Chapter 26—The Synthesis and Degradation of Nucleotides
MULTIPLE CHOICE
1. All of the following statements about nucleotides are true EXCEPT:
a.
They are produced by nearly all organisms.
b.
The rate of proliferation of a cell can determine the amount of DNA and RNA synthesized
per unit of time.
c.
Uridine nucleotide derivatives of carbohydrates are common intermediates in cellular
transformation of carbohydrates.
d.
Many coenzymes are derivatives of nucleotides.
e.
The nitrogenous bases serve as energy sources.
2. Many ____ and ____ are inhibitors of purine and pyrimidine biosynthesis.
a.
eicosanoids; aspirin
b.
NSAIDs; antibiotics
c.
antibiotics; eicosanoids
d.
anticancer drugs; antibiotics
e.
anticancer drugs; eicosanoids
3. What is the limiting substance in the biosynthesis of purines?
a.
ribose-5-phosphate
b.
5-phosphoribosyl- amine
c.
formylglycinamidine ribonucleotide
d.
5-phosphoribosyl--pyrophosphate (PRPP)
e.
-D-ribose-5-phosphate
4. Glutamine phosphoribosyl pyrophosphate amidotransferase catalyzes step 2 in the biosynthesis of
purines. Which of the following is a true statement regarding the regulation of this enzyme?
a.
It is accelerated by an input of ATP or GTP.
b.
It is sensitive to activation by azaserine.
c.
It is accelerated by an input of AMP or GMP.
d.
The G series of nucleotides as well as the adenine nucleotides act as competitive
inhibitors.
e.
It is subject to feed-forward activation by ADP.
5. In purine biosynthesis when glycine adds to 5-phosphoribosyl-–amine, the bond formed is a(n):
a.
ester.
b.
imine.
c.
amine.
d.
amide.
e.
lactam.
6. The coenzyme for two single carbon additions in purine biosynthesis is:
a.
THF.
b.
biotin.
c.
SAM.
d.
TPP.
e.
none are true.
7. Adenylosuccinase (adenylosuccinate lyase) catalyzes the reaction to remove ____ after ____ has
formed an amide with a carbonyl group in purine biosynthesis.
a.
succinate; glutamate
b.
succinate; aspartate
c.
fumarate; alanine
d.
fumarate; aspartate
e.
-ketoglutarate; glutamate
8. All of the following are true concerning the regulation of the purine biosynthetic pathway EXCEPT:
a.
ATP is the energy source for AMP synthesis.
b.
ADP and GDP are feedback inhibitors of ribose-5-phosphate pyrophosphokinase.
c.
“A” and “C” nucleotides allosterically inhibit glutamine phosphoribosyl pyrophosphate
amidotransferase (Gln:PRPP amidotransferase).
d.
The rate of IMP formation is dependent on the levels of the adenine and guanine
nucleotides produced.
e.
PRPP competitively inhibits the enzyme Gln:PRPP amidotransferase.
9. The fate of IMP is regulated by relative levels of ____ and ____; and energy to drive AMP synthesis is
provided by ____, and energy for GMP synthesis by ____.
a.
IMP; PRPP; ATP; GTP
b.
PRPP; ATP; ATP; GTP
c.
AMP; ATP; GTP; ATP
d.
AMP; GMP; GTP; ATP
e.
IMP; GMP; ATP; GTP
10. The reaction, base + PRPP → nucleoside-5-phosphate + PPi is catalyzed by the enzyme:
a.
nucleotide diphosphate kinase.
b.
phosphoribosyltransferase.
c.
ribose-5-phosphate pyrophosphokinase.
d.
adenylate kinase.
e.
GMP synthetase.
