What is Pepsin?

Pepsin is an endopeptidase enzyme that degrades proteins into peptides. It is formed in the chief cells of the stomach lining and is one of the most important digestive enzymes in humans and many other animals' digestive systems, where it aids in the digestion of proteins. Pepsin is an aspartic protease with an active site that contains a catalytic aspartate.

Where is Pepsin found?

Pepsin enzymes are found in the stomach. Pepsin is a stomach enzyme that aids in the digestion of proteins in food. Pepsin is secreted by gastric chief cells as pepsinogen, an inactive zymogen. Hydrochloric acid is secreted by parietal cells in the stomach lining, which lowers the pH of the stomach. Pepsin is activated by a low pH (1.5 to 2).

Structure of Pepsin

Pepsin is a monomer (chain protein) made up of two folding domains that are separated by a deep cleft. Pepsin's catalytic site is formed at the domain junction, with two aspartic acid residues, Asp32 and Asp215, in each domain. The water molecule aids pepsin's catalysis by allowing the active carboxyl group to bear positive and negative charges with aspartic acid 215 and 32, respectively, breaking the peptide bond in the protein.

Activity and Stability of Pepsin

In acidic environments with a pH of 1.5 to 2.5, pepsin is most active. Pepsin works best at temperatures between 37 and 42 degrees Celsius. As a result, the stomach is the main site of synthesis and function (pH 1.5 to 2). 

In humans, pepsin concentrations in the stomach range from 0.5 to 1 mg/mL. At pH 6.5 and above, pepsin is inactive, but it is not completely denatured or irreversibly inactivated until pH 8.0. As a result, re-acidification will reactivate pepsin in solutions up to pH 8.0. The safety of pepsin at high pH has important consequences for laryngopharyngeal reflux disease.

Following a gastric reflux case, pepsin persists in the larynx. Pepsin will be inactive at the mean pH of the laryngopharynx (pH = 6.8), but it may be reactivated during subsequent acid reflux events, causing damage to local tissues. Pepsin has a wide range of cleavage specificity. 

Working of Pepsin Enzyme

Pepsin is expressed as a zymogen called pepsinogen, which has an additional 44 amino acids in its primary structure than the active enzyme.

Pepsinogen is generated by chief cells in the stomach. Hydrochloric acid (HCl), which is released from parietal cells in the stomach lining, activates this zymogen. When food is consumed, the hormone gastrin and the vagus nerve trigger the release of pepsinogen and HCl from the stomach lining. 

The acidic environment produced by hydrochloric acid allows pepsinogen to unfold and cleave itself in an autocatalytic manner, resulting in the development of pepsin (the active form). To make more pepsin, pepsin cleaves the 44 amino acids in pepsinogen.

Classification of Pepsinogen

Pepsinogens are divided into five categories based on their primary structure: pepsinogen A (also known as pepsinogen I), pepsinogen B, progastricsin (also known as pepsinogen II and pepsinogen C), prochymosin (also known as prorennin), and pepsinogen F. (also called pregnancy-associated glycoprotein).

What does Pepsin do? 

Pepsin is a proteolytic enzyme released by chief cells, which are specialized in the stomach. It is a component of gastric juice that aids in food digestion. Pepsin is a digestive enzyme that breaks down large polypeptides into smaller peptide fragments in the stomach's acidic environment (pH 1.5-2.5). 

Pepsin preferentially hydrolyzes peptide bonds containing the aromatic amino acids' amine group (tryptophan, phenylalanine, and tyrosine). Pepsin is a stomach enzyme that aids in the digestion of proteins in food. Pepsin is secreted by gastric chief cells as pepsinogen, an inactive zymogen. Hydrochloric acid is secreted by parietal cells in the stomach lining, which lowers the pH of the stomach.

The function of pepsin in the stomach is to break down proteins in foods like meat and eggs into smaller pieces (polypeptides). It just breaks down proteins at specific stages, so the protein isn't fully digested to the amino acid level.

Applications of Pepsin

  • Pepsin can be used in the food industry. 

  • Pepsin is found in pancreatic curd that condenses and twists during the processing of cheese. 

  • Pepsin may be used to add whipping qualities to soybean protein and gelatin. 

  • It can also turn pre-cooked cereals into instant hot cereals and change the plant protein used in non-dairy snacks. 

  • Pepsin can also be used to make protein hydrolysates from animal and plant sources for seasoning food and beverages. 

  • It is used in the leather industry to extract hair and residual tissue from leather, as well as to retrieve silver from the abandoned photographic film by digesting the gelatin layer that stores the silver.

FAQs (Frequently Asked Questions)

Q1. What is Pepsin made of?

Ans. Pepsin is the mature active form of pepsinogen, which is an inactive protein. Pepsinogen is synthesized and secreted by chief cells (C) in the stomach, which combines with hydrochloric acid secreted by parietal cells to form pepsin (P). Pepsin is produced when pepsinogen reacts with hydrochloric acid. Pepsin is an endopeptidase enzyme that degrades proteins into peptides. It is formed in the chief cells of the stomach lining and is one of the most important digestive enzymes in humans and many other animals' digestive systems, where it aids in the digestion of proteins.

Q2. What is the Source of Pepsin and how is it Prepared?

Ans. The mucous membrane of the ox (Bos taurus), sheep (Ovis aries), or hog (Sus scrofa) stomach is used to make pepsin. There are different methods to prepare pepsin which is mentioned as Pepsin is made by combining hydrochloric acid with minced stomach linings. This solution is clarified, partly evaporated, dialyzed, condensed, and either poured on glass plates to dry (scale pepsin) or carefully evaporated in a vacuum (spongy pepsin). Scale pepsin is made by digesting the mucous lining with about 0.2 percent HCl (or water acidulated with other acids to the same degree of acidity) at about 100°C until the membrane is fully or nearly completely dissolved.