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Chapter 22 Biosynthesis of Amino Acids, Nucleotides,
and Related Molecules
Multiple Choice Questions
1. Which of the following statements about the fixation of atmospheric nitrogen (N2) into NH3 by living
cells is false?
A) It involves the transfer of eight electrons per mol of N2.
B) It occurs in certain microorganisms, but not in humans.
C) It requires a source of electrons, normally ferredoxin.
D) It requires one ATP per mol of N2 fixed.
E) It requires two key protein components, each containing iron.
2. Which of the following is not true about the anammox reaction?
A) Anammox converts ammonia to nitrogen.
B) Anammox is performed by symbiotic bacteria of leguminous plants.
C) Anammox generates the highly reactive molecule hydrazine that is a component of rocket fuel.
D) Anammox occursw anaerobically.
E) The ultimate electron acceptor in anammox is nitrite.
3. Which of the following enzymes is not involved in the assimilation of inorganic nitrogen into an
organic molecule?
A) Dinitrogenase reductase
B) Nitrate reductase
C) Nitrite reductase
D) Nitrile reductase
E) Dinitrogenase
4. Which of the following enzymes is not involved in the assimilation of inorganic nitrogen into an
organic molecule?
A) Arginase
B) Glutamate dehydrogenase
C) Glutamate synthase
D) Glutamine synthetase
E) Dinitrogenase
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260
5. The enzymatic machinery to fix atmospheric N2 into NH4+ is:
A) a means of producing ATP when excess N2 is available.
B) composed of two key proteins, each containing iron.
C) relatively stable when exposed to O2.
D) specific to plant cells.
E) unaffected by the supply of electrons.
6. Glutamine synthetase converts _____ to _____ whereas glutamate synthase converts ____ to _____.
A) formate; glutamine; ammonia; glutamate
B) asparagine; glutamine;
-ketoglutarate; glutamate
C)
-ketoglutarate; glutamine; oxaloacetic acid; glutamate
D)
-ketoglutarate; glutamine;
-ketoglutarate; glutamate
E) glutamate; glutamine;
-ketoglutarate; glutamate
7. Which of the following is not true about glutamine amidotransferases?
A) These enzymes proceed via a covalent intermediate.
B) Ammonia is released into a channel.
C) Glutamate is a product of the reaction.
D) ATP is needed to activate the glutamine.
E) A Cys at the active site is critical for the activity of these enzymes.
8. Nonessential amino acids:
A) are amino acids other than those required for protein synthesis.
B) are not utilized in mammalian proteins.
C) are synthesized by plants and bacteria, but not by humans.
D) can be synthesized in humans as well as in bacteria.
E) may be substituted with other amino acids in proteins.
9. An amino acid that does not derive its carbon skeleton, at least in part, from
-ketoglutarate is:
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261
A) arginine.
B) glutamate.
C) glutamine.
D) proline.
E) threonine.
10. Glutamine, arginine, and proline:
A) do not have a common precursor.
B) may all be derived from a citric acid cycle intermediate.
C) may all be derived from a Cori cycle intermediate.
D) may all be derived from a glycolytic intermediate.
E) may all be derived from a urea cycle intermediate.
11. In which group are all the amino acids closely interrelated metabolically?
A) Arginine, hydroxyproline, and histidine
B) Arginine, tyrosine, and glutamate
C) Glycine, valine, glutamine, and aspartate
D) Ornithine, alanine, glycine, and valine
E) Ornithine, proline, arginine, and glutamate
12. If glucose labeled with 14C at C-1 were the starting material for amino acid biosynthesis, the
product(s) that would be readily formed is (are):
A) serine labeled at the carboxyl carbon.
B) serine labeled at alpha carbon.
C) serine labeled at the R-group carbon.
D) All of the above
E) None of the above
13. An amino acid that does not derive its carbon skeleton, at least in part, from oxaloacetate is:
A) aspartate.
B) lysine.
C) methionine.
D) proline.
E) threonine.
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14. Homoserine is:
A) a precursor of both methionine and threonine.