11. Which of the following statements explains why methotrexate is effective as a cancer
chemotherapeutic?
a.
it serves as an analog of p-aminobenzoic acid and block bacterial synthesis of folate
b.
acts as an inhibitor of dihydrofolate reductase, thus preventing production of
tetrahydrofolate
c.
it acts as a suicide substrates for the enzyme dihydropteroate synthetase
d.
prevents the conversion of N5,N10–methenyl THF to N10-formyl THF which is required in
2 steps of the purine synthesis pathway
e.
both b and d are correct
12. What is the function of phosphoribosyltransferases?
a.
they are involved in the reversible reactions of purine salvage.
b.
they mediate the degradation of AMP.
c.
they are central to the re-synthesis of nucleotides from bases in salvage reactions.
d.
they mediate the degradation of IMP.
e.
they are competitive inhibitors employed in the salvage of purine nucleotides.
13. All are characteristics of Lesch-Nyham Syndrome EXCEPT:
a.
gouty arthritis due to excessive uric acid accumulation
b.
severe malfunctions of the nervous system
c.
complete deficiency in HGPRT activity
d.
congenital, recessive, sex-linked trait manifested only in males
e.
all are true
14. About 30% of severe combined immunodeficiency syndrome (SCIDS) patients suffer from a
deficiency in the activity of the enzyme ____ that is a ____-dependent enzyme and ____ deficiency
can also lead to reduced immune function.
a.
HGPRT; Ca2+; Ca2+
b.
nucleosidase; Ca2+; Ca2+
c.
nucleoside phosphorylase; Pi; Pi
d.
adenosine deaminase; Zn2+; Zn2+
e.
none are true
15. What is the common product of purine catabolism?
a.
xanthine
b.
uric acid
c.
inosine
d.
hypoxanthine
e.
xanthosine
16. What is the importance of the purine nucleoside cycle?
a.
It participates in the conversion of purines into pyrimidines.
b.
It generates fumarate, which inhibits the formation of AMP from IMP.
c.
It provides fumarate as an anaplerotic enhancement of the citric acid cycle.
d.
It inhibits the citric acid cycle in skeletal muscles.
e.
It is used in energy metabolism in the liver.
17. All are characteristics of xanthine oxidase EXCEPT:
a.
present in large amounts in liver, intestinal mucosa and milk.
b.
uses molecular oxygen and produces H2O2.
c.
involved in production of urea.
d.
possesses an electron-transport chain with FAD, Fe−S cluster and molybdenum cofactor.
e.
all are true.
18. Allopurinol is an analog of ____ that tightly binds to ____ and prevents ____ formation.
a.
hypoxanthine; xanthine oxidase; uric acid
b.
guanine; guanine deaminase; xanthine
c.
inosine; xanthine oxidase; xanthine
d.
hypoxanthine; guanine deaminase; xanthine
e.
none of the above
19. Which of the following statements explains why allopurinol is able to prevent gout?
a.
it binds to uric acid thus preventing the formation of uric acid crystals
b.
it acts as a diuretic to increase the elimination of uric acid
c.
it inhibits the production of uric acid and allows for the more highly soluble xanthine and
hypoxanthine to be excreted from the body
d.
it stimulates the purine salvage pathway, thus eliminating the need for production of uric
acid
e.
none of the above
20. All are substrates of carbamoyl phosphate synthetase II (CPS-II) EXCEPT:
a.
H2O.
b.
NH4+.
c.
HCO3−.
d.
glutamine.
e.
2ATP.
21. In mammals CPS-II is the regulated step on pyrimidine biosynthesis; however, in bacteria ____ is the
regulated step.
a.
dihydroorotase
b.
aspartate transcarbamoylase (ATCase)
c.
OMP decarboxylase
d.
dehydroorotate dehydrogenase
e.
none are true
Garrett/Grisham 5e Test Bank 5
22. Which of the following serves as an activator of CPS-II in mammals?
a.
thyrotropin
b.
epidermal growth factor
c.
cortisol
d.
gonadotropin
e.
none of the above
23. What type of reaction gives UMP from OMP?
a.
reduction
b.
decarboxylation
c.
oxidation
d.
condensation
e.
hydrolysis
24. Which of the following is an advantage of metabolic channeling?
a.
It allows the product of one reaction to be available for a reaction later in another pathway.
b.
It allows the diffusion of substrates.
c.
It allows the dilution of substrates.
d.
