A cycle of biochemical processes, which generates urea from ammonia is known as the urea cycle. This takes place in ureotelic species. The urea cycle transforms the highly toxic ammonia into urea for excretion. The urea cycle occurs mainly in the liver, and to some extent in the kidneys as well.
The following are the enzymes that take part in the urea cycle are:
1. Ornithine transcarbamoylase: It transforms carbomyl phosphate to citrulline.
2. Arginosuccinate synthetase: It transforms citrulline and aspartate into arginosuccinate.
3. Arginosuccinase / arginosuccinate lyase: It transforms arginosuccinate to fumarate.
4. Arginase: It transforms arginine to urea.