During digestion, hydrochloric acid (HCl) lowers the stomach's pH and this causes the enzyme pepsinogen to be converted to its active form, called pepsin. Pepsin is a proteolytic enzyme, which means it cleaves (breaks) peptide bonds assocciated with food particles. The enzyme pepsin can also cleave pepsinogen into pepsin, and this causes the rate at which pepsinogen is converted to pepsin to increase. This is an example of:

Catalysis

Explanation:

Pepsin is able to break peptide bonds, turning large protein molecules into small peptide chains.

When pepsin acts to break down pepsinogen (inactive form of pepsin), it is accelerating pepsinogen → pepsin reactions, acting as a catalyst, reducing activation energy and favoring proteolytic reactions at a higher rate.

This process of accelerating reactions is characteristic of enzymes and is known as catalysis.