Synthesis of carbamoyl phosphate- This step takes place in the mitochondria of the liver cells. Here the ammonium ions react with carbon dioxide (product of mitochondrial respiration) to form carbamoyl phosphate catalyzed by the enzyme carbamoyl phosphate synthetase I. This is an irreversible, rate-limiting, ATP-dependent reaction and consumes 2 ATP. Carbamoyl phosphate synthetase I (in mitochondria) is different from carbamoyl phosphate synthetase II (in cytosol) as the latter one has a different role to play and is involved in pyrimidine synthesis.