Chapter 18 Preparation for the Cycle
Matching Questions
Use the following to answer questions 1-10:
Choose the correct answer from the list below. Not all of the answers will be used.
a) lactic acidosis
b) phosphorylation
c) beriberi
d) mitochondria
e) acetyl CoA
f) E3
g) thiamine pyrophosphate
h) lipoamide
i) pyruvate dehydrogenase
j) coenzyme A
k) E1
l) glyoxylate cycle
1.
____________ Where does the citric acid cycle take place in the cell?
2.
____________ The activated carrier of acyl groups.
Ans: j
Section: 18.1
3.
____________ In the presence of oxygen pyruvate is converted to this.
Ans: e
Section: Introduction
4.
____________ This is a large multi-subunit enzyme complex that links glycolysis and the citric
acid cycle under aerobic conditions.
Ans: i
5.
____________ This provides a flexible linker between active sites on the PDH complex.
Ans: h
Section: 18.1
Ans: d
Section: Introduction
Chapter 18 Preparation for the Cycle
2
6.
____________ Pyruvate is decarboxylated at this active site in PDH.
7.
____________ This coenzyme is the prosthetic group in the decarboxylation of pyruvate.
Ans: g
Section: 18.1
8.
____________ Arsenite inhibits the function of this component of the PDH.
Ans: f
Section: 18.2
9.
____________ This type of enzyme regulation process inhibits the pyruvate dehydrogenase
complex.
Ans: b
Section: 18.2
10.
Ans: c
Section: 18.2
____________ is caused by a deficiency in vitamin B1.
Fill–in-the-Blank Questions
11.
is the PDH subunit responsible for the regeneration of the oxidized form of lipoamide.
Ans: Dihydrolipoyl dehydrogenase E3 Section: 18.1
12.
Decarboxylation, oxidation, and are the three steps critical to preserve the free energy
derived from decarboxylation of the PDH complex.
Ans: transfer to CoA Section: 18.1
13.
E1 of the pyruvate dehydrogenase complex requires the coenzyme for proper activity.
Ans: thiamine pyrophospate Section: 18.1
14.
E2 of the pyruvate dehydrogenase complex contains a lipoyl group that is covalently attached to
a residue of the enzyme.
Ans: lysine Section: 18.1
15.
is the critical functional group of lipoamide responsible for carrying acetyl groups and
maintaining the free energy of CoA.
Ans: Sulfhydryl(–SH) group Section: 18.1
16.
provides a flexible linkage for substrate to move between active sites on the PDH.
Ans: k
Section: 18.1
Chapter 18 Preparation for the Cycle
3
17.
inserts the lipoamide arm of the SH domain deep into the channel in E1.
18.
PDH is inactive when it the enzyme is .
Ans: phosphorylated Section: 18.2
19.
High-energy charge, which results in increasing concentration of NADH, the activity of PDH.
Ans: inhibits Section: 18.2
20.
Beriberi is caused by a deficiency of .
Ans: thiamine Section: 18.2
Multiple-Choice Questions
21.
The citric acid cycle is also known as the:
A)
Krebs cycle.
D)
A and C.
B)
Cori cycle.
E)
A, B, and C.
C)
tricarboxylic acid cycle.
Ans: D Section: Introduction
22.
Pyruvate is decarboxylated by which subunit of the PDH?
A)
E1
D)
E4
B)
E2
E)
E5
C)
E3
Ans: A Section: 18.1
23.
What enzyme(s) is (are) responsible for the following reaction?
Pyruvate + CoA + NAD+ → acetyl CoA + NADH + H+ + CO2
A)
acetyl CoA synthetase
D)
A and B.
B)
pyruvate decarboxylase
E)
A, B, and C.
C)
pyruvate dehydrogenase complex
Ans: C Section: 18.1
24.
What are the steps involved (in order) in the conversion of pyruvate to acetyl CoA?