B) a precursor of serine.
C) derived from homocysteine.
D) derived from serine.
E) derived from threonine.
15. If a cell were unable to synthesize or obtain tetrahydrofolic acid (H4 folate), it would probably be
deficient in the biosynthesis of:
A) isoleucine.
B) leucine.
C) lysine.
D) methionine.
E) serine.
16. The nitrogen atom in the side chain of lysine is derived from which amino acid?
A) Aspartic acid
B) Glutamic acid
C) Glutamine
D) Asparagine
E) Arginine
17. The nitrogen atom in the indole ring of tryptophan is derived from which amino acid?
A) Aspartic acid
B) Glutamic acid
C) Glutamine
D) Asparagine
E) Arginine
18. Erythrose 4-phosphate is a precursor of:
A) aspartate.
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263
B) cysteine.
C) phenylalanine.
D) serine.
E) threonine.
19. An important intermediate in the biosynthetic pathway to aromatic amino acids is:
A) benzoic acid.
B) lactate.
C) orotate.
D) shikimate.
E)
-ketoglutarate.
20. Which of the following amino acids derives its nitrogen from a purine ring?
A) Histidine
B) Lysine
C) Arginine
D) Glutamine
) Tryptophan
21. The amino acid __________ and is an intermediate in the biosynthesis of _________.
A) histidine; purines
B) glycine; heme
C) serine; heme
D) serine; sphingosine
E) glutamine; glutathione
22. Bile pigments are:
A) formed in the degradation of heme.
B) generated by oxidation of sterols.
C) responsible for light reception in the vertebrate eye.
D) secreted from the pancreas
E) the products of purine degradation.
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23. Glutathione is a(n):
A) enzyme essential in the synthesis of glutamate.
B) isomer of oxidized glutamic acid.
C) methyl-group donor in many biosynthetic pathways.
D) product of glutamate and methionine.
E) tripeptide of glycine, glutamate, and cysteine.
24. The plant hormone indole-3-acetate (auxin) is formed from:
A) arginine.
B) histidine.
C) phenylalanine.
D) threonine.
E) tryptophan.
25. L-Dopa is an intermediate in the conversion of:
A) phenylalanine to homogentisic acid.
B) phenylalanine to tyrosine.
C) tyrosine to epinephrine.
D) tyrosine to phenylalanine.
E) tyrosine to phenylpyruvate.
26. The amino acid that gives rise to the biological messenger NO is:
A) glutamine.
B) arginine.
C) proline.
D) lysine.
E) histidine.
27. The hormones epinephrine and norepinephrine are derived biosynthetically from:
A) arginine.
B) histidine.
C) isoleucine.
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265
D) tryptophan.
E) tyrosine.
28. One amino acid directly involved in the purine biosynthetic pathway is:
A) alanine.
B) aspartate.
C) glutamate.
D) leucine.
E) tryptophan
29. 5-Phosphoribosyl-
-pyrophosphate (PRPP) is a synthetic precursor for all of the following except:
A) AMP.
B) arginine.
C) histidine.
D) tryptophan.
E) UMP.
30. Glutamine is a nitrogen donor in the synthesis of:
A) CTP.
B) dTTP.
C) inosinic acid (IMP).
D) orotate.
E) UMP.
31. De novo purine biosynthesis is distinguished from de novo pyrimidine biosynthesis by:
A) condensation of the completed purine ring with ribose phosphate
B) incorporation of CO2.
C) inhibition by azaserine (a glutamine analog).
D) participation of aspartate.
E) participation of PRPP (phosphoribosyl pyrophosphate).
32. The ribosyl phosphate moiety needed for the synthesis of orotidylate, inosinate, and guanylate is
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266
provided most directly by:
A) 5-phosphoribosyl 1-pyrophosphate.
B) adenosine 5'-phosphate.
C) guanosine 5'-phosphate.
D) ribose 5-phosphate.
E) ribulose 5-phosphate.
33. The synthesis of purine and pyrimidine nucleotides differ in that:
A) ATP is required in the synthesis of purines but not in the synthesis of pyrimidines.
