BIOC 385 METABOLIC BIOCHEMISTRY
EXAM 3 REVIEW TEST BANK – 170
LATEST REAL EXAM QUESTIONS WITH
CORRECT DETAILED ANSWERS |
UNIVERSITY OF ARIZONA / BIOCHEM
385 EXAM 3
Amino Acid Degradation, Urea Cycle & Nitrogen Metabolism
1. Which of the following amino acids is part of the Group 2 amino acid degradation
pathway?
A. Glycine
B. Proline
C. Phenylalanine
D. Cysteine
Correct Answer: B. Proline
Rationale: Group 2 amino acids (Pro, Arg, His, Glu, Gln) are degraded to α-
ketoglutarate via glutamate. Proline is a classic example of this group .
2. Which of the following is a starting material for the synthesis of heme?
A. Phenylalanine
B. Glycine
C. Histidine
D. Alanine
Correct Answer: B. Glycine
Rationale: Heme biosynthesis begins with the condensation of glycine and succinyl-
CoA to form δ-aminolevulinate (ALA), catalyzed by ALA synthase .
3. Which chemical moieties compose tetrahydrofolate (THF)? (Select all that apply)
A. Indole
B. Serine
C. A pterin
D. Glutamate
Correct Answer: C. A pterin; D. Glutamate
Rationale: THF consists of a pterin ring, p-aminobenzoate (pABA), and a
,polyglutamate tail. The pterin and glutamate moieties are essential for its function as
a one-carbon carrier .
4. THF derivatives transfer C1 groups at different oxidation levels. In addition to
methylene, formyl, and formimino groups, which two other transferable groups are
carried by THF?
A. A carbonyl group (–C=O)
B. A phosphate group (–PO₄)
C. A methenyl group (–CH=)
D. A methyl group (–CH₃)
Correct Answer: C. A methenyl group (–CH=); D. A methyl group (–CH₃)
Rationale: THF can carry one-carbon units at the oxidation levels of methanol
(methyl, –CH₃), formaldehyde (methylene, –CH₂–; methenyl, –CH=), and formate
(formyl, –CHO; formimino, –CH=NH) .
5. Which two reactions utilize tetrahydrofolate? (Select all that apply)
A. Conversion of homocysteine to methionine
B. Conversion of pyruvate to alanine
C. Conversion of glycine to serine
D. Conversion of proline to glutamate
Correct Answer: A. Conversion of homocysteine to methionine; C. Conversion of glycine
to serine
Rationale: THF is required for the methylation of homocysteine to methionine (via
N⁵-methyl-THF) and for the conversion of glycine to serine (via glycine
hydroxymethyltransferase, which uses N⁵,N¹⁰-methylene-THF) .
6. Which amino acid in a target protein for proteasomal degradation is tagged by
ubiquitin?
A. Lysine
B. Tyrosine
C. Arginine
D. Serine
Correct Answer: A. Lysine
Rationale: Ubiquitin is covalently attached via an isopeptide bond between its C-
terminal glycine and the ε-amino group of a lysine residue on the target protein .
7. Which statement most accurately describes the complete urea cycle in a
eukaryotic cell?
A. It occurs between the inner mitochondrial membrane and the matrix.
B. It occurs completely within the cytoplasm.
C. It occurs between the mitochondrial matrix and the cytoplasm.
D. It occurs completely within the mitochondrial matrix.
Correct Answer: C. It occurs between the mitochondrial matrix and the cytoplasm.
Rationale: The urea cycle is compartmentalized: the first two steps (forming
,citrulline) occur in the mitochondrial matrix, while the remaining steps occur in the
cytosol .
8. Which two cellular changes would not encourage the Krebs bicycle (and thus the
urea cycle) to proceed to completion?
A. Increased citrulline transport into the cytoplasm
B. Increased conversion of oxaloacetate into citrate
C. Increased import of ornithine into the mitochondria
D. Increased aspartate charging to mitochondrial tRNAs
Correct Answer: B. Increased conversion of oxaloacetate into citrate; D. Increased
aspartate charging to mitochondrial tRNAs
Rationale: The Krebs bicycle requires aspartate to be exported to the cytosol for
argininosuccinate synthesis. If oxaloacetate is diverted to citrate (via citrate synthase),
aspartate production is reduced. Charging aspartate to mitochondrial tRNAs also
depletes the free aspartate pool needed for the urea cycle .
9. What is the regulatory mechanism by which arginine stimulates flux through the
urea cycle?
A. It inhibits nitrogenase.
B. It stimulates production of N-acetylglutamate, which activates arginase.
C. It stimulates carbamoyl phosphate synthetase I activity.
D. It stimulates production of the allosteric activator N-acetylglutamate.
Correct Answer: D. It stimulates production of the allosteric activator N-
acetylglutamate.
Rationale: Arginine is a positive allosteric effector that promotes the synthesis of N-
acetylglutamate (NAG). NAG is an essential allosteric activator of carbamoyl
phosphate synthetase I (CPS I), the rate-limiting enzyme of the urea cycle .
10. A person with alkaptonuria is easily diagnosed by black urine. What causes the
black color?
