Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Proteins
Amino Acids

Amino Acids are derivatives of organic (carboxylic) acids containing one or more amino groups (-NH₂). In amino acids, the amino group is most frequently located at the a-position of the carbon atom relative to the carboxyl group, meaning it is attached to the first carbon atom bonded to the carboxyl group. Each amino acid can be viewed as a molecule in which an amino group (-NH₂), a carboxyl group (-СООН), a hydrogen atom (H), and a distinct side carbon chain—referred to as a radical (R)—are attached to a carbon atom, as illustrated by the general formula and spatial model of an amino acid:

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All Amino acids differ in their Chemical Composition and The Structure of their side carbon chain, or radical (Table 19). The Structural Features of the side chains in Proteinogenic Amino Acids determine the Properties of Proteins, underlie their diverse biological Functions, and impart biological individuality to them.

TABLE 19. Chemical Structure of individual groups of principal amino acids

Classification of amino acids. Depending on the structure of their radical, amino acids are divided into acyclic (having an open carbon chain) and cyclic (having a closed carbon ring). Based on the number of functional groups (-NH₂ and -СООН), acyclic amino acids are categorized into monoaminomonocarboxylic, monoaminodicarboxylic, diaminomonocarboxylic, and Diaminodicarboxylic Acids. Certain acyclic amino acids may also contain a hydroxyl group (-OH) or a sulfhydryl group (-SH), and are referred to as hydroxy acids (Serine, Tyrosine) or Sulfur-Containing Amino Acids (Cysteine, cystine, Methionine).

Cyclic amino acids are subdivided into homocyclic and heterocyclic. Homocyclic amino acids contain only carbon atoms in their ring structure (phenylalanine, tyrosine). Heterocyclic amino acids contain other chemical elements in the ring as well, such as nitrogen (Tryptophan, Histidine, Proline).

Amino acids vary in their metabolic characteristics and are typically classified as dispensable (non-essential) and indispensable (essential).

Dispensable and indispensable amino acids. Most amino acids are synthesized within the body's Cells during metabolic processes and are called dispensable. Their absence from the diet does not cause significant metabolic disruptions. Other Amino Acids cannot be synthesized by the Organism and are therefore termed indispensable. These amino acids must be obtained through diet. For adults, nine amino acids are indispensable, while children require a tenth—Arginine (whose daily requirement ranges from 0.5 to 6 g).

Amino acids

Dietary proteins containing all indispensable amino acids are referred to as complete proteins. Such proteins are primarily found in foods of animal origin and represent an essential component of a balanced human diet.

Properties of amino acids. Chemically pure amino acids are white, odorless, tasteless powders. However, the salts of Certain amino acids possess distinct tastes and odors. For instance, the sodium salt of glutamic acid has the taste and aroma of chicken broth and is therefore used as a food seasoning. Most amino acids are soluble in Water and insoluble in organic Solvents.

In aqueous solutions, amino acids dissociate into ions:

In a neutral environment, they exist in a zwitterionic (dipolar) form, bearing both positive (-NH₃) and negative (-COO-) charges.

Amino acids are amphoteric compounds because in aqueous solutions they can react with both acids and bases:

Depending on the number of amino or carboxyl groups, amino acid molecules may carry a net positive charge and exhibit basic properties (arginine, lysine, histidine), or a net negative charge and exhibit acidic properties (aspartic and glutamic acids).

The amphoteric properties of amino acids influence the acid-base characteristics of proteins and their biological functions, particularly their buffering capacity. An effective buffer in Blood erythrocytes is the protein Hemoglobin, which contains a high number of histidine residues that impart significant buffer capacity to this protein at neutral pH values.

Biological Role of amino acids. The body's cells maintain a specific metabolic pool of amino acids, which comprises amino acids derived from The breakdown of dietary and tissue proteins, as well as newly synthesized (dispensable) amino acids. The majority of amino acids (400 g ⋅ day^-1) are utilized for the synthesis of body proteins, while a smaller portion (30 g ⋅ day^-1) goes toward synthesizing other nitrogen-containing compounds (Fig. 86). Amino acids can also be converted into CARBOHYDRATES, Fatty acids, Cholesterol, and Ketone Bodies, or oxidized to end products such as СО₂, Н₂О, and NH₃ with the release of energy.

Fig. 86 Formation of the metabolic pool of Amino Acids and their utilization in body Tissues

Many BIOLOGICALLY ACTIVE SUBSTANCES, such as Hormones, Coenzymes, and biogenic amines, are synthesized from individual amino acids. For instance, the catecholamine hormones (adrenaline and noradrenaline) and thyroxine are synthesized from Phenylalanine and Tyrosine. The amino acid alanine is a component of coenzyme A (CoA). Methionine is utilized to synthesize the neurotransmitter acetylcholine, which plays a vital role in the function of The Nervous system. It is used in the Treatment of neurological disorders and to enhance recovery processes, including in sports practice.

Biogenic amines are formed through the Decarboxylation of amino Acids. The primary biogenic amines include y-aminobutyric acid, histamine, serotonin, and creatine. GABA is synthesized in the Brain from glutamic acid; its accumulation in the brain promotes inhibitory processes in the motor centers of the Central Nervous System. Histamine is produced in various tissues via the decarboxylation of histidine, earning it the designation of a tissue hormone. It causes dilation of small Blood Vessels and constriction of large ones, as well as the contraction of smooth Muscles in Internal Organs. Histamine is involved in pain perception and stimulates Hydrochloric acid secretion in The Stomach. Serotonin is synthesized from tryptophan and is involved in the Regulation of Blood pressure, body Temperature, respiratory rate, and renal filtration. In high doses, serotonin stimulates the central nervous system, whereas in low doses, it suppresses its activity. Creatine is synthesized in tissues from the non-Essential Amino Acids arginine and glycine (Fig. 87). Under the action of creatine kinase and ATP, it is converted into creatine phosphate, which is utilized for the resynthesis of ATP in muscles (see chapters 3 and 15). The amount of creatine phosphate is proportional to Muscle mass. Both creatine and creatine phosphate are converted into creatinine, which is excreted in the urine. The amount of creatinine excreted from the body is proportional to the total content of creatine phosphate and can be used to estimate muscle mass. A decrease in muscle mass is accompanied by a reduction in urinary creatinine levels.