B) purine biosynthesis starts with the formation of PRPP, whereas pyrimidines incorporate the
PRPP near the end of the pathway.
C) purine formation requires a THF derivative, whereas pyrimidine formation does not.
D) pyrimidine biosynthesis is tightly regulated in the cell, whereas purine biosynthesis is not.
E) pyrimidines go through many steps, adding a single carbon or nitrogen each time, whereas the
basic skeleton for purines is formed by two main precursors.
34. Which one of the following statements is true of the biosynthetic pathway for purine nucleotides?
A) CO2 does not participate in any of the steps in this pathway.
B) Deoxyribonucleotides are formed from 5-phosphodeoxyribosyl 1-pyrophosphate.
C) Inosinate is the purine nucleotide that is the precursor of both adenylate and guanylate.
D) Orotic acid is an essential precursor for purine nucleotides.
E) The amino acid valine is one of the precursors contributing to purine nucleotides.
35. Orotic aciduria is an inherited metabolic disease in which orotic acid (orotate) accumulates in the
tissues, blood, and urine. The metabolic pathway in which the enzyme defect occurs is:
A) epinephrine synthesis.
B) purine breakdown.
C) purine synthesis.
D) pyrimidine breakdown.
E) pyrimidine synthesis.
36. Precursors for the biosynthesis of the pyrimidine ring system include:
A) carbamoyl phosphate and aspartate.
B) glutamate, NH3, and CO2.
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C) glycine and succinyl-CoA.
D) glycine, glutamine, CO2, and aspartate.
E) inosine and aspartate.
37. The most direct precursors of the nitrogens of UMP are:
A) aspartate and carbamoyl phosphate.
B) glutamate and aspartate.
C) glutamate and carbamoyl phosphate.
D) glutamine and aspartate.
E) glutamine and carbamoyl phosphate.
38. CMP, UMP, and TMP all have ________________ as a common precursor.
A) adenosine
B) aspartate
C) glutamine
D) inosine
E) S-adenosyl methionine
39. Which of the following is not true of the reaction catalyzed by ribonucleotide reductase?
A) Glutathione is part of the path of electron transfer.
B) It acts on nucleoside diphosphates.
C) Its mechanism involves formation of a free radical.
D) There is a separate enzyme for each nucleotide (ADP, CDP, GDP, UDP).
E) Thioredoxin acts as an essential electron carrier.
40. Which one of the following statements correctly describes the biosynthetic pathway for purine
nucleotides?
A) Purine deoxynucleotides are made by the same path as ribonucleotides, followed by reduction of
the ribose moiety.
B) The first enzyme in the path is aspartate transcarbamoylase (ATCase).
C) The nitrogen in the purine base that is bonded to ribose in the nucleotide is derived originally
from glycine.
D) The pathway occurs only in plants and bacteria, not in animals.
E) The purine rings are first synthesized, then condensed with ribose phosphate.
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41. A cell that is unable to synthesize or obtain tetrahydrofolic acid (H4 folate) would probably be
deficient in the biosynthesis of:
A) CMP.
B) GMP.
C) orotate.
D) thymidylate (TMP).
E) UMP.
42. An intermediate of purine degradation in humans is:
A) glutamate.
B) NH4+.
C) succinate.
D) urea.
E) uric acid.
43. Which of the following statements is true about gout?
A) Gout is caused by excessive breakdown of pyrimidine nucleotides.
B) Gout can be treated by administration of fluorouracil.
C) Gout is particularly prevalent in patients lacking xanthine oxidase.
D) Gout leads to deposition of sodium urate crystals in the joints.
E) Eating more liver is an effective way to treat the symptoms of gout.
44. Which of the following is not a chemotherapeutic drug?
A) Azaserine
B) Allopurinol
C) Fluorouracil
D) Methotrexate
E) Acivicin
Short Answer Questions
45. Trace the path of nitrogen from atmospheric N2 into glutamate. Name the intermediates (no
Chapter 22 Biosynthesis of Amino Acids, Nucleotides, and Related Molecules
269
structures necessary) and enzymes, and show any coenzymes involved.