A)
decarboxylation, oxidation, transfer to CoA
B)
decarboxylation, transfer to CoA, oxidation
C)
oxidation, decarboxylation, transfer to CoA
D)
oxidation, transfer to CoA, decarboxylation
E)
None of the above.
Ans: A Section: 18.1
Ans: E2 Section: 18.1
Chapter 18 Preparation for the Cycle
4
25.
Which of the following vitamins are precursors to coenzymes that are necessary for the
formation of acetyl CoA from pyruvate?
A)
thiamine, riboflavin, niacin, lipoic acid, and pantothenic acid
B)
thiamine, riboflavin, niacin, lipoic acid, pantothenic acid, and biotin
C)
thiamine, riboflavin, niacin, and biotin
D)
thiamine, riboflavin, and lipoic acid
E)
None of the above.
26.
Which of the following functions as a “flexible swinging arm” when it transfers the reaction
intermediate from one active site to the next?
A)
FAD
B)
NAD+
C)
lipoamide
D)
thiamine pyrophosphate
E)
coenzyme A
Ans: C Section: 18.1
27.
Pyruvate dehydrogenase is _____________ when ATP/ADP ratios are high.
A)
activated
D)
B and C.
B)
inhibited
E)
A and C.
C)
phosphorylated
Ans: B Section: 18. 2
28.
PDH phosphatase deficiency results in which condition?
A)
overstimulated PDH
D)
high levels of acetyl CoA
B)
low blood glucose
E)
None of the above.
C)
chronic elevated plasma lactate
Ans: C Section: 18.2
29.
Milling and polishing rice results in ______________.
A)
loss of the husk of rice
D)
loss of thiamine pyrophosphase
B)
white rice
E)
All of the above.
C)
potential for beriberi
Ans: E Section: 18.2
30.
Beriberi symptoms are similar to those of which disease?
A)
arsenite poisoning
D)
scurvy
B)
lactic acidosis
E)
cancer
C)
type II diabetes
Ans: A Section: 18.2
31.
Which of the following conditions will activate pyruvate dehydrogenase kinase, which catalyzes
the phorphorylation and inactivation of E1 in the pyruvate dehydrogenase complex?
A)
elevated concentrations of NADH and ATP
B)
elevated concentrations of NAD+ and ADP
C)
Ca2+
D)
insulin
E)
elevated concentrations of acetyl CoA
Ans: A Section: 18.2
Ans: A Section: 18.1 and Table 18.1
Chapter 18 Preparation for the Cycle
5
32.
In addition to pyruvate dehydrogenase, what other enzyme(s) in the citric acid cycle has a key
thiamine pyrophosphate coenzyme?
A)
isocitrate dehydrogenase
D)
A and B.
B)
-ketoglutarate dehydrogenase
E)
A, B, and C.
C)
citrate synthase (in bacteria)
33.
The ΔG°ˊ for the sum of the reactions in the pyruvate dehydrogenase complex is –33.4 kJ/mol.
What is the primary contribution to this large and negative free energy change?
A)
In E3, two electrons are transferred from FADH2 to NAD+, a thermodynamically favored
transfer.
B)
The product of the reaction, acetyl CoA, is shuttled into the mitochondria, keeping the
cytosolic concentration low.
C)
Free energy from the decarboxylation step drives the formation of NADH.
D)
A and B.
E)
A, B, and C.
Ans: C Section: 18.1
34.
Why is it important that the eight catalytic trimers of the transacetylase enzyme, E2, are
assembled to form the hollow cube of the enzyme complex?
A)
Binding of the final coenzyme, NAD+, occurs on the inner surface of the hollow core
structure.
B)
This hollow core separates the intermediates of the three enzyme reactions.
C)
The hollow core is connected to the 20-Å-long hydrophobic channel in the E1 enzyme.
D)
Substrates move from active site to active site across the hollow core.
E)
The E2 enzyme contains the long flexible lipoamide arm needed for the structural
integration of the complex.
