SECTION 7. AMINO ACID METABOLISM

Main subjects:

7.1. Nitrogen balance. Protein diet

7.2. Digestion of proteins in gastrointestinal tract

7.3. Transamination of amino acids

7.4. Deamination of amino acids

7.5. Detoxification of ammonia in tissues

7.6. Urea synthesis (Kreds-Henseleit ornithine cycle)

7.7. Inclusion of nitrogen-free residue of amino acids

into CCP

7.8. Synthesis of non-essential amino acids

7.9. Metabolism of individual amino acids. Inherited disorders of amino acid metabolism

7.10. Decarboxylation of amino acids. Biogenic amines

Amino acids fulfil various functions in the body

(Fig. 7.1):

• structural - most amino acids are used for protein building (up to 400 g/day), the amount of which in adults of normal body-buil is about 12-15 kg;

• anabolic - amino acids are precursors of many metabolically active compounds such as hormones, pyrimidine and purine nucleotides, heme, crea-tine, carnitine, phospholipids, neurotransmitters;

• energetic - amino acids can be energy sources under extreme conditions, for example, during prolonged starvation, or in case of excessive protein consumption.

Fig. 7.1. Biological functions of amino acids

Free amino acids pool in the body is 30-100 g; blood amino acid concentration is 35-65 mg/dL on average.

The sources of free amino acids in the cell are food proteins and tissue proteins; synthesis of non essential amino acids is carried out from carbohydrates and

CCP metabolites (Fig. 7.1).

Amino acids are not stored in the body like glucose (as glycogen) or fatty acids (as TAG). That is why under extreme conditions structural and functional tissue proteins serve as amino acids reserve, and first of all proteins of muscles, liver and blood.

Protein renovation occurs permanently in the human body, about 400 g of proteins are degraded per 24 hours and approximately the same amount is synthesized. Synthesis and renovation of proteins occur with different rate. Thus, connective tissue protein collagen is fully regenerated in 300 days, but blood proteins of clotting system - from several minutes to several days. The main source of amino acid pool is proteins from food. Small amount is generated due to tissue protein catabolism or is synthesized. Only carbon portion of some amino acid molecules is formed from carbohydrates, and amino group must be transferred from other amino acids.