It allows a more diverse interaction of substrates, intermediates, and enzymes.
e.
It does not allow for substrate dilution into the milieu and for the accumulation of
intermediates.
25. Which of the following is a true statement regarding pyrimidine biosynthesis in animals?
a.
PRPP and ATP are allosteric inhibitors.
b.
CTP is a feedback inhibitor.
c.
It is allosterically regulated at aspartate transcarbomoylase.
d.
UDP and UTP are feedback inhibitors of CPS-II.
e.
ACTase catalyzes the committed step in pyrimidine synthesis.
26. Enzymatic ribonucleotide reduction is accomplished via a free radical mechanism. Which of the
following statements is true regarding this process?
a.
The substrate specificity site on the R1 homodimer determines if the enzyme is active or
inactive.
b.
R2 has a single active site containing two Fe atoms that generate the free radicals required
for reduction.
c.
R2, but not R1, function in the delivery of reducing equivalents.
d.
R2 homodimer carries two regulatory sites.
e.
A final decarboxylation forms the deoxyribonucleotide product.
27. The immediate reducing power of ribonucleotide reductase is provided by:
a.
thioredoxin.
b.
Ferredoxin.
c.
NADH.
d.
Fe−S complex.
e.
Cyt P-450.
28. Which of the following statements regarding the activity of ribonucleotide reductase is correct?
a.
the overall activity of the enzyme is inhibited by high levels of ATP
b.
binding of ATP to the specificity site increases the use of GDP as a substrate
c.
binding of dGTP to the specificity site increases the use of CDP as a substrate
d.
binding of dTTP to the specificity site inhibits the use of UDP as a substrate
e.
none of the above
29. Thymine nucleotides are not synthesized directly by cells but indirectly from other pyrimidine
deoxyribonucleotides. All of the following statements regarding thymine nucleotide synthesis are true
EXCEPT:
a.
dUMP is the immediate precursor for dTMP.
b.
The action of dUTPase allows dUTP to serve as a substrate in DNA synthesis.
c.
Thymidylate synthase catalyzes the formation of dTMP from dUMP.
d.
The methyl donor in the reaction of dTMP is N5, N10-methylene-THF.
e.
THF is used as a cofactor.
30. Unlike hydrogen, which is often abstracted from substrates as H+, electronegative ____ cannot be
readily eliminated as ____, thus enzyme inhibitors can be fashioned in which ____ replaces H at
positions where catalysis involves H-removal as H+.
a.
sodium; Na+; Na
b.
potassium; K+; K
c.
deuterium; D+; D
d.
fluorine; F+; F
e.
all are true
31. Thymidylate synthase synthesizes dTMP from ____ by ____ utilizing the coenzyme ____.
a.
dCTP; carboxylation; biotin
b.
dUMP; methylation; THF
c.
dCMP; methylation; THF
d.
dGMP; phosphorylation; ATP
e.
dUMP; phosphorylation; ATP
32. Which of the following is the mechanism of action of 5-fluorouracil?
a.
competitive inhibitor of dihydrofolate reductase
b.
suicide substrate for thymidylate synthase
c.
competitive inhibitor of ribonucleotide reductase
d.
non-competitive inhibitor of thioredoxin reductase
e.
none of the above
33. Which of the following is a source of 2 carbon atoms of the purine ring system?
a.
aspartate
b.
carbon dioxide
c.
glycine
d.
glutamine
e.
glutamic acid
34. Which of the following serves as one of the nitrogen sources for the pyrimidine ring?
a.
glutamate
b.
carbamoyl phosphate
c.
asparagine
d.
glycine
e.
none of the above
35. Which of the following would cause an increase in the conversion of IMP to GMP?
a.
high levels of CMP
b.
high levels of dTTP
c.
high levels of UMP
d.
high levels of AMP
e.
none of the above
36. Which form of the folate coenzyme is directly involved in sythesis of the purine ring?
a.
N5,N10-methenyl THF
b.
N10-formyl THF
c.
N5-formyl THF
d.
tetrahydrofolate
e.
dihydrofolate