urea cycle

The Urea Cycle in E. coli is an important pathway to store excess nitrogen. Mitochondrial nitrogen reacts with solubilized bicarbonate to form carbomyl phosphate. One molecule of ATP is consumed to supply the phosphate group for carbomyl phosphate molecule. Another ATP molecule is consumed to supply the energy for this reaction. Carbamoyl phosphate reacts with ornithine to generate citrulline. An aspartate molecule adds to create arginosuccinate. ATP is cleaved to AMP and pyrophosphate to drive this reaction forward. Fumarate is cleaved off of arginosuccinate. Fumarate can then be used for the ATP cycle. Arginine is the other product of this reaction and can be used for protein synthesis. In other bacteria, hydrolysis of arginine generates ornithine and urea. Urea is then excreted. However, E. coli lacks the enzyme to do this hydrolysis and therefore this cycle is terminated by arginine synthesis.