33. Digestion and absorption of proteins in the gastrointestinal tract. Mechanisms of intracellular degradation of proteins. Nitrogen balance and end products of nitrogen metabolism. General reactions of amino acid metabolism – transamination, oxidative deamination, transdeamination and decarboxylation (biogenic amines). Clinical significance of aminotransferases.

Amino acid catabolism

Amino acid catabolism refers to the set of metabolic pathways that break down amino acids, the building blocks of proteins, in the body. This process is crucial for the removal of excess amino acids and the production of energy, particularly during periods when carbohydrates are scarce or during prolonged exercise.

• Nitrogen Removal: Since amino acids contain nitrogen, their catabolism involves the removal of nitrogen groups. This is primarily done through processes such as transamination and oxidative deamination. The nitrogen removed from amino acids is ultimately converted into urea in the liver and excreted in the urine, which is essential for maintaining nitrogen balance in the body.
• Energy Production: The carbon skeletons of amino acids (once the amino group is removed) can be converted into key metabolic intermediates that enter various metabolic pathways, including the citric acid cycle (TCA cycle), to produce energy in the form of ATP. Some of these intermediates can also be used for gluconeogenesis—the production of glucose from non-carbohydrate sources.
• Biosynthesis: Some of the products of amino acid catabolism are used as substrates for the biosynthesis of other important compounds, such as neurotransmitters, hormones, and other amino acids.