Ans: E Section: 18.1
35.
Which of the following illustrates the role of Ca2+ in PDH complex regulation?
A)
Ca2+ activates phosphatase activity, deactivating PDHase activity.
B)
Ca2+ is elevated intracellularly with the increase of muscle contractions, activating
PDHase activity.
C)
Insulin causes a decrease in Ca2+, activating pyruvate kinase activity.
D)
In liver, Ca2+ stimulates pyruvate kinase, increasing conversion of pyruvate to acetyl
CoA.
E)
None of the above.
Ans: B Section: 18.2
Ans: B Section: 18.2
Chapter 18 Preparation for the Cycle
6
36.
Why does acetyl CoA, a product of fatty acid catabolism, influence the pyruvate dehydrogenase
complex (PDH), a control point in carbohydrate catabolism?
A)
When fatty acid breakdown is inhibited, ADP is low, causing a decrease in activity.
B)
When fatty acid breakdown is inhibited, PDH is inhibited by acetyl CoA due to
phosphatase activity.
C)
When fatty acid breakdown is high, ADP is low and PDH is inhibited by phosphatase
activity.
D)
When fatty acid breakdown is high, PDH is inhibited by acetyl CoA due to pyruvate
kinase activity.
E)
When fatty acid breakdown is high, NADH levels drop, causing an increase in E2 activity.
Short-Answer Questions
37.
Review the fates of pyruvate and the cellular conditions that dictate these fates.
decarboxylation to form acetyl CoA and then enters the citric acid cycle for further
oxidation.
Section: Introduction
38.
What reaction serves to link glycolysis and the citric acid cycle?
pyruvate + CoA + NAD+ → acetyl CoA + NADH + H++ CO2
Section: 18.1
39.
How does the term “mad as a hatter” realistically reflect the condition?
would react with sulfhydryl groups, such as those on the dihydrolipoyl groups. This
resulted in neurological pathologies.
Section: 18.2
40.
What are the three enzyme subunits in the PDH complex and the reactions they catalyze?
dihydrolipoyl dehydrogenase, catalyzes the regeneration of the oxidized form of
lipoamide.
Section: 18.1
41.
What coenzyme is required for the decarboxylation reaction in PDH and what is the vitamin
precursor?
Ans:
Thiamine pyrophosphate, derived from vitamin B1 (thiamine) is required for the
decarboxylation reaction in PDH.
Section: 18.1
Ans: D Section: 18.2
Chapter 18 Preparation for the Cycle
7
42.
Identify the coenzyme, and its vitamin precursor, that is responsible for the transfer of a two–
carbon group to form a high-energy thioester bond in coenzyme A.
43.
Identify the coenzyme, and its vitamin precursor, that accepts electrons to reoxidize lipoamide
in PDH.
Ans:
FAD, derived from riboflavin, is reduced to FADH2 in the reoxidaiton of lipoamide.
Section: 18.1
44.
How are the three active sites of PDH linked?
Ans:
The long flexible arm of the E2 subunit acts like a long robotic arm and carries the
substrate from active site to active site.
Section: 18.1
45.
What are the two advantages that are derived from the coordinated actions of the three enzymes
in the PDH complex?
Ans:
The proximity of one enzyme to another enzyme increases the overall reaction rate and
minimizes side reactions.
Section: 18.1
46.
If NADH levels are high, what is the fate of acetyl CoA?
Ans:
The PDH complex is inhibited by high levels of NADH, so glucose is spared and acetyl
CoA is used in the synthesis of fatty acids.
Section: 18.1
47.
Why do beriberi and mercury poisoning present with similar symptoms even though they have
different molecular causes?
depends on glucose as an energy source, nervous tissue is impacted the most and
symptoms such as pain in peripheral tissues are common.
Section: 18.2
Ans:
Lipoamide, derived from lipoic acid, transfers and acetyl group to CoA to form acetyl
CoA.
Section: 18.1