Pepsin: what it is, characteristics and functions

The human being and the rest of the animals are open systems, since we require the intake of organic matter to obtain energy. 50% of our diet is made up of carbohydrates, 30% fat and 10-15% protein.

All these macronutrients are broken down by hydrolysis into small biomolecules., which cross the plasma membrane of cells and are oxidized in the mitochondrial environment, in order to obtain energy for all tissues and reactions necessary for life.

Digestion, known as the process by which a food is transformed in the digestive system into a substance that the body assimilates, it is essential for the food to end up converted into energy and heat metabolic. For this, the food is ingested through the mouth, subjected to a series of mechanical and chemical changes, transported to the stomach, then to the intestines and, finally, the waste is ejected into the medium in the form of feces.

This general process describes the passage of food through the digestive system in an extremely brief way, but it can be emphasize that each of these sections of the system is characterized by a series of chemical and physical reactions of great interest. Today we tell you everything about

pepsin, one of those enzymes essential to understand digestion at the gastric level.

• Related article: "Digestive system: anatomy, parts and functioning"

What is pepsin?

First of all, it is necessary to emphasize that pepsin is an endopeptidase, that is, an enzyme that breaks down the proteins obtained in the dietary intake into smaller peptides. These types of enzymatic molecules break the peptide bonds between amino acids within the protein chain, following a series of very specific guidelines. Pepsin is not the only endopeptidase in charge of digestion, since trypsin, chymotrypsin, elastase or thermolysin, among others, also stand out in this group.

Despite the variety of endopeptidases in the gastric environment, pepsin is considered one of the most important, along with trypsin and chymotrypsin.. In addition, its environment of action is very clear and delimited: it works at its best between a pH of 1.5 and 2, the exact ideal conditions of the stomach. Once it reaches the portion of the duodenum (with a pH of 6), this enzyme is inactivated and its functionality comes to an end (although it maintains its three-dimensional conformation up to a pH of 8).

In any case, it is necessary to clarify that protein digestion also continues at the intestinal level, due to the effects of pancreatic enzymes such as trypsin, chymotrypsin, elastase and carboxypeptidase. Thus, despite its essentiality, pepsin is not essential for life: if this enzyme is missing, others can take care of protein metabolism, with more or less effort.

Curiously, the enzymatic activity of pepsin and other enzymes could self-degrade the body's own tissue if there were no preventive mechanisms clear and effective. Fortunately, the mucous barrier of the stomach secretes a mucus-bicarbonate-like substance, which gives the gastric wall an almost neutral pH environment and inactivates pepsin. The stomach itself must protect itself from the enzymatic activity that takes place within it, counterintuitive as it sounds.

The synthesis of pepsin

Pepsin is synthesized in the stomach, as we have hinted in previous lines. Anyway, stomach cells (main cells of the gastric glands) do not secrete pepsin itself, but pepsinogen. This compound is an inactive zymogen or proenzyme, containing 44 "extra" amino acids, compared to the actual enzyme.

The hormone gastrin, secreted by the G cells of the gastric apparatus, stimulates the secretion of pepsinogen and hydrochloric acid, which creates a very acidic pH environment within the chamber stomach. When pepsinogen comes into contact with this acid conglomerate, it undergoes an autocatalytic reaction, in which it is released from the "tail" of amino acids that kept it inactive. Thus, thanks to the presence of stomach acids, pepsinogen is transformed into its active variant pepsin and this can begin to break down proteins into smaller molecules.

Furthermore, it is necessary to point out that pepsinogen is synthesized thanks to the instructions present in the genes, that is to say, the chromosomes within cells. In humans, there are 3 different genes that code for the same form of pepsinogen A: PGA3, PGA4, and PGA5. All of them have the directions of the synthesis of the zymogen, which is then transformed into the enzyme by the stimulation of gastric acids.

On the other hand, some compounds (such as pepstatin) are capable of inhibiting pepsin at very low concentrations. Pepstatin was first isolated in cultures of actinomycete fungi, but little else is known about it beyond its activity as a protease.

• You may be interested in: "Table of amino acids: functions, types and characteristics"

Pepsin function

At this point, it is essential to emphasize that Pepsin is dedicated to breaking down proteins, but being composed of amino acids, this enzyme is also a protein in itself. Amino acids are the basic unit of any protein, as they are joined in specific orders by peptide bonds to give rise to peptides (less than 10 amino acids), polypeptides (10 to 50 amino acids) and proteins (more than 50 amino acids).

For its part, pepsin "cuts" the protein chain that is to be degraded at the level of amino acids leucine (leu) phenylalanine (phe), tryptophan (trp), or tyrosine (tyr), unless one of them is preceded by proline (pro). We remember that it is an endopeptidase, which means that it cuts "inside" (between amino acids that are not part of the terminal protein section).

Proteins only make up 10-15% of our diet (since carbohydrates are the richest source of energy), but these they account for 50% of the dry weight of almost all biological tissues, since there is no metabolic process that does not depend in some way on they. That is why pepsin and the rest of the enzymes that degrade proteins are so essential: not only for obtaining energy, but for the integration of amino acids in biological tissues, such as muscles and skin.

The role of pepsin in pathologies

Like any element of the human body, pepsin can fail or perform activities at times when it is not necessary, which leads to pathologies. In this case, this and other enzymes play an essential role in the development of symptoms of laryngopharyngeal reflux (LPR) and gastroesophageal reflux (GERD).

A person with a weakened lower esophageal sphincter (LES) may experience these conditions, as the food bolus mixed with gastric juices recedes into the esophagus if the environment of the stomach. This causes acids, pepsin, and other enzymes to travel backward through the esophageal tube, even reaching the larynx and, in worst cases, the lung environment.

To further complicate matters, LPR patients have local neural sensitivity altered, so they cannot respond with coughs and rales to the presence of acid in the environment laryngeal. Being in its active form and not being excreted, pepsin begins to break down the laryngeal tissues, resulting in chronic dysphagia (inability to swallow), a harsh voice, and repeated coughs. The more the pepsin is in contact with the laryngeal environment, the worse the damage.

Resume

As you may have seen, pepsin is a very interesting enzyme at a physiological level, since it is self-activated by itself with the acidic environment of the stomach and its functionality is regulated in a completely pH-dependent way environmental. If the pH is kept between 1.5 and 2, the enzyme remains in its active form and does its job. When this value changes, it maintains its three-dimensional conformation, but does not break down proteins like it does inside the stomach.

Thanks to pepsin and many other biomolecules of an enzymatic nature, human beings can transform the proteins that we consume in energy and, above all, in amino acids useful for the formation and repair of tissues. Of course, it is clear to us that without our internal metabolism we are nothing.