Introduction:
Enterokinase is an enzyme that plays a crucial role in the process of protein digestion. It is primarily found in the small intestine, specifically in the brush border membrane of the duodenum. Enterokinase helps in the conversion of trypsinogen into its active form trypsin.

Conversion of Trypsinogen into Trypsin:
Step 1:
Trypsinogen is an inactive zymogen produced by the pancreas. It is released into the small intestine along with other pancreatic enzymes. Trypsinogen is synthesized and stored in the pancreatic acinar cells as an inactive precursor to prevent the premature activation of trypsin within the pancreas.

Step 2:
When chyme (partially digested food) enters the duodenum, it triggers the release of enterokinase from the brush border membrane of the duodenal epithelial cells. Enterokinase acts as a protease enzyme and specifically cleaves a peptide bond between the amino acids lysine and isoleucine (Lys-Ile) in the trypsinogen molecule.

Step 3:
This cleavage by enterokinase leads to the production of active trypsin. The newly formed trypsin then undergoes an autocatalytic process, where it further activates more trypsinogen molecules by cleaving them at specific sites. This process is known as trypsinogen activation cascade or trypsinogen autolysis.

Importance of Trypsin Activation:
Role in Protein Digestion:
Trypsin is a proteolytic enzyme that plays a crucial role in the digestion of proteins. It acts on peptide bonds and breaks down proteins into smaller polypeptides and amino acids, which can be easily absorbed by the small intestine.

Activation of Other Pancreatic Enzymes:
Trypsin also activates other pancreatic enzymes such as chymotrypsinogen, proelastase, and procarboxypeptidase. These enzymes are also synthesized and secreted in their inactive forms to prevent self-digestion within the pancreas. The activation of these enzymes by trypsin ensures efficient protein digestion.

Conclusion:
Enterokinase is an important enzyme that facilitates the conversion of trypsinogen into its active form trypsin. This activation of trypsin is a crucial step in the process of protein digestion, as trypsin acts on proteins to break them down into smaller polypeptides and amino acids.

The correct option is (a) because trypsinogen is activated by an enzyme,enterokinase, secreted by the intestinal mucosa into active trypsin. Trypsinogen is a zymogen from pancreas.