For conversion of inactive trypsinogen to active trypsin _____ enzyme is required.

Question

Accepted Answer

The correct option is 3: Enterokinase

Enterokinase, which is also known as enteropeptidase, is the specific and critical enzyme required for the initial conversion of inactive *trypsinogen* to its active form, *trypsin*. Trypsinogen is an inactive enzyme precursor, or **zymogen**, which is synthesized and secreted by the pancreas into the duodenum of the small intestine. It is produced in this inactive form to protect the pancreatic tissue from self-digestion, as active trypsin would destroy the cells of the pancreas; this preventative measure is vital for health. Once trypsinogen enters the small intestine via the pancreatic duct, the enzyme Enterokinase, secreted by the mucosal cells of the duodenal brush border, acts as a molecular "key." Enterokinase precisely cleaves a small peptide segment from the trypsinogen molecule, resulting in a conformational change that transforms it into the fully active enzyme, trypsin. This activation is the central initiating event of the entire protein digestion cascade in the small intestine, as active trypsin then goes on to auto-catalyze the activation of all remaining trypsinogen, as well as activating other pancreatic proenzymes like chymotrypsinogen and procarboxypeptidase.

The incorrect option is 1: Hydrochloric acid

Hydrochloric acid (HCl) is a strong chemical, not an enzyme, secreted by the parietal cells of the stomach, which is the organ preceding the small intestine. Its primary function is to denature or unfold the complex structure of dietary proteins, making them more accessible to enzymes, and to provide the highly acidic environment (pH 1.5–3.5) necessary for its specific action. Critically, HCl is the substance that converts the inactive zymogen *pepsinogen* into its active form, *pepsin*, within the stomach. However, the activation of trypsinogen occurs later, in the small intestine (duodenum), a location where the acidic chyme from the stomach has already been neutralized by bicarbonate from the pancreas. Therefore, HCl does not function in the alkaline environment of the small intestine where trypsinogen is converted to trypsin.

The incorrect option is 2: Erepsin

Erepsin is an older, collective term that refers to a mixture of final-stage proteolytic enzymes found in the intestinal juice (succus entericus), secreted by the small intestinal mucosa. This complex includes enzymes such as **dipeptidases** and **aminopeptidases**. The function of these enzymes is to complete the process of protein digestion, by taking the small peptide chains that result from the action of earlier enzymes like trypsin and chymotrypsin and breaking them down into individual, absorbable amino acids. While Erepsin is essential for the final absorption of nutrients, it is not involved in the crucial initial activation of the pancreatic zymogen, trypsinogen.

The incorrect option is 4: Pepsin

Pepsin is the major protein-digesting enzyme (protease) that functions within the stomach. It is secreted as the inactive zymogen *pepsinogen* and activated by the acid, hydrochloric acid (HCl). Pepsin's role is to break down large, denatured proteins into smaller fragments called proteoses and peptones. Its activity is dependent on the highly acidic environment of the stomach, and it is quickly inactivated when the partially digested food mass (chyme) enters the small intestine and is neutralized. Therefore, its activity is confined to the stomach, and it is not responsible for the conversion of trypsinogen, which is initiated by Enterokinase in the duodenum.