Chapter 18 Amino Acid Oxidation and the Production of Urea
Multiple Choice Questions
1. Under which circumstances are amino acids not metabolized via oxidative degradation?
A) Starvation
B) Plants growing in nutrient-rich soils
C) Normal protein turnover
D) A diet rich in proteins
E) Uncontrolled diabetes
2. Which of these is not a protease that acts in the small intestine?
A) Chymotrypsin
B) Elastase
C) Enteropeptidase
D) Secretin
E) Trypsin
3. In the digestion of protein that occurs in the small intestine, which enzyme is critical in the activation
of zymogens?
A) Enteropeptidase
B) Hexokinase
C) Papain
D) Pepsin
E) Secretin
4. Which of the following is a zymogen that can be converted to an endopeptidase that hydrolyzes
peptide bonds adjacent to Lys and Arg residues?
A) Chymotrypsinogen
B) Pepsin
C) Pepsinogen
D) Trypsin
E) Trypsinogen
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5. In amino acid catabolism, the first reaction for many amino acids is a(n):
A) decarboxylation requiring thiamine pyrophosphate (TPP).
B) hydroxylation requiring NADPH and O2.
C) oxidative deamination requiring NAD+.
D) reduction requiring pyridoxal phosphate (PLP).
E) transamination requiring pyridoxal phosphate (PLP).
6. The coenzyme required for all transaminations is derived from:
A) niacin.
B) pyridoxine (vitamin B6).
C) riboflavin.
D) thiamin.
E) vitamin B12.
7. The coenzyme involved in a transaminase reaction is:
A) biotin phosphate.
B) lipoic acid.
C) nicotinamide adenine dinucleotide phosphate (NADP+).
D) pyridoxal phosphate (PLP).
E) thiamine pyrophosphate (TPP).
8. Transamination from alanine to
-ketoglutarate requires the coenzyme:
A) biotin.
B) NADH.
C) No coenzyme is involved.
D) pyridoxal phosphate (PLP).
E) thiamine pyrophosphate (TPP).
9. Which of the following reactions involving an amino acid cannot be catalyzed via a PLP-dependent
mechanism?
A) Hydrolysis
B) Decarboxylation
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C) Racemization
D) Transamination
E) Transimination
10. Which of the following is not true of the reaction catalyzed by glutamate dehydrogenase?
A) It is similar to transamination in that it involves the coenzyme pyridoxal phosphate (PLP).
B) NH4+ is produced.
C) The enzyme can use either NAD+ or NADP+ as a cofactor.
D) The enzyme is glutamate-specific, but the reaction is involved in oxidizing other amino acids.
E)
-Ketoglutarate is produced from an amino acid.
11. Glutamate is metabolically converted to
-ketoglutarate and NH4+ by a process described as:
A) deamination.
B) hydrolysis.
C) oxidative deamination.
D) reductive deamination.
E) transamination.
12. The conversion of glutamate to an
-ketoacid and NH4+:
A) does not require any cofactors.
B) is a reductive deamination.
C) is accompanied by ATP hydrolysis catalyzed by the same enzyme.
D) is catalyzed by glutamate dehydrogenase.
E) requires ATP.
13. Urea synthesis in mammals takes place primarily in tissues of the:
A) brain.
B) kidney.
C) liver.
D) skeletal muscle.
E) small intestine.
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14. Which substance is not involved in the production of urea from NH4+ via the urea cycle?
A) Aspartate
B) ATP
C) Carbamoyl phosphate
D) Malate
E) Ornithine
15. Which of these directly donates a nitrogen atom for the formation of urea during the urea cycle?
A) Adenine
B) Aspartate
C) Creatine
D) Glutamate
E) Ornithine
16. Conversion of ornithine to citrulline is a step in the synthesis of:
A) aspartate.
B) carnitine.
C) pyruvate.
D) tyrosine.
E) urea.
17. In the urea cycle, ornithine transcarbamoylase catalyzes:
A) cleavage of urea to ammonia.
B) formation of citrulline from ornithine and another reactant.
C) formation of ornithine from citrulline and another reactant.
D) formation of urea from arginine.
E) transamination of arginine.
18. Which of the following statements is false in reference to the mammalian synthesis of urea?
A) Krebs was a major contributor to the elucidation of the pathway involved.
B) The amino acid arginine is the immediate precursor to urea.
C) The carbon atom of urea is derived from mitochondrial HCO3–.
D) The precursor to one of the nitrogens of urea is aspartate.
E) The process of urea production is an energy-yielding series of reactions.
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19. Which of the following amino acids are essential for humans?
A) Alanine
B) Aspartic acid
C) Asparagine
D) Serine
E) Threonine
20. If a person’s urine contains unusually high concentrations of urea, which one of the following diets
has he or she probably been eating recently?
A) High carbohydrate, very low protein
B) Very high carbohydrate, no protein, no fat
C) Very very high fat, high carbohydrate, no protein
D) Very high fat, very low protein
E) Very low carbohydrate, very high protein
21. Which of these amino acids can be directly converted into a citric acid cycle intermediate by
transamination?
A) Glutamic acid
B) Serine
C) Threonine
D) Tyrosine
E) Proline
22. Which of these amino acids are both ketogenic and glucogenic?
1. Isoleucine
2. Valine
3. Histidine
4. Arginine
5. Tyrosine
A) 1 and 5
B) 1, 3, and 5
C) 2 and 4
D) 2, 3, and 4
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E) 2, 4, and 5
23. Tetrahydrofolate (THF) and its derivatives shuttle between different substrates.
A) electrons
B) H+
C) acyl groups
D) one carbon units
E) NH2 groups
24. Which of the following is not a form of the most oxidized state of tetrahydrofolate (THF)?
F) N10-formyl THF
G) N5N10-methenyl THF
H) N5N10-methylene THF
I) N5-formyl THF
J) N5-formimino THF
25. The amino acids serine, alanine, and cysteine can be catabolized to yield:
A) fumarate.
