CHEM 210 BIOCHEMISTRY MODULE 8 EXAM 2026/2027 |
Nitrogen Metabolism, Urea Cycle & Metabolic Integration |
Latest Q&A Verified | Geneva College | Pass Guaranteed - A+
Graded
Q1: Which enzyme catalyzes the committed step in the urea cycle and is exclusively
located in the mitochondrial matrix of hepatocytes?
A. Ornithine transcarbamylase
B. Arginase
C. Carbamoyl phosphate synthetase I [CORRECT]
D. Argininosuccinate synthetase
Correct Answer: C
Rationale: Carbamoyl phosphate synthetase I (CPS I) catalyzes the rate-limiting,
committed step of the urea cycle: the ATP-dependent synthesis of carbamoyl
phosphate from bicarbonate and ammonia. It is exclusively mitochondrial and
liver-specific, requiring N-acetylglutamate (NAG) as an essential allosteric activator. CPS
I deficiency is the most severe urea cycle disorder, causing lethal neonatal
hyperammonemia. Option A (ornithine transcarbamylase) is mitochondrial but not the
committed step. Option B (arginase) is cytosolic and produces the final product urea.
Option D (argininosuccinate synthetase) is cytosolic and catalyzes step 3. The
distinction between CPS I (mitochondrial, urea cycle) and CPS II (cytosolic, pyrimidine
synthesis) is critical—CPS II uses glutamine, not ammonia, and is not activated by NAG.
Q2: A 3-day-old neonate presents with lethargy, poor feeding, and rapidly progressing
encephalopathy. Laboratory studies reveal serum ammonia of 850 μmol/L (normal <50).
,Which enzyme deficiency is most likely, given that this represents the most common
inherited urea cycle disorder?
A. Carbamoyl phosphate synthetase I deficiency
B. Ornithine transcarbamylase deficiency [CORRECT]
C. Argininosuccinate lyase deficiency
D. N-acetylglutamate synthase deficiency
Correct Answer: B
Rationale: Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder and the
most common urea cycle defect (1:30,000-80,000). Hemizygous males present with
catastrophic neonatal hyperammonemia; heterozygous females may present later or
with milder symptoms. The elevated ammonia without acidosis (distinguishing it from
organic acidemias) points to urea cycle dysfunction. OTC deficiency causes
accumulation of carbamoyl phosphate, which shunts to pyrimidine synthesis, increasing
orotic acid in urine—a diagnostic key. Option A (CPS I) is severe but less common
(autosomal recessive). Option C presents later with chronic course and argininosuccinic
aciduria. Option D is extremely rare and causes similar presentation to CPS I deficiency
but is treatable with carbamoylglutamate analogs.
Q3: In the transamination reaction catalyzed by alanine aminotransferase (ALT), which
cofactor is required for the reversible transfer of the amino group?
A. Thiamine pyrophosphate
B. Pyridoxal phosphate (PLP) [CORRECT]
C. Biotin
D. Tetrahydrofolate
Correct Answer: B
Rationale: All aminotransferases (transaminases) require pyridoxal phosphate (PLP),
the active form of vitamin B6, as an essential cofactor. PLP forms a Schiff base
(aldimine) with the ε-amino group of a conserved lysine residue in the enzyme active
site. The amino group from the amino acid substrate is transferred to PLP, forming
, pyridoxamine phosphate (PMP) and the α-keto acid. The reaction is reversible. ALT
specifically transfers the amino group from alanine to α-ketoglutarate, producing
pyruvate and glutamate. Option A (TPP) is used by decarboxylases and
dehydrogenases. Option C (biotin) is for carboxylases. Option D (THF) carries
one-carbon units.
Q4: Which intermediate of the urea cycle serves as the direct link between the urea
cycle and the citric acid cycle, allowing for metabolic integration?
A. Ornithine
B. Citrulline
C. Argininosuccinate
D. Fumarate [CORRECT]
Correct Answer: D
Rationale: Fumarate is produced when argininosuccinate lyase cleaves
argininosuccinate into arginine and fumarate (step 4 of urea cycle). Fumarate is
hydrated to malate by fumarase, then oxidized to oxaloacetate by malate
dehydrogenase—both citric acid cycle enzymes. Oxaloacetate can be transaminated to
aspartate (providing the second nitrogen of urea via argininosuccinate synthetase) or
enter gluconeogenesis. This "aspartate-argininosuccinate shunt" connects nitrogen
disposal with energy metabolism. Options A, B, and C are urea cycle-specific
intermediates without direct citric acid cycle entry. This metabolic integration allows the
liver to coordinate amino acid catabolism with glucose production during fasting.
Q5: Which amino acids are classified as purely ketogenic, meaning their carbon
skeletons can only be converted to acetyl-CoA or acetoacetyl-CoA and cannot
contribute to net glucose synthesis?
