8.1. Digestion of Proteins and Amino Acid Absorption. Essential Amino Acids
Major proteins of the body are constantly degraded up to amino acids and resynthesized. The assembly of new proteins requires a source of amino acids. These building blocks are generated by the digestion of the dietary proteins in the gastrointestinal tract and the degradation of proteins within the cell.
Amino acids perform the following functions in the body:
► anabolic: in addition to their role as the building blocks of proteins, amino acids are precursors of many specialized biomolecules, including hormones, coenzymes, nucleotides, alkaloids, cell wall polymers, porphyrins, antibiotics, pigments, and neurotransmitters;
► energetic: amino acids are the source of energy under extreme conditions (during starvation or in uncontrolled diabetes mellitus, when carbohydrates are either unavailable or not properly utilized, cellular proteins are used as fuel).
Proteins cannot be stored in the body and don't form any tissue depot such as glucose (in the form of glycogen) or fatty acids (as TAG). That is why the amino acid reserve is mainly muscular and blood plasma proteins. In human body, approximately 400 g of proteins are degraded and the same amount synthesized every day. Amino acids that are not incorporated into a new protein and unnecessary for immediate requirements can?t be stored and undergo oxidative degradation. One important feature distinguishes amino acid degradation from other catabolic processes described to this point: every amino acid contains an amino group, and the pathways for amino acid degradation, therefore, include a key step in which the