CHEM210 Biochemistry
Final Assessment Review
Module 8 (Questions & Solutions)
2025
©2025
, 1. Case Study – Urea Cycle Deficiency Diagnosis:
A pediatric patient presents with hyperammonemia, lethargy, and
vomiting. Laboratory analysis reveals elevated blood levels of citrulline.
Genetic testing indicates a defect in one of the urea cycle enzymes.
Question: Which enzyme deficiency is most commonly associated with
these findings?
A. Carbamoyl phosphate synthetase I (CPS I)
B. Ornithine transcarbamylase (OTC)
C. Argininosuccinate synthetase
D. Arginase
ANS: B. Ornithine Transcarbamylase (OTC)
Rationale: OTC deficiency is the most common urea cycle disorder,
often presenting with hyperammonemia and elevated citrulline. Defects
in OTC lead to an inability to combine carbamoyl phosphate and
ornithine, resulting in accumulation of ammonia and citrulline.
---
2. Case Study – Allosteric Regulation in the Urea Cycle:
In a liver homogenate study, the activity of carbamoyl phosphate
synthetase I (CPS I) is measured. The enzyme is found to be inactive
unless an allosteric activator is added.
Question: Which molecule is the required allosteric activator for CPS I?
A. Citrate
B. N‑acetylglutamate (NAG)
C. Arginine
D. Glutamine
ANS: B. N‑acetylglutamate (NAG)
Rationale: N‑acetylglutamate is the essential allosteric activator of CPS
I, the rate‑limiting enzyme in the urea cycle, ensuring that the cycle is
responsive to the organism’s nitrogen load.
©2025
,---
3. Case Study – ATP Cost in the Urea Cycle:
A metabolic study demonstrates that urea synthesis consumes energy.
When calculating the energetics, the formation of carbamoyl phosphate
and argininosuccinate are noted to be ATP‑dependent.
Question: How many ATP equivalents are consumed for every urea
molecule synthesized in the liver?
A. 2 ATP equivalents
B. 3 ATP equivalents
C. 4 ATP equivalents
D. 5 ATP equivalents
ANS: C. 4 ATP equivalents
Rationale: Urea synthesis requires roughly 4 ATP equivalents. Two are
used for carbamoyl phosphate formation (each costly 1 ATP, but later in
the cycle one ATP is consumed, and ATP is concentrated as AMP/PPi in
the argininosuccinate synthesis step, which uses 2 ATP-equivalents)
yielding an overall cost of approximately 4 ATP per urea produced.
---
4. Case Study – ATP Synthesis and Electron Transport:
Isolated mitochondria are used to study oxidative phosphorylation.
When the P/O ratio is measured (number of ATP molecules produced per
atom of oxygen reduced), the experimental value for NADH oxidation
approximates 2.5 ATP.
Question: What does the P/O ratio represent?
A. ATP produced per phosphate group added
B. ATP produced per NADH molecule oxidized
C. ATP produced for every proton translocated
D. ATP produced per oxygen atom reduced
ANS: B. ATP produced per NADH molecule oxidized
©2025
, Rationale: The P/O ratio reflects the efficiency of oxidative
phosphorylation. For NADH, the P/O ratio is typically about 2.5, meaning
that approximately 2.5 ATP molecules are produced per NADH oxidized.
---
5. Case Study – Enzyme Activity in the TCA Cycle:
In high-energy state conditions, a mitochondrial enzyme in the TCA cycle
shows reduced activity. The reduction is thought to be caused by
feedback inhibition from its reaction products.
Question: Which enzyme is most likely inhibited by high levels of
NADH and acetyl‑CoA?
A. Citrate synthase
B. Isocitrate dehydrogenase
C. α‑Ketoglutarate dehydrogenase
D. Succinate dehydrogenase
ANS: C. α‑Ketoglutarate dehydrogenase
Rationale: α‑Ketoglutarate dehydrogenase is notably inhibited by high
NADH and acetyl‑CoA levels, signaling an energy replete state and
slowing the TCA cycle.
---
6. Case Study – Beta‑Oxidation and ATP Production:
In a metabolic study, researchers are following the complete oxidation of
palmitate. They note that fatty acid oxidation requires activation and
sequential cycles of beta‑oxidation.
Question: Which step prior to beta‑oxidation is ATP-dependent?
