Module 8 Exam: Biochemistry - Amino Acid Metabolism, Nitrogen
Balance, Urea Cycle, and Vitamins 2026/2027 UPDATE
1. Which enzyme is primarily responsible for the transamination of most amino
acids?
A. Aminotransferase
B. Glutamate dehydrogenase
C. Arginase
D. Carbamoyl phosphate synthetase I
Answer: A
Rationale: Aminotransferases (or transaminases) catalyze the transfer of an amino group
from an amino acid to an alpha-keto acid.
2. Which vitamin serves as the essential cofactor for all transamination
reactions?
A. Thiamine (B1)
B. Riboflavin (B2)
C. Pyridoxal phosphate (B6)
D. Cobalamin (B12)
Answer: C
Rationale: Pyridoxal phosphate (PLP), a derivative of vitamin B6, is the required cofactor
for transamination and many other amino acid transformations.
,3. In the glucose-alanine cycle, alanine is transported from the muscle to which
organ?
A. Kidney
B. Brain
C. Liver
D. Heart
Answer: C
Rationale: Alanine travels from the muscle to the liver, where it is converted back to
pyruvate for gluconeogenesis.
4. A patient in a state of starvation would most likely exhibit which nitrogen
balance status?
A. Positive nitrogen balance
B. Zero nitrogen balance
C. Neutral nitrogen balance
D. Negative nitrogen balance
Answer: D
Rationale: Starvation leads to muscle protein breakdown for energy, resulting in more
nitrogen being excreted than consumed, which is negative nitrogen balance.
5. Which of the following amino acids is exclusively ketogenic?
A. Alanine
B. Serine
C. Glycine
D. Leucine
Answer: D
Rationale: Leucine and Lysine are the only two amino acids that are exclusively ketogenic,
meaning they are degraded into acetyl-CoA or acetoacetate.
, 6. Which molecule serves as the primary carrier of nitrogen from peripheral
tissues to the liver in a non-toxic form?
A. Ammonia
B. Glutamine
C. Urea
D. Alpha-ketoglutarate
Answer: B
Rationale: Glutamine is the major non-toxic carrier of ammonia in the blood, synthesized
from glutamate and ammonia.
7. Where in the cell does the first step of the urea cycle take place?
A. Cytosol
B. Endoplasmic reticulum
C. Mitochondria
D. Lysosome
Answer: C
Rationale: The first two reactions of the urea cycle (CPS I and Ornithine
transcarbamoylase) occur within the mitochondrial matrix.
8. The rate-limiting step of the urea cycle is catalyzed by which enzyme?
A. Arginase
B. Ornithine transcarbamoylase
C. Carbamoyl phosphate synthetase I
D. Argininosuccinate synthase
Answer: C
Rationale: Carbamoyl phosphate synthetase I (CPS I) is the regulatory and rate-limiting
enzyme of the urea cycle, requiring N-acetylglutamate for activation.
Balance, Urea Cycle, and Vitamins 2026/2027 UPDATE
1. Which enzyme is primarily responsible for the transamination of most amino
acids?
A. Aminotransferase
B. Glutamate dehydrogenase
C. Arginase
D. Carbamoyl phosphate synthetase I
Answer: A
Rationale: Aminotransferases (or transaminases) catalyze the transfer of an amino group
from an amino acid to an alpha-keto acid.
2. Which vitamin serves as the essential cofactor for all transamination
reactions?
A. Thiamine (B1)
B. Riboflavin (B2)
C. Pyridoxal phosphate (B6)
D. Cobalamin (B12)
Answer: C
Rationale: Pyridoxal phosphate (PLP), a derivative of vitamin B6, is the required cofactor
for transamination and many other amino acid transformations.
,3. In the glucose-alanine cycle, alanine is transported from the muscle to which
organ?
A. Kidney
B. Brain
C. Liver
D. Heart
Answer: C
Rationale: Alanine travels from the muscle to the liver, where it is converted back to
pyruvate for gluconeogenesis.
4. A patient in a state of starvation would most likely exhibit which nitrogen
balance status?
A. Positive nitrogen balance
B. Zero nitrogen balance
C. Neutral nitrogen balance
D. Negative nitrogen balance
Answer: D
Rationale: Starvation leads to muscle protein breakdown for energy, resulting in more
nitrogen being excreted than consumed, which is negative nitrogen balance.
5. Which of the following amino acids is exclusively ketogenic?
A. Alanine
B. Serine
C. Glycine
D. Leucine
Answer: D
Rationale: Leucine and Lysine are the only two amino acids that are exclusively ketogenic,
meaning they are degraded into acetyl-CoA or acetoacetate.
, 6. Which molecule serves as the primary carrier of nitrogen from peripheral
tissues to the liver in a non-toxic form?
A. Ammonia
B. Glutamine
C. Urea
D. Alpha-ketoglutarate
Answer: B
Rationale: Glutamine is the major non-toxic carrier of ammonia in the blood, synthesized
from glutamate and ammonia.
7. Where in the cell does the first step of the urea cycle take place?
A. Cytosol
B. Endoplasmic reticulum
C. Mitochondria
D. Lysosome
Answer: C
Rationale: The first two reactions of the urea cycle (CPS I and Ornithine
transcarbamoylase) occur within the mitochondrial matrix.
8. The rate-limiting step of the urea cycle is catalyzed by which enzyme?
A. Arginase
B. Ornithine transcarbamoylase
C. Carbamoyl phosphate synthetase I
D. Argininosuccinate synthase
Answer: C
Rationale: Carbamoyl phosphate synthetase I (CPS I) is the regulatory and rate-limiting
enzyme of the urea cycle, requiring N-acetylglutamate for activation.