A. Leucine and lysine [CORRECT]
B. Isoleucine and phenylalanine
Nitrogen Metabolism, Urea Cycle & Metabolic Integration |
Latest Q&A Verified | Geneva College | Pass Guaranteed - A+
Graded
Q1: Which enzyme catalyzes the committed step in the urea cycle and is exclusively
located in the mitochondrial matrix of hepatocytes?
A. Ornithine transcarbamylase
B. Arginase
C. Carbamoyl phosphate synthetase I [CORRECT]
D. Argininosuccinate synthetase
Correct Answer: C
Rationale: Carbamoyl phosphate synthetase I (CPS I) catalyzes the rate-limiting,
committed step of the urea cycle: the ATP-dependent synthesis of carbamoyl
phosphate from bicarbonate and ammonia. It is exclusively mitochondrial and
liver-specific, requiring N-acetylglutamate (NAG) as an essential allosteric activator. CPS
I deficiency is the most severe urea cycle disorder, causing lethal neonatal
hyperammonemia. Option A (ornithine transcarbamylase) is mitochondrial but not the
committed step. Option B (arginase) is cytosolic and produces the final product urea.
Option D (argininosuccinate synthetase) is cytosolic and catalyzes step 3. The
distinction between CPS I (mitochondrial, urea cycle) and CPS II (cytosolic, pyrimidine
synthesis) is critical—CPS II uses glutamine, not ammonia, and is not activated by NAG.
Q2: A 3-day-old neonate presents with lethargy, poor feeding, and rapidly progressing
encephalopathy. Laboratory studies reveal serum ammonia of 850 μmol/L (normal <50).
,Which enzyme deficiency is most likely, given that this represents the most common
inherited urea cycle disorder?
A. Carbamoyl phosphate synthetase I deficiency
B. Ornithine transcarbamylase deficiency [CORRECT]
C. Argininosuccinate lyase deficiency
D. N-acetylglutamate synthase deficiency
Correct Answer: B
Rationale: Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder and the
most common urea cycle defect (1:30,000-80,000). Hemizygous males present with
catastrophic neonatal hyperammonemia; heterozygous females may present later or
with milder symptoms. The elevated ammonia without acidosis (distinguishing it from
organic acidemias) points to urea cycle dysfunction. OTC deficiency causes
accumulation of carbamoyl phosphate, which shunts to pyrimidine synthesis, increasing
orotic acid in urine—a diagnostic key. Option A (CPS I) is severe but less common
(autosomal recessive). Option C presents later with chronic course and argininosuccinic
aciduria. Option D is extremely rare and causes similar presentation to CPS I deficiency
but is treatable with carbamoylglutamate analogs.
Q3: In the transamination reaction catalyzed by alanine aminotransferase (ALT), which
cofactor is required for the reversible transfer of the amino group?
A. Thiamine pyrophosphate
B. Pyridoxal phosphate (PLP) [CORRECT]
C. Biotin
D. Tetrahydrofolate
Correct Answer: B
Rationale: All aminotransferases (transaminases) require pyridoxal phosphate (PLP),
the active form of vitamin B6, as an essential cofactor. PLP forms a Schiff base
(aldimine) with the ε-amino group of a conserved lysine residue in the enzyme active
site. The amino group from the amino acid substrate is transferred to PLP, forming
, pyridoxamine phosphate (PMP) and the α-keto acid. The reaction is reversible. ALT
specifically transfers the amino group from alanine to α-ketoglutarate, producing
pyruvate and glutamate. Option A (TPP) is used by decarboxylases and
dehydrogenases. Option C (biotin) is for carboxylases. Option D (THF) carries
one-carbon units.
Q4: Which intermediate of the urea cycle serves as the direct link between the urea
cycle and the citric acid cycle, allowing for metabolic integration?
A. Ornithine
B. Citrulline
C. Argininosuccinate
D. Fumarate [CORRECT]
Correct Answer: D
Rationale: Fumarate is produced when argininosuccinate lyase cleaves
argininosuccinate into arginine and fumarate (step 4 of urea cycle). Fumarate is
hydrated to malate by fumarase, then oxidized to oxaloacetate by malate
dehydrogenase—both citric acid cycle enzymes. Oxaloacetate can be transaminated to
aspartate (providing the second nitrogen of urea via argininosuccinate synthetase) or
enter gluconeogenesis. This "aspartate-argininosuccinate shunt" connects nitrogen
disposal with energy metabolism. Options A, B, and C are urea cycle-specific
intermediates without direct citric acid cycle entry. This metabolic integration allows the
liver to coordinate amino acid catabolism with glucose production during fasting.
Q5: Which amino acids are classified as purely ketogenic, meaning their carbon
skeletons can only be converted to acetyl-CoA or acetoacetyl-CoA and cannot
contribute to net glucose synthesis?
A. Leucine and lysine [CORRECT]
B. Isoleucine and phenylalanine
