Amino Acids and Their Diversity, Classification, Action, and Significance in the Body

From a chemical perspective, amino acids are a comprehensive group of organic compounds that share the common characteristic of having at least one basic amino group and one acidic hydroxyl group in a molecule. For this reason, amino acids belong to the group of ampholytes, which behave as both acids and bases. Amino acids serve as the fundamental building blocks of nearly all protein structures. However, it turns out that only 20 out of the 300 known amino acids fulfill the function of a building block in the body. These are referred to as proteinogenic amino acids. The majority of these, with the exception of proline, belong to the alpha-amino acids. Besides proteinogenic amino acids, there are also non-proteinogenic ones that also perform significant physiological functions. An example of a non-proteinogenic amino acid is beta-alanine, which is part of a strong antioxidant, coenzyme A. Below is depicted the general chemical structure of alpha-, beta-, and gamma-amino acids. Source: Hames H. , Biochemistry. Brief lectures, Warsaw 2007. Due to their chemical structure, which resembles that of salts, amino acids are crystalline substances of white color. The melting point of amino acids is very high. All amino acids dissolve well in water, with proline being particularly soluble.

The criteria for the classification of amino acids are very diverse, just like the entire group. Within this group, many subdivisions can be made, but the most important one is the division of primary protein amino acids into endogenous and exogenous amino acids (this division applies to the majority of vertebrates, there is no such division in plants): – Endogenous amino acids are those that the human body can produce on its own, without the need to be taken up with human sources. These include: alanine, asparagine, cysteine, glycine, glutamine, glutamic acid, proline, serine, tyrosine, cystine, citrulline, homoserine, hydroxylysine, hydroxyproline, carnitine, gamma-aminobutyric acid, ornithine and taurine; – exogenous amino acids are those that the human body cannot synthesize on its own. It is our responsibility to provide the right amount of them to our body. The exogenous amino acids include: phenylalanine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, valine. In addition to these amino acids, there are also arginine and histidine, which are classified as relatively exogenous amino acids. This means that the body of a young, growing person is not able to produce a sufficient amount of them (in the case of an adult person, the amount is however sufficient). Apart from the division into endogenous and exogenous amino acids, there are also divisions into aminoketogenic and non-aminoketogenic amino acids as well as into alpha-, beta- and gamma-aminoketones and a division with regard to the polarity of the side chain (nonpolar side chain, polar side chain with negative or positive formal charge, polar side chain without formal charge).

Each of the protein amino acids in the human body performs specific functions. The most important functions and food sources are as follows: - Tryptophan - is an exogenous amino acid that is found in human proteins. It is a component of serotonin, commonly known as the hormone of happiness. The body is converted into melatonin, the hormone responsible for the proper functioning of the circadian cycle. Tryptophan is also necessary for the proper synthesis of pyridoxine and niacin. It has an effect on the regulation of metabolism. Its best food sources are bananas and dairy products. Lack of tryptophan in the body can reduce the immune system and increase the risk of insomnia, depression, and anxiety; - Methionine - is an exogenous amino acid that contains a sulfur atom. In the body, it is essential for the proper synthesis of choline and lecithin. Methionine is an amino acid that prevents the occurrence of atherosclerosis and metabolic diseases, such as fatty liver disease. Adequate intake of methionine prevents brittle nails, hair loss, and improves overall skin condition. Long-term deficiency of methionine predisposes to the development of mental disorders, including depression. The best food sources of methionine are rice, barley, wheat bran, mushrooms, walnuts, legumes, fish, and meat; - Phenylalanine - is an exogenous amino acid that is an important building block of body proteins. It is necessary for the proper functioning and development of the nervous system. Phenylalanine is used in the treatment of migraine headaches. In addition, this amino acid plays an important role in the production of thyroid hormones and adrenal cortex hormones. Both excess and deficiency of phenylalanine can be dangerous to health. Excess occurs most often in the case of a metabolic disease, phenylketonuria. People suffering from this disease must avoid its main food sources, such as meat, fish, dairy products, cheese, large amounts of grain products, fruits and vegetables; - Lysine - is an exogenous amino acid that is necessary for the synthesis of body proteins. It is the main component of proteins in the skin, tendons, cartilage, and bones. Adequate intake of lysine in the diet increases the uptake of calcium from the small intestine. Lysine strengthens the immune system. Its food sources are mainly eggs, poultry, fish, meat, yellow cheese, and legumes. Lack of lysine in the diet is extremely rare. It occurs in exhausted, severely malnourished, or long-term restrictive elimination diets.

Isoleucine, leucine, and valine form a branched group of amino acids known as BCAAs. They are exogenous amino acids that are found in large quantities in protein-rich products (both animal and plant). BCAA deficiencies are extremely rare in the population, occurring only in individuals who consume insufficient protein in general or in patients with severe metabolic disorders. The requirements for adults for leucine, valine, and isoleucine are 40, 17-25, and 19 mg/kg body weight, respectively. BCAAs are commonly supplemented by individuals who engage in active sports to increase muscle strength and support recovery processes. Amino acids are a diverse group of compounds that are utilized in various industrial sectors, including the food and pharmaceutical industries.