How does pepsinogen become active pepsin?

Pepsinogen is a proenzyme that is converted into the active enzyme pepsin in the stomach. This conversion is catalyzed by hydrogen ions (H+), which are abundant in the acidic environment of the stomach.

The conversion of pepsinogen to pepsin involves several steps:

1. Protonation: Hydrogen ions bind to specific amino acid residues in the pepsinogen molecule, causing it to undergo a conformational change. This conformational change exposes the active site of the enzyme.

2. Autocatalytic Activation: The exposed active site of pepsinogen can cleave the peptide bond between two specific amino acids within the pepsinogen molecule. This cleavage results in the release of a small peptide, which activates the pepsinogen into pepsin.

3. Further Activation by Pepsin: Once a small amount of pepsin has been activated, it can further activate additional pepsinogen molecules through a positive feedback mechanism. Pepsin cleaves the peptide bond between specific amino acids in pepsinogen, converting it into active pepsin.

This process ensures that pepsin is only activated in the acidic environment of the stomach, where it can function optimally to break down proteins into smaller peptides.