Which Enzymes catalyze key steps of the urea cycle?

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Detailed Solution

The urea cycle, which takes place in the liver of terrestrial vertebrates, relies on five key enzymes. These enzymes work together to turn harmful ammonia into urea, which can be safely excreted. This process is essential for removing excess nitrogen from the body.

The five enzymes and their roles:

Carbamoyl phosphate synthetase I (CPS1): Found in the mitochondria, CPS1 kicks off the cycle. It uses ammonia, bicarbonate, and ATP to produce carbamoyl phosphate. This step is irreversible and sets the pace for the entire cycle.
Ornithine transcarbamylase (OTC): Also located in the mitochondria, OTC combines carbamoyl phosphate with ornithine to form citrulline.
Argininosuccinate synthetase (ASS): Working in the cytosol, ASS joins citrulline with aspartate, using ATP, to form argininosuccinate.
Argininosuccinate lyase (ASL): Another cytosolic enzyme, ASL breaks down argininosuccinate into arginine and fumarate.
Arginase: The final enzyme in the cytosol. Arginase splits arginine into urea (ready for excretion) and ornithine, which is sent back to the mitochondria to restart the cycle.

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