Pepsin – What It Serves: Function and Structure of this Proteolytic Enzyme

Definition: Pepsin is a proteolytic enzyme that breaks down proteins into peptides.

It was discovered by the German physiologist Theodor Schwann, in 1836. In 1930, it was isolated in crystalline form by John H. Northrop, of the Rockefeller Institute of Medical Research.

The lining of the human stomach has millions of gastric glands. After each meal, these glands perform the vital function of gastric juice secretion. There are different types of cells within these glands.

These include the parietal cells, the main cells, the mucus-secreting cells and the hormone-secreting cells.

The parietal cells secrete hydrochloric acid and intrinsic factor, while mucus secreting cells secrete mucin. The main cells produce and secrete pepsinogen, which is the precursor of the proteolytic enzyme pepsin.

It should be noted that hormone-secreting cells in the stomach stimulate the gastric glands to secrete the gastric juice.

After ingestion of food and its arrival in the stomach, the vagus nerve sends signals that stimulate the release of gastrin in the bloodstream.

Once it comes in contact with the gastric glands in the stomach, it triggers the secretion of gastric juice, which includes hydrochloric acid, mucus and digestive enzymes such as renin and pepsin.

Function

Pepsin, chymotrypsin and trypsin are classified in the category of proteolytic enzymes. These enzymes are involved in the hydrolysis of proteins into peptides and amino acids by breaking them down into peptide bonds.

Pepsin is quite effective in the decomposition of peptide bonds in the case of amino acids such as phenylalanine, tryptophan and tyrosine.

Pepsin is secreted in the form of pepsinogen, which is a zymogen (proenzyme or an inactive precursor).

It is the release of hydrochloric acid by the parietal cells in the lining of the stomach that causes the inactive precursor pepsinogen to change to the active form of pepsin. Hydrochloric acid helps maintain the optimum acidity (pH 1-3) for the function of pepsin.

In the stomach, the pepsinogen molecules are partially digested with each other, thus eliminating the segments of the polypeptide chains and converting the pepsinogen into pepsin.

Pepsin breaks the peptide bonds between the amino acids with the hydrophobic side chains in the middle of the polypeptides. Therefore, it changes the long polypeptides into short polypeptides.

The process of protein conversion into peptones can be achieved with the help of commercially prepared pepsin. One of the applications of pepsin is found in the food industry, where the enzyme is used to curdle milk in cheese making.

It could also be used to process soy protein and gelatin, in order to give them a shake quality. It can be used for the preparation of animal and vegetable protein hydrolysates which can be used as flavoring agents in foods or beverages.

In food chemistry, pepsin can be used to evaluate the digestibility of proteins. Pepsin was historically an additive to the chewing gum brand Beemans by Dr. Edward E. Beeman.

In the leather industry, it is used to remove the remains of tissue and hair from the skins and soften them. It is also used in the laboratory analysis of several proteins.

Structure

Basically, proteins are large polymers linked by peptide bonds. The peptide bond is an amide bond that binds the amino group of one amino acid to the carboxyl group of another.

Proteins comprise several types of amino acids, with the difference in the chemical nature of the side chain. When many amino acids bind in this way, it leads to the formation of a peptide.

Polypeptides are formed when hundreds of amino acids are joined in this way. In the case of proteins, one or more polypeptide chains are held together by non-covalent interactions.

They have an N-terminus, where the amino group of the final amino acid is unlinked, and a C-terminus, where the carboxyl group of the final amino acid is unlinked.

The pepsinogen has 44 additional amino acids at its N-terminal end. During the transformation of the pepsinogen into pepsin, these 44 amino acids are released.

While pepsin has less basic amino acid residues, it has 44 acid residues. This is the reason why it remains stable at an extremely low pH.

To avoid self-digestion, pepsins should be stored at very low temperatures ranging from -80 ° C to -20 ° C.

Pepsin hydrolyzes peptide bonds in proteins, breaking them down into smaller polypeptide fragments. It is active in the acidic conditions provided by the presence of gastric juice in the stomach.

The low pH of the stomach is due to the secretion of HCl by the gastric glands. The strong acidity of the stomach denatures the proteins of the ingested food, which increases the exposure of the peptide bonds of the protein.

Pepsin works best with the acidity of normal gastric juice, which has a pH that ranges between 1.5 and 2.5. It should be noted that the reaction for the conversion of pepsinogen to pepsin requires a pH value of less than 5. Pepsin is not effective in the intestine, since the gastric acids are neutralized, with a pH value of 7.

The impulses of the vagus nerve, as well as the secretion of gastrin and secretin hormones, stimulate the release of pepsinogen in the stomach, where it is mixed with hydrochloric acid and rapidly converted to the enzyme pepsin.

It should be noted that pepsin is only involved in the partial degradation of proteins. The main site of protein digestion is the intestine, where trypsin, chymotrypsin (secreted by the pancreas) and others work in the digestion of proteins, decomposing them into peptides, which in turn are converted into amino acids.