From a biological perspective, a molecule containing an amino group and a carboxyl group in its physical structure is termed an amino acid (Sánchez Amador, 2020). This organic compound serves as the foundation for proteins. When considering the concept of amino acids from a purely physiological standpoint, it may initially seem unexciting; however, the situation becomes fascinating when discovering that proteins constitute the most prevalent molecules throughout the human body, representing 50% of the dry weight of all tissues. These nutrients are distributed throughout all cells, shaping organs, muscles, tissues, hair, and skin. In conjunction with nucleic acids (RNA and DNA), proteins stand as the cornerstone of the life of all living beings. Therefore, according to Sánchez Amador (2020), amino acids play an essential role in the concept of human "existence" and that of all organisms.
¿Qué es un Aminoácido?
Paying attention to a functional approach, it could be stated that each of the amino acids contributes to the formation of proteins, which, in turn, integrate into cells and give rise to the complex tissues that make up the human body. These protein structures, fundamental for existence, take the form of polymeric chains formed by amino acids joined by peptide bonds (Sánchez Amador, 2020). The term "amino acid" is derived from the abbreviation of α-amino (alpha-amino) carboxylic acid (Reddy, 2024). Each molecule is composed of a central carbon atom (C), known as the α-carbon, to which both an amino group and a carboxyl group are attached. The remaining two bonds of the α-carbon atom are usually satisfied by a hydrogen atom (H) and the R group. Amino acids show variations in their chemical structure, mainly in the R group. According to Reddy (2024), the general formula for an amino acid is:
The Classification and Role of Amino Acids
In general terms, it is often assumed that all amino acids play a crucial role as fundamental components of proteins, thus establishing a clear distinction between those considered "essential" and those classified as "non-essential" (Sánchez Amador, 2020). However, according to Sánchez Amador (2020), not all amino acids participate in widely accepted protein complexes.
Non-Protein Amino Acids
All metabolic intermediates and neurotransmitters have a characteristic amino acid structure but are not linked to the polymeric chain that makes up proteins (Sánchez Amador, 2020). According to Sánchez Amador (2020), examples of these are Ornithine and Citrulline, which function as intermediate compounds in the urea cycle, as well as Homocysteine and Homoserine, essential molecules for various metabolic processes. Worth mentioning is the precursor substrate Dihydroxyphenylalanine (DOPA), which initiates metabolic pathways leading to neurotransmitters such as Dopamine and Adrenaline.
Although these compounds play a more discreet role compared to those directly associated with protein polymers, their importance is undeniable. These hormones increase the heart rate of living beings and stimulate fight-or-flight responses, thus theoretically enhancing individual survival. Sánchez Amador (2020) mentions that, even though they are not structural amino acids per se, their function is unquestionable.
Protein Amino Acids
These are encoded in the genome, meaning their assembly instructions are stored in DNA (Sánchez Amador, 2020). Through processes like transcription and translation, mediated by messenger and transfer RNAs, these synthesis instructions lead to the formation of the desired protein. This formation is based on the concatenation of amino acids in a specific order. Common amino acids for all living beings include Alanine, Arginine, Asparagine, Aspartate, Cysteine, Phenylalanine, Glycine, Glutamate, Glutamine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Proline, Serine, Tyrosine, Threonine, Tryptophan, and Valine. These twenty molecules represent the pillars for life. Given the anthropocentric classification of biological terms, Sánchez Amador (2020) indicates that these canonical amino acids have been categorized as "essential" and "non-essential" based on their consumption necessity.
Essential amino acids are those the human body cannot produce on its own and must be consumed in the form of proteins in the diet (Sánchez Amador, 2020). Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine, in other words, nine of the previously named twenty, are examples of these amino acids. According to Sánchez Amador (2020), it is essential to emphasize that the "essentiality" of these amino acids depends on the species, as not all living organisms follow the same metabolic pathways.