A. Reduction of acetoacetyl-CoA
B. Oxidation of phenylalanine
C. Accumulation of phenylpyruvate and phenylacetate
D. Oxidation of homogentisate
Correct Answer: D. Oxidation of homogentisate
Rationale: Alkaptonuria is caused by a deficiency in homogentisate 1,2-dioxygenase.
Homogentisate accumulates and oxidizes to a black pigment (alkapton) upon
exposure to air .
11. Why is ingesting large amounts of L-arginine a viable treatment for
argininosuccinase deficiency?
A. Arginine directly inhibits CPS I.
B. Arginine is converted to ornithine, providing urea cycle intermediates.
C. Arginine is a substrate for arginase.
, D. Arginine stimulates N-acetylglutamate synthesis.
Correct Answer: B. Arginine is converted to ornithine, providing urea cycle
intermediates.
Rationale: In argininosuccinase deficiency, argininosuccinate cannot be cleaved to
arginine and fumarate. Supplementing arginine provides a substrate for arginase,
which produces ornithine—a critical intermediate that helps keep the urea cycle
running and facilitates nitrogen excretion .
12. Which metabolite unique to the urea cycle connects it to the citrate cycle
through the "Krebs bicycle" pathway?
A. Malate
B. Arginine
C. Ornithine
D. Fumarate
Correct Answer: D. Fumarate
Rationale: Argininosuccinate is cleaved to arginine and fumarate. Fumarate enters
the citrate cycle, is converted to malate and then oxaloacetate, which can be
transaminated to aspartate—linking the two cycles .
13. What are the sources of the two nitrogen atoms in urea?
A. Aspartate and carbamoyl phosphate
B. Glutamate and glutamine
C. Asparagine and glutamine
D. NH₄⁺ and carbamoyl phosphate
Correct Answer: A. Aspartate and carbamoyl phosphate
Rationale: One nitrogen enters via carbamoyl phosphate (derived from ammonia),
and the second nitrogen is donated by aspartate during the synthesis of
argininosuccinate .
14. A negative nitrogen balance in a person would be an indicator of _____________.
A. A diet heavy in protein
B. Starvation
C. Overall good health
D. Accumulation of nitrogen in the body
Correct Answer: B. Starvation
Rationale: A negative nitrogen balance means nitrogen excretion exceeds intake,
which occurs when body protein is being catabolized for energy—as seen in
starvation or severe illness .
15. Arginine, leucine, and lysine are all essential amino acids. This means they are
_____________.
A. The main intermediates to the urea cycle
B. Only available from dietary sources
C. Produced from other intermediates
EXAM 3 REVIEW TEST BANK – 170
LATEST REAL EXAM QUESTIONS WITH
CORRECT DETAILED ANSWERS |
UNIVERSITY OF ARIZONA / BIOCHEM
385 EXAM 3
Amino Acid Degradation, Urea Cycle & Nitrogen Metabolism
1. Which of the following amino acids is part of the Group 2 amino acid degradation
pathway?
A. Glycine
B. Proline
C. Phenylalanine
D. Cysteine
Correct Answer: B. Proline
Rationale: Group 2 amino acids (Pro, Arg, His, Glu, Gln) are degraded to α-
ketoglutarate via glutamate. Proline is a classic example of this group .
2. Which of the following is a starting material for the synthesis of heme?
A. Phenylalanine
B. Glycine
C. Histidine
D. Alanine
Correct Answer: B. Glycine
Rationale: Heme biosynthesis begins with the condensation of glycine and succinyl-
CoA to form δ-aminolevulinate (ALA), catalyzed by ALA synthase .
3. Which chemical moieties compose tetrahydrofolate (THF)? (Select all that apply)
A. Indole
B. Serine
C. A pterin
D. Glutamate
Correct Answer: C. A pterin; D. Glutamate
Rationale: THF consists of a pterin ring, p-aminobenzoate (pABA), and a
,polyglutamate tail. The pterin and glutamate moieties are essential for its function as
a one-carbon carrier .
4. THF derivatives transfer C1 groups at different oxidation levels. In addition to
methylene, formyl, and formimino groups, which two other transferable groups are
carried by THF?
A. A carbonyl group (–C=O)
B. A phosphate group (–PO₄)
C. A methenyl group (–CH=)
D. A methyl group (–CH₃)
Correct Answer: C. A methenyl group (–CH=); D. A methyl group (–CH₃)
Rationale: THF can carry one-carbon units at the oxidation levels of methanol
(methyl, –CH₃), formaldehyde (methylene, –CH₂–; methenyl, –CH=), and formate
(formyl, –CHO; formimino, –CH=NH) .
5. Which two reactions utilize tetrahydrofolate? (Select all that apply)
A. Conversion of homocysteine to methionine
B. Conversion of pyruvate to alanine
C. Conversion of glycine to serine
D. Conversion of proline to glutamate
Correct Answer: A. Conversion of homocysteine to methionine; C. Conversion of glycine
to serine
Rationale: THF is required for the methylation of homocysteine to methionine (via
N⁵-methyl-THF) and for the conversion of glycine to serine (via glycine
hydroxymethyltransferase, which uses N⁵,N¹⁰-methylene-THF) .
