Pepsin: what it is, characteristics and functions

Pepsin is one of the key elements to understand the digestion and absorption of nutrients.

Humans and all other animals are open systems, as we require the ingestion of organic matter to obtain energy. Fifty percent of our diet is composed of carbohydrates, 30% of fats and 10-15% of proteins.

All these macronutrients are broken down by hydrolysis into small biomolecules, which cross the membrane and are then broken down into small molecules.which cross the plasma membrane of the 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 tract into a substance that is assimilated by the organism, is essential for the food to be converted into energy and metabolic heat. To do this, food is ingested by mouth, undergoes a series of mechanical and chemical changes, is transported to the stomach, then to the intestines and, finally, waste is ejected into the environment in the form of feces.

This general process describes the passage of food through the digestive tract in an extremely cursory manner, but it should be noted 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 all about pepsin, one of those enzymes essential to understand digestion at the gastric level..

What is pepsin?

First of all, it is necessary to emphasize that pepsin is an endopeptidase, i.e. an enzyme that breaks down proteins obtained in dietary intake into smaller peptides. This type 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 responsible for digestion, as this group also includes trypsin, chemotrypsin, elastase or thermolysin, among others.

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

However, 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 indispensable 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 even self-degrade the organism's own tissue if there were no clear and effective preventive mechanisms. and effective preventive mechanisms. Fortunately, the mucosal barrier of the stomach secretes a mucus-bicarbonate type substance, which gives the gastric wall a near pH-neutral environment and deactivates pepsin. The stomach itself must protect itself from the enzymatic activity that takes place within it, counterintuitive as it may sound.

The synthesis of pepsin

Pepsin is synthesized in the stomach, as we have mentioned above. However, the stomach cells (main cells of the gastric glands) do not secrete pepsin in the stomach, the stomach cells (main cells of the gastric glands) do not secrete pepsin per se, but pepsinogen.. This compound is a zymogen or inactive proenzyme, which contains 44 "extra" amino acids, compared to the real enzyme.

The hormone gastrin, secreted by the G-cells of the gastric apparatus, stimulates the secretion of pepsinogen and hydrochloric acid, which generates a very acidic pH environment within the stomach chamber. When pepsinogen comes into contact with this acidic 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 pepsin can begin to break down proteins into smaller molecules.

In addition, it should be noted that pepsinogen is synthesized thanks to the instructions present in the genes, i.e., the chromosomes inside the cells.the chromosomes inside the cells. In humans, there are 3 different genes coding for the same form of pepsinogen A: PGA3, PGA4 and PGA5. All of them have the directions for the synthesis of zymogen, which is then transformed into the enzyme by gastric acid stimulation.

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

Function of pepsin

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

For its part, pepsin "cuts" the protein chain to be degraded at the level of the amino acids leucine (leu), phenylalanine (phe), tryptophan (trp) or tyrosine (tyr), unless one of them is preceded by proline (pro). Recall that it is an endopeptidase, which means that it cuts "on the 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 they make up 50% of the dry weight of almost all Biological tissues, since there is no metabolic process that does not depend to some extent on them. That is why pepsin and the other protein-degrading enzymes are so essential: not only for obtaining energy, but also for the integration of amino acids in biological tissues, such as muscles and skin..

The role of pepsin in pathologies

Like any element in the human body, pepsin can fail or perform activities at times when it is not needed, leading to pathologies. In this case, this and other enzymes play an essential role in the development of reflux symptoms. 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 backs up into the esophagus if the stomach environment is not well demarcated. This causes acids, pepsin and other enzymes to travel backwards through the esophageal tube, even reaching the larynx and, in the worst cases, the pulmonary environment.

To complicate matters further, patients with LPR have impaired local neural sensitivity, so they cannot respond with coughs and rales to the presence of acid in the laryngeal environment. Since it is in its active form and is not excreted, pepsin begins to degrade, pepsin begins to degrade laryngeal tissues, resulting in chronic dysphagia (inability to swallow), raspy voice and repeated coughing.. The more pepsin is in contact with the laryngeal environment, the worse the damage.

Summary

As you may have noticed, pepsin is a most interesting enzyme at the physiological level, since it is self-activating with the acidic environment of the stomach and its functionality is regulated in a completely pH-dependent manner. If the pH is maintained between 1.5 and 2, the enzyme remains in its active form and performs its work. When this value changes, it maintains its three-dimensional conformation, but does not break down proteins as it does in the stomach.

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