Phenylalanine is associated with a sense of well-being as it regulates endorphins (García-Allen, 2017). Among its notable functions are reducing excess appetite and alleviating pain. It is also involved in the synthesis of catecholamines such as Adrenaline, Dopamine, and Noradrenaline, promoting alertness, improving memory and learning, and increasing vitality. According to García-Allen (2017), supplements containing this amino acid can be used to improve symptoms of Parkinson's, vitiligo, chronic pain, or for the comprehensive treatment of depression.
A deficiency in Isoleucine seems to be implicated in some mental and physical disorders, such as depression, behavior alterations, and decreased muscle mass, among others (García-Allen, 2017). This amino acid is essential for the formation of hemoglobin and muscle tissue, and it stabilizes and regulates blood sugar and energy levels. Additionally, according to García-Allen (2017), it contributes to wound healing, skin, and bone health.
Lysine inhibits the development of viruses in the body and is used in the treatment of herpes and viruses associated with chronic fatigue syndrome (García-Allen, 2017). It participates in the synthesis of L-carnitine along with vitamin C. Moreover, it contributes to collagen formation, the connective tissue present in bones, ligaments, tendons, and joints. It promotes calcium absorption, essential for children as it is crucial for bone formation. According to García-Allen (2017), it also plays a role in hormone production and reduces serum triglyceride levels.
Threonine is indispensable for collagen formation and proves to be an essential factor in the antibody production process (García-Allen, 2017). Additionally, according to García-Allen (2017), it is essential for the normal functioning of the gastrointestinal tract and can convert into Glycine, a neurotransmitter in the central nervous system.
Tryptophan, an essential amino acid, has caught the attention of psychologists and health professionals due to its crucial role in serotonin and melatonin synthesis (García-Allen, 2017). Consequently, according to García-Allen (2017), it plays a fundamental role in mood regulation and directly contributes to improving sleep quality.
Valine competes with tyrosine and tryptophan when crossing the blood-brain barrier (García-Allen, 2017). The higher the valine level, the lower the levels of the other two amino acids in the brain. Valine is actively absorbed and used directly by the muscle as an energy source, so it is not processed by the liver before entering the bloodstream. According to García-Allen (2017), valine deficiency leads to lower absorption of other amino acids (and proteins) by the gastrointestinal tract.
Histidine proves beneficial in the treatment of anemia due to its connection with hemoglobin (García-Allen, 2017). Its role as a precursor to histamine has made it a resource used in allergy treatment. Finally, according to García-Allen (2017), this amino acid contributes to maintaining the proper blood pH and has proven useful in the treatment of rheumatoid arthritis.
Methionine actively plays a role in fat breakdown, facilitating the reduction of cholesterol in the blood (García-Allen, 2017). Also, its influence is highlighted in preventing disorders in hair, skin, and nails. According to García-Allen (2017), it has antioxidant properties and actively participates in RNA and DNA synthesis.
Non-essential amino acids are produced through metabolic pathways included in human physiology (Sánchez Amador, 2020). These include Alanine, Tyrosine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine, Asparagine, and Arginine, that is, 11 of the 20 canonical ones. Various metabolic pathways even vary among mammals. For example, Sánchez Amador (2020) mentions that cats lack an essential enzyme to synthesize Taurine, derived from Cysteine, making it essential for them.
Arginine plays a fundamental role in normal immune system activity and wound healing (García-Allen, 2017). It also participates in growth hormone release, increases insulin and glucagon release, and is a precursor to Gamma-Aminobutyric Acid (GABA). According to García-Allen (2017), it is also known for reducing tumor size and being necessary for spermatogenesis.
Aspartic Acid, an amino acid recognized for its ability to increase endurance and physical performance, is beneficial for combating chronic fatigue (García-Allen, 2017). García-Allen (2017) mentions that, as one of the two main excitatory amino acids, along with Glutamic Acid, Aspartic Acid contributes to protecting the liver, participates in DNA and RNA metabolism, and enhances the immune system.