B) pyruvate.
C) succinate.
D)
-ketoglutarate.
E) None of the above
26. Serine or cysteine may enter the citric acid cycle as acetyl-CoA after conversion to:
A) oxaloacetate.
B) propionate.
C) pyruvate.
D) succinate.
E) succinyl-CoA.
27. The human genetic disease phenylketonuria (PKU) can result from:
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A) deficiency of protein in the diet.
B) inability to catabolize ketone bodies.
C) inability to convert phenylalanine to tyrosine.
D) inability to synthesize phenylalanine.
E) production of enzymes containing no phenylalanine.
28. In the human genetic disease maple syrup urine disease, the metabolic defect involves:
A) a deficiency of the vitamin niacin.
B) oxidative decarboxylation.
C) synthesis of branched chain amino acids.
D) transamination of an amino acid.
E) uptake of branched chain amino acids into liver.
29. Which of these amino acids are converted to
-ketoglutarate?
1. Glycine
2. Glutamate
3. Histidine
4. Arginine
5. Proline
A) 1, 3, and 5
B) 2, 3, and 4
C) 2, 3, and 5
D) 2, 3, 4, and 5
E) 3, 4, and 5
30. Which of these amino acids are converted to succinyl CoA?
1. Isoleucine
2. Valine
3. Methionine
4. Arginine
5. Threonine
A) 2 and 4
B) 2, 3, and 4
C) 2, 4, and 5
D) 1 and 5
E) 1, 3, and 5
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31. Which of these amino acids are converted to oxaloacetate?
1. Asparagine
2. Glutamine
3. Serine
4. Arginine
5. Aspartate
A) 2 and 4
B) 2, 3, and 4
C) 2, 4, and 5
D) 1 and 5
E) 1, 3, and 5
Short Answer Questions
32. Describe (briefly) the role of (a) gastrin, (b) pepsinogen, (c) cholecystokinin, and (d) enteropeptidase
in protein digestion.
33. In the treatment of diabetes, insulin is given intravenously. Why can’t this hormone, a small protein,
be taken orally?
34. The bacterium responsible for gastric ulcers, Helicobacter pylori, survives the acidic pH of the
stomach surprisingly well. It does so in part by synthesizing and excreting large amounts of the
enzyme urease, which hydrolyzes urea to bicarbonate and ammonia. Explain how these reaction
products lead to a less acidic environment for H. pylori.
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35. Define zymogen and describe the role of one zymogen in protein digestion.
36. Transamination reactions are catalyzed by a family of enzymes, all of which require __________ as a
coenzyme. In the first step of a transamination, the coenzyme in the aldehyde form condenses with
the _________ group of an amino acid to form a(n) _________.
37. Draw the structures of reactants and products in the transamination in which glutamate and pyruvate
are the starting materials. What cofactor is required for this reaction?
38. Give the name and draw the structure of the
-keto acid resulting when the following amino acids
undergo transamination with
-ketoglutarate: (a) glutamate; (b) aspartate; (b) alanine.
39. Describe, by showing the chemical intermediates, the role of pyridoxal phosphate (PLP) in the
transamination of an amino acid.
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40. Describe the roles of glutamine synthetase and glutaminase in the metabolism of amino groups in
mammals.
41. Show the reaction in which ammonia is formed from glutamate; include any required cofactors and/or
intermediates.
42. Describe the reactions and the role of the glucose-alanine cycle.
43. Why does a mammal go to all of the trouble of making urea from ammonia rather than simply
excreting ammonia as many bacteria do?
44. Describe the three different forms of nitrogen by which different organisms dispose of excess
nitrogen obtained in the diet. Give examples of organisms that produce these different forms.
excretion of urea (land-dwelling animals).
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45. Amino acid catabolism involves the breakdown of 20 amino acids all of which contain nitrogen but
have different carbon skeletons. What overall strategy is used to deal with this problem? Illustrate
the strategy with two examples.
46. During starvation, more urea production occurs. Explain this observation.
47. Describe (a) the fundamental nutritional problem faced by individuals with genetic defects in
enzymes involved in urea formation and (b) two approaches to treatment of these diseases.
48. Explain why one might provide high concentrations of the amino acid Arg to patients who have a
deficiency in one of the first three enzymes of the urea cycle.
49. Name four amino acids that can be converted directly (in one step) into pyruvate or a citric acid cycle
intermediate, and name the intermediate formed from each.
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50. Name one amino acid whose oxidation proceeds via the intermediate shown: (a) pyruvate; (b)
oxaloacetate; (c)
-ketoglutarate; (d) succinyl-CoA; (e) fumarate.
51. Degradation of amino acids yields compounds that are common intermediates in the major metabolic
pathways. Explain the distinction between glucogenic and ketogenic amino acids in terms of their
metabolic fates.
52. There are bacteria for which alanine can serve as the chief energy source; they oxidize the carbon
skeleton of this amino acid, thereby generating ATP. Describe the first step in alanine degradation;
show any cofactors that participate.
53. If you received a laboratory report showing the presence of a high concentration of phenylalanine and
its metabolites in the urine of a patient, what disease would you suspect? What defect(s) in
metabolism account(s) for the accumulation of phenylalanine in such patients?
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54. Suppose you are responsible for formulating the diet for a 4-year-old boy with phenylketonuria. How
do you decide what kind and amount of protein to include in the diet?
55. Diagram the degradative pathway from proline to an intermediate of either glycolysis or the citric
acid cycle. Show structures of intermediates and indicate where cofactors are involved.