6. Which amino acid in a target protein for proteasomal degradation is tagged by
ubiquitin?
A. Lysine
B. Tyrosine
C. Arginine
D. Serine
Correct Answer: A. Lysine
Rationale: Ubiquitin is covalently attached via an isopeptide bond between its C-
terminal glycine and the ε-amino group of a lysine residue on the target protein .
7. Which statement most accurately describes the complete urea cycle in a
eukaryotic cell?
A. It occurs between the inner mitochondrial membrane and the matrix.
B. It occurs completely within the cytoplasm.
C. It occurs between the mitochondrial matrix and the cytoplasm.
D. It occurs completely within the mitochondrial matrix.
Correct Answer: C. It occurs between the mitochondrial matrix and the cytoplasm.
Rationale: The urea cycle is compartmentalized: the first two steps (forming
,citrulline) occur in the mitochondrial matrix, while the remaining steps occur in the
cytosol .
8. Which two cellular changes would not encourage the Krebs bicycle (and thus the
urea cycle) to proceed to completion?
A. Increased citrulline transport into the cytoplasm
B. Increased conversion of oxaloacetate into citrate
C. Increased import of ornithine into the mitochondria
D. Increased aspartate charging to mitochondrial tRNAs
Correct Answer: B. Increased conversion of oxaloacetate into citrate; D. Increased
aspartate charging to mitochondrial tRNAs
Rationale: The Krebs bicycle requires aspartate to be exported to the cytosol for
argininosuccinate synthesis. If oxaloacetate is diverted to citrate (via citrate synthase),
aspartate production is reduced. Charging aspartate to mitochondrial tRNAs also
depletes the free aspartate pool needed for the urea cycle .
9. What is the regulatory mechanism by which arginine stimulates flux through the
urea cycle?
A. It inhibits nitrogenase.
B. It stimulates production of N-acetylglutamate, which activates arginase.
C. It stimulates carbamoyl phosphate synthetase I activity.
D. It stimulates production of the allosteric activator N-acetylglutamate.
Correct Answer: D. It stimulates production of the allosteric activator N-
acetylglutamate.
Rationale: Arginine is a positive allosteric effector that promotes the synthesis of N-
acetylglutamate (NAG). NAG is an essential allosteric activator of carbamoyl
phosphate synthetase I (CPS I), the rate-limiting enzyme of the urea cycle .
10. A person with alkaptonuria is easily diagnosed by black urine. What causes the
black color?
A. Reduction of acetoacetyl-CoA
B. Oxidation of phenylalanine
C. Accumulation of phenylpyruvate and phenylacetate
D. Oxidation of homogentisate
Correct Answer: D. Oxidation of homogentisate
Rationale: Alkaptonuria is caused by a deficiency in homogentisate 1,2-dioxygenase.
Homogentisate accumulates and oxidizes to a black pigment (alkapton) upon
exposure to air .
11. Why is ingesting large amounts of L-arginine a viable treatment for
argininosuccinase deficiency?
A. Arginine directly inhibits CPS I.
B. Arginine is converted to ornithine, providing urea cycle intermediates.
C. Arginine is a substrate for arginase.
, D. Arginine stimulates N-acetylglutamate synthesis.
Correct Answer: B. Arginine is converted to ornithine, providing urea cycle
intermediates.
Rationale: In argininosuccinase deficiency, argininosuccinate cannot be cleaved to
arginine and fumarate. Supplementing arginine provides a substrate for arginase,
which produces ornithine—a critical intermediate that helps keep the urea cycle
running and facilitates nitrogen excretion .
12. Which metabolite unique to the urea cycle connects it to the citrate cycle
through the "Krebs bicycle" pathway?
A. Malate
B. Arginine
C. Ornithine
D. Fumarate
Correct Answer: D. Fumarate
Rationale: Argininosuccinate is cleaved to arginine and fumarate. Fumarate enters
the citrate cycle, is converted to malate and then oxaloacetate, which can be
transaminated to aspartate—linking the two cycles .
13. What are the sources of the two nitrogen atoms in urea?
A. Aspartate and carbamoyl phosphate
B. Glutamate and glutamine
C. Asparagine and glutamine
D. NH₄⁺ and carbamoyl phosphate
Correct Answer: A. Aspartate and carbamoyl phosphate
Rationale: One nitrogen enters via carbamoyl phosphate (derived from ammonia),
and the second nitrogen is donated by aspartate during the synthesis of
argininosuccinate .
14. A negative nitrogen balance in a person would be an indicator of _____________.
A. A diet heavy in protein
B. Starvation
C. Overall good health
D. Accumulation of nitrogen in the body
Correct Answer: B. Starvation
Rationale: A negative nitrogen balance means nitrogen excretion exceeds intake,
which occurs when body protein is being catabolized for energy—as seen in
starvation or severe illness .
15. Arginine, leucine, and lysine are all essential amino acids. This means they are
_____________.
A. The main intermediates to the urea cycle
B. Only available from dietary sources
C. Produced from other intermediates