Glutamic Acid, along with Aspartic Acid, stands out as an exciter, sharing numerous functions (García-Allen, 2017). It contributes to improving physical performance and reducing fatigue. According to García-Allen (2017), its role is essential in DNA and RNA synthesis, providing protection to the body and strengthening the immune system.
Alanine plays a significant role in muscle growth and serves as a significant energy source for the muscle (García-Allen, 2017). According to García-Allen (2017), its involvement in sugar metabolism, strengthening the immune system through antibody production, and its fundamental role in connective tissue highlight it as a key component for overall health.
Asparagine is formed by the combination of Aspartic Acid with adenosine triphosphate (ATP) (García-Allen, 2017). According to García-Allen (2017), this substance plays a crucial role in short-term memory processes, contributes to ammonia removal from the body, reduces fatigue, and actively participates in DNA synthesis.
Cysteine is an antioxidant that protects against radiation, pollution, ultraviolet light, and other phenomena that generate free radicals (García-Allen, 2017). It acts as a natural "detoxifier" and is essential for the growth, maintenance, and repair of skin and hair. Additionally, according to García-Allen (2017), it is a precursor to the amino acid Taurine and Chondroitin Sulfate, the latter being the main component of cartilage.
Glycine is part of the hemoglobin structure and is one of the two main inhibitory neurotransmitters of the nervous system, the other being Gamma-Aminobutyric Acid (GABA) (García-Allen, 2017). It also participates in the formation of cytochromes, enzymes involved in energy production. According to García-Allen (2017), it contributes to the production of Glucagon, which supports Glycogen metabolism.
Glutamine, recognized as a precursor to two significant neurotransmitters of the central nervous system, Glutamate and Gamma-Aminobutyric Acid (GABA), plays a vital role in maintaining normal and constant blood sugar levels, as well as muscle strength and endurance (García-Allen, 2017). Similarly, according to García-Allen (2017), this amino acid is essential for proper gastrointestinal function.
Proline, a fundamental component of cartilage, emerges as a crucial element for joint, tendon, and ligament health (García-Allen, 2017). This amino acid contributes to maintaining heart health and stands out as the main precursor of glutamate. García-Allen (2017) mentions that among the notable functions of this amino acid is sustaining skin and joint health.
Serine actively participates in enhancing the immune system by contributing to antibody and immunoglobulin production, as well as playing a fundamental role in the development of the myelin sheath (García-Allen, 2017). Also, according to García-Allen (2017), this amino acid is crucial for muscle growth and maintenance.
Tyrosine, a precursor amino acid to the hormone thyroxine, participates in metabolic processes (García-Allen, 2017). Additionally, it stands out as a precursor to growth hormone and neurotransmitters such as Dopamine, Norepinephrine, Epinephrine (Adrenaline), and Serotonin. These elements, according to García-Allen (2017), positively influence mood, sleep, clarity of thought, concentration, and memory.
The third category corresponds to conditional amino acids (Sánchez Amador, 2020). The question of what an amino acid is carries certain considerations, and one of them highlights the existence of conditional amino acids. These are not essential under normal conditions but may be necessary in illness or special conditions. A clear example of this is arginine, considered non-essential under normal circumstances. However, according to Sánchez Amador (2020), its intake should be monitored in the diet when certain conditions, such as obesity disorders and sickle cell anemia, arise.
References
García - Allen, J. (2017, abril 12). Tabla de Aminoácidos: Funciones, Tipos y Características. Psicología y Mente. https://psicologiaymente.com/neurociencias/tabla-de-aminoacidos
Reddy, M. K. (2024, enero 3). Amino Acid. Enciclopedia Británica. https://www.britannica.com/science/amino-acid
Sánchez Amador, S.A. (2020, agosto 20). ¿Qué es un Aminoácido? Características de Este Tipo de Moléculas. Psicología y Mente. https://psicologiaymente.com/salud/que-es-aminoacido
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