46. Give the overall reaction that results from the combined action of glutamate synthase and glutamine
synthetase.
47. Give the equations for the two-step reaction sequence catalyzed by glutamine synthetase.
Ans:
48. Describe two types of regulation of the enzyme glutamine synthetase and explain why the regulation
of this enzyme is so complex.
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49. Why is it necessary to have protein in our (human) diets?
50. Give the name and structure of the glycolytic or citric acid cycle intermediate that has the same
carbon skeleton as (a) alanine, (b) glutamate, (c) aspartate.
51. Show the biosynthetic pathway for the conversion of a citric acid cycle intermediate into proline.
Indicate where any cofactors participate.
52. Show the reaction catalyzed by glycine synthase, indicating the role of any cofactors that participate.
53. An animal cell is capable of converting alanine into serine. What is the shortest pathway using
known enzymes by which this conversion could be accomplished? Show intermediates and cofactors;
no enzyme names are required. (Hint: The first step is removal of the nitrogen by transamination.)
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271
54. Show the steps by which an intermediate of glycolysis can be converted into serine.
55. Show the two-step reaction catalyzed by tryptophan synthetase.
56. In bacteria, the amino acids listed below can be derived directly or indirectly from serine, alanine,
aspartate, glutamate, or chorismate. Indicate below which of these “parent” compounds provides the
carbon skeleton for each amino acid:
Parent compound
Asparagine __________________
Tryptophan __________________
Glycine __________________
Methionine __________________
Threonine __________________
Cysteine __________________
Proline __________________
Isoleucine __________________
Phenylalanine __________________
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272
57. Describe and contrast, with diagrams, concerted (cumulative) feedback regulation and sequential
feedback inhibition.
58. Show how you can form
-aminolevulinate from either glycine and succinyl-CoA or from glutamate.
59. Match the signaling molecule with its amino acid precursor (a given precursor may be used more than
once or not at all):
Signal Molecule Amino Acid Precursor
(a) auxin (1) histidine
(b) epinephrine (2) glutamic acid
(c)
-amino butyrate (3) tyrosine
(d) histamine (4) tryptophan
(e) serotonin (5) arginine
(f) NO
60. Draw the structure of 5'-IMP. Indicate with arrows those carbon atoms donated by derivatives of
Chapter 22 Biosynthesis of Amino Acids, Nucleotides, and Related Molecules
273
tetrahydrofolate and circle the atoms derived from glycine.
61. Draw the structure of 5'-GMP. Indicate with an arrow the atom(s) derived from glutamine’s amide
group(s).
62. Draw the structure of 5'AMP. What is the first “committed” step in the biosynthetic sequence that
leads to 5'AMP? How is this step regulated?
63. Draw the structure of 5'-UMP (uridylic acid). Circle those carbon atoms donated by atoms derived
from aspartate.
64. Draw the structure of deoxythymidylic acid (dTMP). Indicate the source of each N and each C in the
thymine ring, including its substituents.
65. Describe the pathway by which GMP is converted into GTP; show the co-substrates that are involved
and name the enzymes.
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66. Diagram the biosynthetic pathway from UMP to dTTP. Use abbreviations (e.g., UMP), not complete
structures, and indicate where any co-substrates participate.
67. Show the reaction catalyzed by thymidylate synthase and explain with a simple diagram how the
chemotherapeutic agents fluorouracil and methotrexate inhibit the synthesis of dTMP.
68. Explain how lack of adenosine deaminase leads to a 100-fold increase in dATP, yet also causes a
deficiency in dNTPs required for replication of T-cells and B-cells required for a functioning immune
system.
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69. Azaserine is a structural analog of glutamine. It is a competitive inhibitor of many enzymes that use
glutamine as substrates. Name) three biosynthetic products whose synthesis you would expect to be
inhibited by azaserine. Do you think that eating azaserine would be immediately fatal? Why or why
not?
