What is an amino acid? Characteristics of this type of molecules

From a biological point of view, an amino acid is a molecule that contains an amino group and a carboxyl group in its physical structure. This organic compound is the basis of proteins.

If we look at the concept of amino acid from a purely physiological point of view, it can leave us a little cold, but things get interesting when We know that, for example, proteins are the most abundant molecules in the entire human body, since they represent 50% of the dry weight of all our tissues.

These nutrients can be found in all cells, so they make up organs, muscles, tissues, hair and skin. It could be said that, together with nucleic acids (RNA and DNA), proteins are the basis of the life of all living beings.

Thus, amino acids play an essential role in the concept of human "existence" and of all organisms as we know it today. If you want to know what an amino acid is and what its types are, keep reading.

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What is an amino acid? Coding life

We have already defined the term from a purely physical point of view, but paying attention to a more functional approach, one could say that each amino acid is one more “brick” in the construction of the foundations of a building, in this case, each of the proteins that will later become part of the cells, which will make up the tissues that will give rise to the highly complex body human.

These essential protein structures for existence are polymeric chains made up of amino acids linked by peptide bonds., that is, the amino group (-NH2) of one attached to the carboxyl group (-COOH) of another. The bond resulting from this union of both molecules receives a chemical nomenclature CO-NH, and in the process a molecule of water (H2O) is lost. Without entering the complex world of organic chemistry, we will limit ourselves to saying that this type of bond presents intermediate characteristics between a double and a single one.

Once we have defined exactly how amino acids associate to give rise to proteins, it is time for us to define the types of amino acids present in nature.

Types of amino acids

It is taken for granted that all amino acids are part of proteins, and therefore the simplest and fastest classification of them is between "essential" and "non-essential". Still, many readers will be surprised to learn that not all amino acids are part of the protein complexes assumed by all. These require a special mention.

1. Non-protein amino acids

Some metabolic intermediates and neurotransmitters have a characteristic amino acid structure, but they do not appear associated with the polymer chain that makes up the proteins.

An example of this are Ornithine and Citrulline, intermediate compounds of the urea cycle, or Homocysteine ​​and Homoserine, essential molecules for different metabolic processes. Another precursor substrate that needs to be named is dihydroxyphenylalanine (DOPA), the initiator of metabolic pathways that give rise to neurotransmitters as important as dopamine and the adrenalin.

Although these compounds act more "behind the scenes" compared to those directly associated with protein polymers, it is clear that we cannot imagine life without a hormone such as adrenaline (and therefore DOPA), which increases the heart rate of living beings and promotes fight and flight responses, thus increasing the theoretical survival of the individual. Although they are not structural amino acids per-se, of course their function is essential.

Once this atypical group has been discussed, it is clear that the bulk of the space and the informative importance is carried by the amino acids that are part of the proteins. We show them below.

2. Protein amino acids

To the question of what an amino acid is, these are the first to come to mind. Canonical or encodable protein amino acids are those that are encoded in the genome, that is, whose assembly instructions are stored in the DNA.

By processes such as transcription and translation (mediated by messenger and transfer RNAs) these Synthesis instructions give rise to the desired protein, based on a concatenation of amino acids in an order concrete. This is applicable when we are moving in realms of "standard genetic code".

These common amino acids for all living beings are the following: alanine, arginine, asparagine, aspartate, cysteine, phenylalanine, glycine, glutamate, glutamine, histidine, isoleucine, leucine, lysine, methionine, proline, serine, tyrosine, threonine, tryptophan and valine, is say, 20 organic molecules that are one of the essential pillars of life.

As the classification of biological terms follows a clearly anthropocentric nomenclature, beings Humans have divided these canonical amino acids into "essential" and "non-essential" according to the need of their consumption.

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2.1. Essential amino acids

They are those that the human body cannot produce by itself, and therefore must be consumed in the form of proteins with the diet.. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine, that is, 9 of the 20 canonical names mentioned above.

We are lucky that these amino acids are found in the environment that surrounds us. For example, histidine is synthesized in abundant amounts by plants, fungi, and bacteria. By concatenating the elements of the food chain of ecosystems, dairy products, beef and chicken contain histidine. We ingest it and this amino acid will be the precursor of histamine, an essential compound in the mediation of allergic responses by the immune system. This is where its “essential” name comes from, as it is an example that we literally could not live without it.

Legumes and cereals are generally assumed to be foods rich in essential amino acids. It is necessary to emphasize that this "essentiality" depends on the species in which we look, because naturally not all living beings follow the same metabolic routes.

2.2. Nonessential amino acids

In this case, the amino acids are produced by metabolic pathways included in the physiology of the human being. They are the following: alanine, tyrosine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, asparagine and arginine, that is, 11 of the 20 canonicals.

As mentioned above, the different metabolic pathways are different even within mammals themselves. For example, cats need an essential enzyme to synthesize taurine, which is an acid derived from cysteine. In this case, this compound would become essential for them even though humans can synthesize it ourselves.

Even so, the fact that amino acids can be synthesized by the human being himself does not mean that they are not also ingested with the diet, since they compose, naturally, the tissue of many other mammals on which we feed. For example, proline is found in both animal products (meat, fish, dairy products, and eggs) and plant-based foods (legumes, seeds, whole grains, and fruits).

2.3. Conditional amino acids

But didn't the classification end with essentials and nonessentials? The question of what an amino acid is must be followed by certain considerations, and one of them is that conditional amino acids exist.

These are those that They are not essential in a moment of normality, but they may be required in illness or special conditions. Undoubtedly an example of this is arginine (not essential in normal times), since only its ingestion is needed monitored in the diet in the presence of certain diseases, such as obesity disorders and anemia sickle cell.

Conclusions

As we have been able to observe in these lines, the world of amino acids is extensive and complex, but its most widespread classification has been made based on the need (or not) of the intake by humans in their diet.

Still, there are many other divisions based, for example, on the properties of its chain (polar neutrals, neutral nonpolar, negatively or positively charged) or depending on the location of the amino group (alpha, beta or gamma amino acids). In any case, we leave these classifications for another opportunity, since they are focused on a vision much more biochemical than functional.

Thus, amino acids are organic molecules that make up the most basic “pillar” of the human being: proteins, cells and tissues. Therefore, it is essential to know them and to know which ones have to be consumed properly in the diet during the day to day.

Bibliographic references:

• What types of amino acids exist, aminoacidos.eu. Picked up on August 19 in https://www.aminoacido.eu/aminoacidos/que-tipos-de-aminoacidos-existen.html
• Amino Acids, United States National Library of Medicine. Picked up on August 19 in https://medlineplus.gov/spanish/ency/article/002222.htm#:~:text=Los%20amino%C3%A1cidos%20no%20esenciales%20incluyen,%2C%20prolina%2C%20serina%20y%20tirosina.
• Biomolecules (amino acids), University of the Basque Country. Picked up on August 19 in http://www.ehu.eus/biomoleculas/aa/tema8.htm#index