Pepsin

Pepsin is an aspartic protease and an endopeptidase that breaks down proteins into smaller peptides. Pepsin is one of the three main proteases in the human digestive system. It is produced in the stomach and helps digest proteins in food. The other two are chymotrypsin and trypsin. Each digestive enzyme can cut off the relationship between specific types of amino acids, in which pepsin can effectively break down the peptide bond between hydrophobic amino acids and aromatic amino acids. These digestive enzymes work together to break down food proteins into peptides and amino acids, which are eventually absorbed by the small intestine.

Precursor

Pepsinogen is the precursor of pepsin, which is released by chief cells of intestinal wall. Its primary structure is composed of 44 amino acids. Compared with pepsin, pepsinogen is stable in neutral and weak alkaline environment, but when exposed to hydrochloric acid in gastric juice, these 44 amino acids are removed in an autocatalytic manner and activated into pepsin. Parietal cells of the gastric wall release hydrochloric acid (HCl), pepsinogen can be activated by hydrochloric acid. Gastrin and vagus nerves trigger the release of pepsinogen and hydrochloric acid from the gastric wall when eating. Hydrochloric acid produces acidic environment, which makes pepsinogen unfold and cleave in an autocatalytic manner, thus producing pepsin. Pepsin cuts 44 amino acids in pepsinogen into more pepsin.

Structure

Pepsin is a chain protein (monomer) composed of two similar folding domains separated by a deep cleft. The catalytic site of pepsin is formed at the junction of the domain, each domain contains two aspartic acid residues, Asp32 and Asp215. Under the catalysis of pepsin, the water molecule helps the active carboxyl group to bear positive and negative charges with aspartic acid 215 and aspartic acid 32, respectively, which breaks the peptide bond in the protein.

Stability

Pepsin was most active in acidic environment from 37 °C to 42 °C. The activity of pepsin was the highest in pH2.0, inactive above pH6.5, and completely denatured or irreversibly inactivated above pH8.0. Therefore, in the solution below pH8.0, pepsin can be reactivated after reacidification. The stability of pepsin at high pH value is of great significance to the diseases caused by pharynx and larynx reflux.

Physiological effects

Pepsin is one of the main causes of mucosal injury in pharynx and larynx reflux. Pepsin still stays in the pharynx and larynx after pharyngeal reflux. Although the enzyme is in a neutral environment, it can be reactivated in the subsequent acid reflux event. After pepsin is activated, laryngeal mucosa is exposed to active pepsin, resulting in a decrease in the expression of protective proteins, thus increasing the susceptibility to laryngeal injury. In addition, pepsin may also cause mucosal damage in weak acid or non-acid reflux events. Pepsin can be internalized in the upper airway by receptor-mediated endocytosis. When cells ingest pepsin, pepsin is stored in intracellular vesicles with a low pH value, at which the enzyme activity of pepsin recovers. The exposure of pepsin to neutral pH and the internalization of pepsin lead to changes in the expression of genes related to inflammation, which are the basis of signs and symptoms of reflux and tumor progression. Pepsin in airway specimens is considered to be a sensitive and specific marker for pharyngeal reflux.

Inhibitors

Pepstatin is a low molecular weight compound, which has a strong inhibitory effect on acid protease. Statine is a potential transition state analogue of acid protease catalysis. The Styl residue of pepstatin is considered to be related to the inhibition of pepsin. Pepstatin cannot covalently bind to pepsin, so the inhibitory effect of pepstatin on pepsin is reversible. 1-bis (diazoacetyl)-2-phenylethane can inactivate pepsin during pH5.0. Sucralfate can also inhibit pepsin activity.

Applications

Pepsin can be used in the food industry. Pepsin is a component of pancreatic curd that condenses and twists during cheese production. Pepsin can be used to modify soybean protein and gelatin and provide whipping qualities. It can also modify the plant protein used in non-dairy snacks and make pre-cooked cereals into instant hot cereals. Pepsin can also be used to prepare animal and plant protein hydrolysates for seasoning food and beverages. In the leather industry, it is used to remove hair and residual tissue from leather and to recover silver from abandoned photographic film by digesting the gelatin layer in which silver is stored.