UREA CYCLE

The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH₂)₂CO from ammonia (NH₃). This cycle occurs in ureotelic organisms. The urea cycle converts highly toxic ammonia to urea for excretion.^[1] This cycle was the first metabolic cycle to be discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the TCA cycle. 

Steps of the urea cycle

1. Carbamoyl phosphate is converted to citrulline. With catalysis by ornithine transcarbamoylase, the carbamoyl phosphate group is donated to ornithine and releases a phosphate group.
2. A condensation reaction occurs between the amino group of aspartate and the carbonyl group of citrulline to form argininosuccinate. This reaction is ATP dependent and is catalyzed by argininosuccinate synthetase.
3. Argininosuccinate undergoes cleavage by argininosuccinase to form arginine and fumarate

       4. Arginine is cleaved by arginase to form urea and ornithine. The ornithine is then transported back to the mitochondria to begin the urea cycle again. 

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