Amino Acids

Unlike fats and carbohydrates, amino acids are not stored by the body. That is, no protein exists whose sole function is to maintain a supply of amino acids for future use. Therefore, amino acids must be obtained from the diet, synthesized de novo, or produced from the degradation of body protein. Any amino acids in excess of the biosynthetic needs of the cell are rapidly degraded. The first phase of catabolism involves the removal of the α-amino groups (usually by transamination and subsequent oxidative deamination), forming ammonia and the corresponding α-keto acids, the carbon skeletons of amino acids. A portion of the free ammonia is excreted in the urine, but most is used in the synthesis of urea (Fig. 1), which is quantitatively the most important route for disposing of nitrogen from the body. In the second phase of amino acid catabolism, the carbon skeletons of the α-keto acids are converted to common intermediates of energy-producing metabolic pathways. These compounds can be metabolized to carbon dioxide (CO₂) and water (H₂O),  glucose, fatty acids, or ketone bodies by the central pathways of metabolism.

Figure 1: Urea cycle shown as part of the essential pathways of energy metabolism. NH₃ = ammonia; CO₂ = carbon dioxide.