Which enzyme initiates protein digestion?

Protein digestion takes place in two different phases: in the stomach and in the small intestine. HCl (hydrochloric acid) and pepsinogen secreted by stomach interact to create pepsin, an enzyme that plays a very important role in protein digestion. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin. Pepsin is an enzyme whose zymogen (pepsinogen) is released by the chief cells in the stomach and that degrades food proteins into peptides.