🧬 The Urea Cycle – Nitrogen Removal & Key Enzymes
The urea cycle is a biochemistry favorite on the DAT. It’s the body’s main way to safely eliminate toxic nitrogen—a byproduct of amino acid metabolism. This pathway involves both mitochondrial and cytosolic steps, and it connects to several areas of clinical importance (like hyperammonemia).
Today’s breakdown includes:
Urea cycle purpose & overview
Step-by-step enzymes
ATP cost and transport
DAT-style questions
🔬 What Is the Urea Cycle?
The urea cycle, also called the ornithine cycle, converts ammonia (NH₃) into urea, which is excreted via the urine. It prevents ammonia buildup, which is toxic to the brain.
🧠 Key inputs: NH₃ + CO₂ → Urea
🔁 Steps and Enzymes of the Urea Cycle
| Step | Enzyme | Key Product | Location |
|---|---|---|---|
| 1 | Carbamoyl phosphate synthetase I (CPS I) | Carbamoyl phosphate | Mitochondria |
| 2 | Ornithine transcarbamylase (OTC) | Citrulline | Mitochondria |
| 3 | Argininosuccinate synthetase | Argininosuccinate | Cytosol |
| 4 | Argininosuccinate lyase | Arginine + Fumarate | Cytosol |
| 5 | Arginase | Urea + Ornithine | Cytosol |
🧠 Mnemonic: "Ordinarily, Careless Crappers Are Also Frivolous About Urination"
⚙️ Energy and Transport Notes
The cycle uses 3 ATP equivalents
Citrulline moves from mitochondria → cytosol
Ornithine is regenerated and cycles back in
🩺 DAT Clinical Tie-In: Hyperammonemia
| Type | Cause |
|---|---|
| Type I | CPS I deficiency |
| Type II | Ornithine transcarbamylase deficiency (OTC) |
| Acquired | Liver dysfunction (e.g., cirrhosis) |
Symptoms: Vomiting, confusion, lethargy, coma (due to ammonia buildup)
✅ Know enzyme names and where disruption causes backup of substrates.
📈 DAT-Style Practice Question
Q: A patient has elevated blood ammonia and citrulline. Which enzyme is likely deficient?
A: Argininosuccinate synthetase
🎯 DAT Strategy Tips
✅ Memorize the enzyme sequence (CPS I → OTC → ASS → ASL → Arginase)
✅ Know which steps happen in mitochondria vs cytosol
✅ Understand ATP cost and ammonia detox
✅ Review clinical correlations (especially OTC deficiency)
See more biochem visuals at kingofthecurve.org/studyscience
🚀 Call-to-Action
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