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 (catalyzed by the enzyme encoded by carA and carB). Another ATP molecule is consumed to supply the energy for this reaction. Carbamoyl phosphate reacts with ornithine to generate citrulline (catalyzed by the enzyme encoded by argF and argI). An aspartate molecule adds to create arginosuccinate (catalyzed by the enzyme encoded by argG). ATP is cleaved to AMP and pyrophosphate to drive this reaction forward. Fumarate is cleaved off of arginosuccinate by the enzyme encoded by argH. Fumarate can then be used in the TCA cycle. Arginine is the other product of this reaction and can be used for protein synthesis. In other bacteria and animals, 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 at arginine synthesis.
