Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter III. METABOLISM OF MAJOR CLASSES OF BIOMOLECULES

CHAPTER 17. AMINO ACID METABOLISM I. GENERAL PATHWAYS OF CONVERSION

The body of an adult human contains on average 12-15 kg of Proteins, which perform numerous catalytic, regulatory, and structural Functions. Approximately half of this mass (6-7 kg) consists of extracellular proteins of supportive Tissues, among which Collagen ranks first in quantity. The second half of body proteins comprises intracellular tissue proteins and Blood proteins with a high turnover rate. Due to protein turnover in Catabolic and anabolic reactions, i.e., the continuous processes of proteolysis and Protein Synthesis, the body maintains a constant pool of free Amino Acids that undergo various biochemical transformations.

17.1. PATHWAYS OF AMINO ACID CONVERSION IN TISSUES

Like other Biomolecules, the proteins that make up a living Organism exist in a steady state of constant renewal, involving the Cleavage of proteins by tissue proteases and the synthesis of new polypeptide molecules.

The total amino acid pool in The Human Body consists of fluxes that ensure the influx of free Amino Acids and their utilization in various anabolic and catabolic processes. The total amount of amino acids undergoing conversion per day in a healthy adult in nitrogen balance is 300-500 g, while their steady-state concentration is about 50-100 g per body mass.

The influx of amino acids entering The amino acid pool comprises the following sources:

1. Amino acids absorbed by intestinal enterocytes As a result of dietary Protein Hydrolysis in the digestive tract (Stomach, Small Intestine); biochemical processes of protein proteolysis in the digestive tract will be considered separately. The quantitative value of this component is (depending on the diet) 60-100 g per day. An additional component to this influx (from 35 to 200 g of protein) is contributed by the proteolysis of endogenous proteins from shed enterocyte epithelium (I. Halkerston, 1988).

2. Amino acids released as a result of The breakdown of the body's own cellular and extracellular proteins. The average half-life (T_1/2) of individual proteins varies significantly, ranging from a few hours for certain hepatocyte Enzymes to several years for the structural protein collagen.

In a healthy adult, the average daily turnover of tissue proteins accounts for 1-2 % of total body protein mass and occurs primarily through the degradation of Muscle Proteins into amino acids (30-40 g of proteins per day). The breakdown of tissue proteins is catalyzed by lysosomal proteases and increases significantly under conditions of protein and total starvation, as well as during debilitating diseases (especially infectious and oncological diseases) that disrupt METABOLISM/35.html">Protein Biosynthesis and cause catabolic processes to predominate over anabolic ones.

3. Amino acids synthesized within the organism. The human body has limited capacity for de novo amino acid formation: its enzyme systems are capable of synthesizing from other intermediates, in quantities sufficient for the synthesis of its own proteins, only eight "dispensable" ("nonessential") L-amino acids. Amino acids that are not synthesized in the human body and must be obtained exclusively from food are "indispensable" ("essential").

Dispensable amino acids include: Alanine, aspartic acid, asparagine, glutamic acid, glutamine, Proline, Glycine, Serine. During The biosynthesis of dispensable amino acids, their carbon Skeleton is formed from glucose oxidation and citrate cycle intermediates, while the amino group is supplied by Other Amino Acids via Transamination reactions. "Conditionally dispensable" Amino acids are Cysteine and Tyrosine, which can be synthesized from the indispensable amino acids Methionine and phenylalanine, respectively; "partially dispensable" amino acids (Histidine, Arginine) are synthesized in insufficient quantities.

The outflow of amino acids leaving the amino acid pool includes anabolic and Catabolic pathways of free amino acid conversion and consists of the following components:

1. Utilization of amino acids for the synthesis of body proteins. In adults consuming A balanced diet, this flux compensates for the proteolysis of endogenous proteins—a state of nitrogen balance. About 75-80 % of the amino acids released during tissue protein breakdown and those coming from the intestine are used for the Synthesis of the body's own enzymatic and structural proteins, as well as physiologically active compounds of protein and peptide nature (for anabolic needs).

2. Utilization of amino acids not included in anabolic processes through catabolic reactions. In this process, amino acid molecules are broken down to form carbon dioxide, Water (via The Citric Acid Cycle), and End products of Nitrogen metabolism (in humans, primarily urea). A certain portion of the nitrogen-free carbon skeleton of amino acids is used for glucose formation (Gluconeogenesis) and Ketone Bodies (ketogenesis).

All amino acids not utilized for the synthesis of proteins or physiologically active compounds (PACs) are subject to degradation regardless of their source, since protein reserves are not formed in animal organisms (unlike Introduction/36.html">CARBOHYDRATES and Lipids). At the same time, the bioenergetic significance of Amino Acid Catabolism in a healthy human is negligible compared to carbohydrates and lipids (about 10 % of total Energy Requirements), increasing significantly only under conditions of starvation.

The First stage of free L-amino acid catabolism is the removal of the α-amino group via transamination and deamination reactions. Some amino acids participate in decarboxylation reactions to form amines with hormonal and neurotransmitter activity, which subsequently undergo degradation via deamination.

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Fig. 17.1. Scheme of General Pathways of amino acid conversion.

Solid arrows indicate reactions of amino acid utilization in anabolic and catabolic processes; dashed arrows indicate reactions of amino acid influx into the general amino acid pool.