A. Transport of fatty acids into the mitochondria
B. Activation of fatty acids to fatty acyl-CoA
C. Dehydrogenation of acyl-CoA
D. Thiolysis of acyl-CoA
ANS: B. Activation of fatty acids to fatty acyl-CoA
©2025
Final Assessment Review
Module 8 (Questions & Solutions)
2025
©2025
, 1. Case Study – Urea Cycle Deficiency Diagnosis:
A pediatric patient presents with hyperammonemia, lethargy, and
vomiting. Laboratory analysis reveals elevated blood levels of citrulline.
Genetic testing indicates a defect in one of the urea cycle enzymes.
Question: Which enzyme deficiency is most commonly associated with
these findings?
A. Carbamoyl phosphate synthetase I (CPS I)
B. Ornithine transcarbamylase (OTC)
C. Argininosuccinate synthetase
D. Arginase
ANS: B. Ornithine Transcarbamylase (OTC)
Rationale: OTC deficiency is the most common urea cycle disorder,
often presenting with hyperammonemia and elevated citrulline. Defects
in OTC lead to an inability to combine carbamoyl phosphate and
ornithine, resulting in accumulation of ammonia and citrulline.
---
2. Case Study – Allosteric Regulation in the Urea Cycle:
In a liver homogenate study, the activity of carbamoyl phosphate
synthetase I (CPS I) is measured. The enzyme is found to be inactive
unless an allosteric activator is added.
Question: Which molecule is the required allosteric activator for CPS I?
A. Citrate
B. N‑acetylglutamate (NAG)
C. Arginine
D. Glutamine
ANS: B. N‑acetylglutamate (NAG)
Rationale: N‑acetylglutamate is the essential allosteric activator of CPS
I, the rate‑limiting enzyme in the urea cycle, ensuring that the cycle is
responsive to the organism’s nitrogen load.
©2025
,---
3. Case Study – ATP Cost in the Urea Cycle:
A metabolic study demonstrates that urea synthesis consumes energy.
When calculating the energetics, the formation of carbamoyl phosphate
and argininosuccinate are noted to be ATP‑dependent.
Question: How many ATP equivalents are consumed for every urea
molecule synthesized in the liver?
A. 2 ATP equivalents
B. 3 ATP equivalents
C. 4 ATP equivalents
D. 5 ATP equivalents
ANS: C. 4 ATP equivalents
Rationale: Urea synthesis requires roughly 4 ATP equivalents. Two are
used for carbamoyl phosphate formation (each costly 1 ATP, but later in
the cycle one ATP is consumed, and ATP is concentrated as AMP/PPi in
the argininosuccinate synthesis step, which uses 2 ATP-equivalents)
yielding an overall cost of approximately 4 ATP per urea produced.
---
4. Case Study – ATP Synthesis and Electron Transport:
Isolated mitochondria are used to study oxidative phosphorylation.
When the P/O ratio is measured (number of ATP molecules produced per
atom of oxygen reduced), the experimental value for NADH oxidation
approximates 2.5 ATP.
Question: What does the P/O ratio represent?
A. ATP produced per phosphate group added
B. ATP produced per NADH molecule oxidized
C. ATP produced for every proton translocated
D. ATP produced per oxygen atom reduced
ANS: B. ATP produced per NADH molecule oxidized
©2025
, Rationale: The P/O ratio reflects the efficiency of oxidative
phosphorylation. For NADH, the P/O ratio is typically about 2.5, meaning
that approximately 2.5 ATP molecules are produced per NADH oxidized.
---
5. Case Study – Enzyme Activity in the TCA Cycle:
In high-energy state conditions, a mitochondrial enzyme in the TCA cycle
shows reduced activity. The reduction is thought to be caused by
feedback inhibition from its reaction products.
Question: Which enzyme is most likely inhibited by high levels of
NADH and acetyl‑CoA?
A. Citrate synthase
B. Isocitrate dehydrogenase
C. α‑Ketoglutarate dehydrogenase
D. Succinate dehydrogenase
ANS: C. α‑Ketoglutarate dehydrogenase
Rationale: α‑Ketoglutarate dehydrogenase is notably inhibited by high
NADH and acetyl‑CoA levels, signaling an energy replete state and
slowing the TCA cycle.
---
6. Case Study – Beta‑Oxidation and ATP Production:
In a metabolic study, researchers are following the complete oxidation of
palmitate. They note that fatty acid oxidation requires activation and
sequential cycles of beta‑oxidation.
Question: Which step prior to beta‑oxidation is ATP-dependent?
A. Transport of fatty acids into the mitochondria
B. Activation of fatty acids to fatty acyl-CoA
C. Dehydrogenation of acyl-CoA
D. Thiolysis of acyl-CoA
ANS: B. Activation of fatty acids to fatty acyl-CoA
©2025
