Pepsin is an enzyme that is released by the chief cells in the stomach and that degrades food proteins into peptides
The substrate is proteins, and the products are peptides. Pepsin is a protease, i.e. an enzyme that catalyzes the breakdown of proteins. In humans, it is secreted by certain cells (called chief...
I am not an expert, but I believe you can find it in your stomach. More info here: http://biology.clc.uc.edu/courses/bio115/pepsin.htm
breaks down complex protein molecules into simpler proteins
hydrochloric acid