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

Amino acids are fundamental elements for the maintenance of the organism. Let's see what they are.

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, this may 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, representing 50% of the dry weight of all our tissues.

These nutrients can be found in every cellThey 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 life in all living beings.

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

What is an amino acid? Encoding life

We have already defined the term from a purely physical point of view, but paying attention to a more functional approach, it could be said 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 form part of the cells, which will make up the tissues that will give rise to the very complex human body.

These protein structures essential for existence are polymeric chains made up of amino acids joined by peptide bonds.The amino group (-NH2) of one bonded to the carboxyl group (-COOH) of the other. 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 going into the complex world of organic chemistry, we will limit ourselves to saying that this type of bond has intermediate characteristics between a double and a single bond.

Once we have defined exactly how amino acids are associated to give rise to proteins, it is time 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 quickest classification of them is between "essential" and "non-essential". However, many readers may be surprised to learn that not all amino acids are part of proteins. not all amino acids are part of the commonly assumed protein complexes.. These require special mention.

1. Non-protein amino acids

Some metabolic intermediates and neurotransmitters have a characteristic amino acid structure, but are not associated with the polymeric chain that makes up proteins. but are not associated with the polymeric chain that makes up proteins..

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

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 or flight responses, thus increasing the theoretical survival of the individual. Although they are not structural amino acids per se, their function is certainly indispensable.

Having discussed this atypical group, it is clear that the bulk of the space and informative importance goes to the amino acids that are part of proteins. They are shown below.

2. Protein amino acids

When asked what an amino acid is, these are the first to spring to mind. The canonical or codifiable protein amino acids are are those that are encoded in the genome, i.e., 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 a particular order. This is applicable when we are moving on "standard genetic code" grounds.

These amino acids common to 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, ie, 20 organic molecules that are one of the essential pillars of life..

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

2.1. Essential amino acids

They are those that the human body cannot produce by itself, and therefore, they have to be consumed in the form of proteins in the diet.. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine, i.e. 9 of the 20 canonical ones mentioned above.

We are fortunate that these amino acids are found in the environment around us. For example, histidine is synthesized in abundant quantities by plants, fungi and bacteria. By concatenation of the elements of the ecosystem food chain, 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 the name "essential" comes from, as it is an example that we literally could not live without it.

In general, legumes and cereals are assumed to be foods rich in essential amino acids.. It is necessary to emphasize that this "essentiality" depends on the species we are looking at, as naturally not all living beings follow the same metabolic pathways.

2.2. Non-essential amino acids

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

As mentioned above, the different metabolic pathways are different even within mammals themselves. For example, cats lack 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 we humans can synthesize it ourselves.

However, the fact that amino acids can be synthesized by humans themselves does not mean that they are not ingested. does not mean that they are not also ingested in the diet.They naturally make up 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 foods (legumes, seeds, whole grains and fruits).

2.3. Conditional amino acids

But didn't the classification end with essential and non-essential? The question of what an amino acid is has to be followed by certain appreciations, and one of them is that conditional amino acids exist.

These are those that are not essential at a time of normality, but may be required in disease or special conditions.. Undoubtedly an example of this is arginine (not essential at normal times), as its monitored dietary intake is only required in the presence of certain diseases, for example, obesity disorders and sickle cell anemia.

Conclusions

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

Even so, there are many other divisions based, for example, on the properties of their chain (polar neutrals, non-polar neutrals, negatively or positively charged) or according to the location of the amino group (alpha, beta or gamma amino acids). However, we will leave these classifications for another opportunity, since they are focused on a much more biochemical than functional vision.

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 adequately consumed in the diet on a daily basis.

Bibliographic references:

• What types of amino acids exist, aminoacidos.eu. Retrieved Aug. 19 from https://www.aminoacido.eu/aminoacidos/que-tipos-de-aminoacidos-existen.html.
• Amino Acids, U.S. National Library of Medicine. Retrieved August 19 from https://medlineplus.gov/spanish/ency/article/002222.htm#:~:text=The%20amino%C3%A1acids%20non%20essential%20include,%2C%20proline%2C%20serine%20and%20tyrosine.
• Biomolecules (amino acids), University of the Basque Country. Retrieved August 19 from http://www.ehu.eus/biomoleculas/aa/tema8.htm#index.