Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Metabolism of Simple Proteins
Absorption of Protein Digestion Products
Fate of Absorbed Amino Acids

The diagram below illustrates the diverse Metabolic pathways of Amino Acids following intestinal absorption. Transported via the portal vein to the Liver, they undergo a series of primary transformations, although a significant portion is distributed by the bloodstream throughout the body for various physiological needs. In the liver, Amino acids are utilized not only for the synthesis of its own Proteins and Blood Plasma Proteins, but also for The production of specific nitrogen-containing compounds, such as purine and pyrimidine NUCLEOTIDES, creatine, uric acid, NAD, and others. Furthermore, the liver maintains a balanced pool of free amino acids in the body through the synthesis of non-Essential Amino Acids and the redistribution of nitrogen via Transamination reactions.

Class="center">

As seen in the diagram, absorbed amino acids are primarily utilized as building blocks for the Synthesis of specific tissue proteins, Enzymes, Hormones, and other biologically active compounds. A certain amount of amino acids undergoes degradation to yield the End products of Protein METABOLISM (CO₂, H₂O, and NH₃) with the release of energy. It is estimated that in an adult on A balanced diet, approximately 1200 kJ per day is generated through The oxidation of about 70 g of amino acids (derived from both dietary sources and endogenous amino acids produced during The breakdown of tissue proteins). This amount accounts for about 10% of the daily human energy requirement. The amount of amino acids undergoing degradation depends on both dietary habits and the physiological state of the Organism. For example, even during complete starvation or partial protein deprivation, a small amount of nitrogenous waste is continuously excreted in the urine, indicating that the degradation of body proteins is an ongoing process. Unlike fats and Glycogen, Amino Acids and Proteins are not stored or accumulated in Tissues; in healthy adults consuming adequate dietary protein, a relatively constant blood concentration of amino acids is maintained (see Chapter 16).

The Role of amino acids in Protein Synthesis is discussed in detail in Chapter 14.