Urea Cycle
Start with the big picture
The cycle begins in mitochondria, where CPS I uses ammonia, carbon dioxide, and ATP to form carbamoyl phosphate; this rate-limiting enzyme requires N-acetylglutamate, whose synthesis is stimulated by arginine. OTC then forms citrulline, which moves to the cytosol. There, aspartate contributes nitrogen as reactions produce argininosuccinate and then arginine and fumarate. Arginase releases urea and regenerates ornithine, which returns to mitochondria. The pathway consumes three ATP, equivalent to four high-energy bonds, per urea. Its nitrogen comes from ammonia and aspartate, and its carbon from carbon dioxide. Genetic defects and acquired liver-related conditions can impair ammonia disposal; OTC deficiency is associated with increased orotic acid, while CPS I deficiency is not.
What you'll learn
- Describe the liver and cellular compartments involved in the urea cycle.
- Trace the five enzymatic steps and the movement of key intermediates.
- Explain CPS I regulation by N-acetylglutamate and arginine.
- Identify the cycle’s nitrogen and carbon sources and its energy cost.
- Distinguish key clinical features of OTC and CPS I deficiencies.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
