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Pepsin is expressed as a zymogen called pepsinogen, whose primary structure has an additional 44 amino acids compared to the active enzyme. In the stomach, gastric chief cells release pepsinogen. This zymogen is activated by hydrochloric acid (HCl), which is released from parietal cells in the stomach lining.
The gastric chief cell (also known as a zymogenic cell or peptic cell) is a cell in the stomach that releases pepsinogen [1] and chymosin.Pepsinogen is activated into the digestive enzyme pepsin when it comes in contact with hydrochloric acid produced by gastric parietal cells. [2]
Pepsin is the main gastric enzyme. It is produced in the stomach by gastric chief cells in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids.
Gastric juice is the combination of gastric gland secretions including the main component of hydrochloric acid (gastric acid), gastric lipase and pepsinogen. [4] Once in the stomach pepsinogen is changed by gastric acid to the digestive enzyme pepsin adding this enzyme to the gastric juice. [5]
The gastric mucosa is covered in surface mucous cells that produce the mucus necessary to protect the stomach's epithelial lining from gastric acid secreted by parietal cells in the glands, and from pepsin, a secreted digestive enzyme. Surface mucous cells follow the indentations and partly line the gastric pits.
Hydrochloric acid is formed in the following manner: Hydrogen ions are formed from the dissociation of carbonic acid. Water is a very minor source of hydrogen ions in comparison to carbonic acid. Carbonic acid is formed from carbon dioxide and water by carbonic anhydrase.
The breakdown of protein begins in the stomach through the actions of hydrochloric acid, and the enzyme pepsin. The stomach can also produce gastric lipase, which can help digesting fat. The contents of the stomach are completely emptied into the duodenum within two to four hours after the meal is eaten.
As a result, the stomach's secretion of essential substances such as hydrochloric acid, pepsin, and intrinsic factor is impaired, leading to digestive problems. The most common are pernicious anemia possibly leading to vitamin B 12 deficiency; and malabsorption of iron, leading to iron deficiency anaemia. [2]