 Dear students, in this topic we shall discuss what are pro-enzymes. The pro-enzymes are the inactivated forms of enzymes. Most of the proteolytic enzymes are synthesized, stored and released in inactive form. This inactive molecular form is called pro-enzyme or zymogen. They are released in this form to prevent self-digestion of the lining of the digestive tract. It is actually a protective mechanism. As you know, the proteolytic enzymes are capable of digesting the proteins present in the lining of the digestive tract, stomach and intestine itself. This is why these enzymes are released in inactivated form to prevent self-damage. These pro-enzymes are activated by cleaving a certain portion of the molecule. This cleavage happens by another enzyme or by changing the pH. Dear students, now we shall take few examples of enzymes secreted as pro-enzymes. First we shall discuss pepsinogen. The active form of pepsinogen is pepsin, which is a gastric proteolytic enzyme. This enzyme is synthesized by the chief cells of gastric glands of stomach. It is released in the inactive form pepsinogen, which is a pro-enzyme. This pro-enzyme pepsinogen is activated in the lumen of stomach by raising the pH by hydrochloric acid. The increase in acidity converts pepsinogen into its active form pepsin. The second example of a pro-enzyme is pepsinogen, which is activated to form pepsin. Pepsinogen is secreted in pancreatic juice as a pro-enzyme. It is a polypeptide which contains 249 residues. It is activated in the duodenum by cleaving it. A 6-residue segment is cleaved from the amino terminal end. After cleaving this 6 amino acid residue, it becomes activated. This cleavage is done by two types of enzymes. One is the anterokinase enzyme, which is an intestinal proteolytic enzyme. The second way of activating pepsinogen is to activate the already activated pepsin molecules on the inactivated pepsinogen and cleavage it. As a result, it is converted into activated pepsin. The third example of pro-enzymes is the chymotrepsinogen, which is activated into chymotrepsin. Chymotrepsinogen is secreted as pro-enzyme in the pancreatic juice. It is activated by trypsin and its hydrolytic active form is the chymotrypsin.