The process by which excess ammonia present in the liver cells is converted into urea is called the ornithine cycle or the urea cycle. The ornithine cycle is completed in five steps. The steps are-
1. Formation of carbamyl phosphate: Carbamyl phosphate is produced from CO2 and NH3 in the presence of carbamyl synthetase enzyme.
CO2 + NH3→ carbamyl phosphate
2. Formation of citrulline: Carbamyl phosphate reacts with ornithine in the presence of ornithine transcarbamylase enzyme to produce citrulline.
Carbamyl phosphate + ornithine → citrulline
3. Formation of argininosuccinate: Citrulline reacts with aspartate in the presence of argininosuccinate synthetase enzyme to form argininosuccinate.
Citrulline + aspartate → argininosuccinate
4. Arginine production: Argininosuccinate is cleaved to fumarate and arginine by the enzyme argininosuccinase.
Argininosuccinate → fumarate + arginine
5. Urea produced: Arginine reacts with water in the presence of arginase enzyme to produce urea.
Arginine + water → urea