Stoichiometry of urea synthesis — What are the inputs required for one complete turn of the urea cycle (to produce one urea)?

Introduction / Context:The urea cycle has a defined stoichiometry because it converts toxic nitrogen into urea. Understanding its inputs is crucial for biochemistry, nutrition, and clinical medicine (e.g., hyperammonemia management).

Given Data / Assumptions:

• One urea molecule is produced per cycle turn.
• Two nitrogens are required: one from free ammonia, one from aspartate.
• Energy is supplied by three ATP molecules (four high-energy phosphate bonds because one ATP → AMP + PPi).

Concept / Approach:Key reactions: CPS I uses NH3, CO2, and 2 ATP to form carbamoyl phosphate. Later, argininosuccinate synthetase uses aspartate and 1 ATP (to AMP) to add the second nitrogen. Overall, one NH3, one aspartate, and one CO2 are consumed, with 3 ATP hydrolyzed (equivalent to 4 phosphoanhydride bonds), yielding one urea and one fumarate.

Step-by-Step Solution:

Verification / Alternative check:Metabolic flux analyses and textbook conventions consistently list these inputs for one urea molecule produced.

Why Other Options Are Wrong:

• Option B mixes inputs and products incorrectly.
• Option C lists products rather than the correct reactants and confuses ATP with AMP accounting.
• “None” is incorrect because Option A is accurate.

Common Pitfalls:Confusing “3 ATP” with “3 high-energy bonds”; the cycle actually consumes 4 high-energy bonds due to one ATP → AMP.

Final Answer:1 aspartic acid, 1 ammonia, 1 carbon dioxide, and 3 ATP