The urea cycle is the main pathway that converts toxic ammonia into urea for safe excretion in urine. This cheat sheet helps college biology students follow each step, compartment, enzyme, substrate, and product in order. It is especially useful for connecting amino acid catabolism, liver metabolism, and nitrogen balance.
A step-by-step reference reduces confusion because the pathway moves between the mitochondrion and cytosol.
The cycle begins in the mitochondrial matrix when ammonia and bicarbonate form carbamoyl phosphate through carbamoyl phosphate synthetase I. Ornithine accepts the carbamoyl group to form citrulline, which then moves to the cytosol for the remaining reactions. Aspartate supplies the second nitrogen of urea through argininosuccinate formation and cleavage.
The overall process consumes 3 ATP molecules but uses 4 high-energy phosphate bonds to produce one urea molecule.
Key Facts
- The overall urea cycle reaction is NH4+ + HCO3- + aspartate + 3 ATP + 2 H2O -> urea + fumarate + 2 ADP + 2 Pi + AMP + PPi.
- Carbamoyl phosphate synthetase I occurs in the mitochondrial matrix and catalyzes NH4+ + HCO3- + 2 ATP -> carbamoyl phosphate + 2 ADP + Pi.
- Ornithine transcarbamylase occurs in the mitochondrial matrix and catalyzes ornithine + carbamoyl phosphate -> citrulline + Pi.
- Argininosuccinate synthetase occurs in the cytosol and catalyzes citrulline + aspartate + ATP -> argininosuccinate + AMP + PPi.
- Argininosuccinate lyase occurs in the cytosol and catalyzes argininosuccinate -> arginine + fumarate.
- Arginase occurs in the cytosol and catalyzes arginine + H2O -> ornithine + urea.
- The two nitrogen atoms in urea come from free ammonia and aspartate, while the carbon atom comes from bicarbonate.
- N-acetylglutamate activates carbamoyl phosphate synthetase I and is required for normal entry of ammonia into the urea cycle.
Vocabulary
- Urea cycle
- A liver pathway that converts toxic nitrogen from ammonia and aspartate into urea for excretion.
- Carbamoyl phosphate
- A high-energy intermediate made from ammonia and bicarbonate that donates a carbamoyl group to ornithine.
- Ornithine
- A carrier molecule that enters the mitochondrion, accepts a carbamoyl group, and is regenerated at the end of the cycle.
- Citrulline
- A urea cycle intermediate formed in the mitochondrion and transported to the cytosol for later reactions.
- Aspartate
- An amino acid that donates the second nitrogen atom incorporated into urea.
- N-acetylglutamate
- An essential allosteric activator of carbamoyl phosphate synthetase I in the mitochondrial matrix.
Common Mistakes to Avoid
- Mixing up CPS I and CPS II is wrong because CPS I is mitochondrial and functions in the urea cycle, while CPS II is cytosolic and functions in pyrimidine synthesis.
- Saying both urea nitrogens come from ammonia is wrong because one nitrogen comes from free ammonia and the other comes from aspartate.
- Counting only 3 high-energy bonds is wrong because 3 ATP molecules are used but ATP to AMP in the argininosuccinate synthetase step uses two high-energy phosphate bonds.
- Placing the entire pathway in the mitochondrion is wrong because only the CPS I and ornithine transcarbamylase steps occur in the mitochondrial matrix, while later steps occur in the cytosol.
- Treating ornithine as a consumed reactant is wrong because ornithine is regenerated by arginase and functions as a reusable cycle carrier.
Practice Questions
- 1 Write the balanced overall urea cycle reaction, including NH4+, HCO3-, aspartate, ATP, water, urea, fumarate, ADP, Pi, AMP, and PPi.
- 2 If 6 molecules of urea are produced, how many ATP molecules are consumed and how many high-energy phosphate bonds are used?
- 3 A liver cell produces 10 molecules of urea. How many nitrogen atoms came from ammonia, and how many came from aspartate?
- 4 A patient has low N-acetylglutamate production. Explain how this would affect carbamoyl phosphate synthetase I activity and blood ammonia levels.