Degradation of Amino Acids

Multiple pathways exist for the catabolism of amino acids (AAs) to CO₂, ammonia, water, and SO₄ ²⁻ in animals and microorganisms in a species-, tissue-, and cell-specific manner. Reactions spanning multiple organs (e.g., skeletal muscle and liver), as well as their cytoplasm and mitochondria, are required for degrading most AAs in mammals, birds, fish, and crustaceans. Based on their metabolic fates, AAs can be classified into three groups: ketogenic (e.g., leucine and lysine), glucogenic (e.g., alanine and serine), and ketogenic plus glucogenic (e.g., isoleucine and threonine). The complete oxidation of most AAs to CO₂ involves the formation of acetyl-CoA, which enters the Krebs cycle for oxidation to CO₂, NADH, and FADH₂. NADPH is generated from the catabolism of certain AAs (e.g., arginine and proline). NAD(P)H and FADH₂ are oxidized to water via the mitochondrial electron transport system, where ATP is synthesized from ADP plus Pi. Because of the different lengths of carbon skeletons and different numbers of N atoms, the efficiency of oxidation of different AAs for ATP production varies greatly. Microbes degrade AAs via pathways that are either the same as, similar to, or different than those in animals. Substantial catabolism of AAs by animal tissues and gastrointestinal bacteria host affects the efficiency of the utilization of dietary protein for protein synthesis and other metabolic pathways and also regulates AA homeostasis in the body.