Amino acid oxidation is a crucial metabolic process where the body utilizes the carbon skeletons of amino acids to fuel the citric acid cycle. Before these carbon skeletons can be used, the body must address the nitrogen components of the amino acids. Directly releasing nitrogen as ammonia would disrupt cellular pH and could be toxic, so the body employs the urea cycle to safely process these nitrogens.
The urea cycle primarily occurs in the liver and involves the conversion of two nitrogen atoms into one molecule of urea, a process that consumes three ATP molecules. The cycle begins in the mitochondria, where the enzyme carbamoyl phosphate synthetase catalyzes the reaction between bicarbonate and ammonium (NH4+) to form carbamoyl phosphate, using two ATP in the process.
Next, ornithine enters the mitochondria and combines with carbamoyl phosphate, resulting in the formation of citrulline and the release of an inorganic phosphate group. Citrulline then exits the mitochondria, where it undergoes further reactions. In the cytosol, citrulline reacts with aspartate, facilitated by the breakdown of ATP into pyrophosphate (PPi), which is subsequently converted into two inorganic phosphates by the enzyme pyrophosphatase. This reaction produces argininosuccinate.
Argininosuccinate is then cleaved into fumarate and arginine. In the final step of the cycle, arginine is converted into urea, releasing the nitrogen atoms, and regenerating ornithine, which re-enters the mitochondria to continue the cycle. This cyclical process is essential for the detoxification of ammonia and the regulation of nitrogen levels in the body.
