level phosphorylation.
b.
it is coupled with a strong reduction.
c.
it is coupled with ATP hydrolysis.
d.
oxaloacetate is used in the next reaction, which has a negative G.
e.
the previous reaction has a large negative G.
31. In the TCA cycle, ____ oxidation(s) use(s) NAD+ as the oxidizing agent while ____ oxidation(s)
use(s) FAD as the oxidizing agent.
a.
0; 4
b.
1; 3
c.
2; 2
d.
3; 1
e.
4; 0
32. Which of the following represents a reaction that could be used to refill the CAC?:
a.
OAA → PEP
b.
citrate → OAA + acetyl-CoA
c.
Asp → -ketoglutarate
d.
pyruvate → OAA
e.
none of the above
33. The anaplerotic reactions associated with the TCA cycle are a result of the:
a.
use of many of the TCA cycle intermediates in biosynthesis.
b.
oxidative nature of the TCA cycle.
c.
decarboxylation reactions.
d.
production of GTP and reduced coenzymes.
e.
irreversible nature of some of the TCA cycle reactions.
34. When acetyl-CoA levels exceed the ____ supply, allosteric activation of ____ by ____ raises the
oxaloacetate (OAA) levels for condensation with acetyl-CoA to form ____.
a.
citrate; citrate synthase; acetyl-CoA; citrate
b.
malate; malate dehydrogenase; ATP; citrate
c.
OAA; citrate synthase; acetyl-CoA; isocitrate
d.
OAA; pyruvate carboxylase; acetyl-CoA; citrate
e.
Acetyl-CoA; pyruvate carboxylase; citrate; acetyl-CoA
35. All are true for pyruvate carboxylase EXCEPT:
a.
an anaplerotic reaction.
b.
in mitochondria of plants, but not animals.
c.
is tetrameric.
d.
contains covalently bound biotin.
e.
absolute allosteric requirement for acetyl-CoA.
36. The reaction CO2 + PEP + GDP OAA + GTP is catalyzed by:
a.
PEP carboxylase.
b.
PEP carboxykinase.
c.
malic enzyme.
d.
pyruvate carboxylase.
e.
None are true.
Garrett/Grisham 5e Test Bank 8
37. The three reactions of the citric acid cycle with large negative G values include:
A.
citrate synthase.
B.
aconitase.
C.
isocitrate dehydrogenase.
D.
-ketoglutarate dehydrogenase.
E.
succinyl-CoA synthetase.
F.
succinyl-CoA dehydrogenase.
G.
fumarase.
H.
malate dehydrogenase.
a.
A, B, C
b.
A, C, D
c.
C, E, F
d.
B, C, E
e.
A, C, H
38. All are principal allosteric regulatory “signals” controlling the TCA cycle activity EXCEPT:
a.
acetyl-CoA.
b.
NADH.
c.
NAD+.
d.
ATP.
e.
all are true.
39. It is crucial that regulation occur at pyruvate dehydrogenase because:
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.
e.
all of the above.
40. Mammalian pyruvate dehydrogenase is regulated by phosphorylation using the enzyme ____ which is
allosterically activated by high levels of ____ and ____.
a.
pyruvate kinase; ATP; acetyl-CoA
b.
pyruvate kinase; NADH; NADPH
c.
pyruvate carboxylase; acetyl-CoA; ATP
d.
pyruvate dehydrogenase kinase; NADH; acetyl-CoA
e.
pyruvate dehydrogenase kinase; ADP; NAD+
41. In mammalian tissues, isocitrate dehydrogenase is allosterically stimulated by:
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.
e.
none of the above.
42. Inhibition of the citric acid cycle at isocitrate dehydrogenase increases the levels of citrate and
isocitrate that may increase the production of:
a.
amino acids.
b.
pyruvate and oxaloacetate.
c.
glyoxylate and cytosolic acetyl-CoA.
d.
succinate and fumarate.
e.
lactate and alanine.
43. All of the following are inhibitors of citrate synthase EXCEPT:
a.
acetyl-CoA
b.
NADH
c.
succinyl-CoA
d.
ATP
e.
all are inhibitors
44. An essential feature of the glyoxylate cycle, which is not associated with the TCA cycle is:
a.
it utilizes condensation of acetyl-CoA with glyoxylate
b.
each complete cycle produces a net gain of two 4-carbon molecules
c.
it occurs in chloroplasts and the mitochondria of plant seeds
d.
mammals use the glyoxylate cycle for gluconeogenesis
e.
it is a reductive pathway that utilizes NADPH as an electron source
45. All are true statements for the glyoxylate pathway EXCEPT:
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 of the enzymes for the glyoxylate cycle.
d.
bypasses the oxidative-decarboxylation steps of the TCA cycle.
e.
isocitrate is routed through isocitrate lyase and malate synthase.
46. The isocitrate lyase catalyzed reaction cleaves isocitrate into:
a.
glyoxylate and fumarate.
b.
succinate and acetyl-CoA.
c.
malate and acetyl-CoA.
d.
succinate and glyoxylate.
e.
glyoxylate and acetyl-CoA.
47. The glyoxylate cycle reaction that is catalyzed by malate synthase involves the condensation of acetyl
CoA with _____. The mechanism is most similar to the TCA cycle enzyme _____.
a.
succinate; fumarase
b.
isocitrate; succinyl-CoA synthetase
c.
glyoxylate; citrate synthase
d.
oxaloacetate; aconitase
e.
acetyl CoA; isocitrate lyase
48. Which of the following coenzymes is NOT used by pyruvate dehydrogenase?
a.
NAD+
b.
FAD
c.
thiamine pyrophosphate
d.
tetrahydrofolate
e.
lipoic acid
49. Which of the following is correctly paired with its inhibitor?
a.
pyruvate dehydrogenase: phosphorylation
b.
isocitrate dehydrogenase: AMP
c.
-ketoglutarate dehydrogenase: NAD+
d.
citrate synthase: acetyl CoA
e.
none of the above are correct
50. Which of the following statements regarding the glyoxylate cycle is true?
a.
the process takes place in the mitochondria
b.
two molecules of acetyl CoA can be converted to one molecule of oxaloacetate
c.
isocitrate dehydrogenase and malate dehydrogenase are not used
d.
the primary role is to allow for the synthesis of amino acids from fatty acids
e.
none of the above
51. Which of the following enzymes catalyzes an endergonic step of the citric acid cycle?
a.
citrate synthase
b.
malate dehydrogenase
c.
-ketoglutarate dehydrogenase
d.
isocitrate dehydrogenase
e.
none of the above