Pepsin is a digestive enzyme produced by the stomach. It breaks down the proteins you eat into amino acid monomers. Pepsin works best in an acidic environment. The pH of stomach contents is usually around 2 due to the high concentration of hydrochloric acid produced by the lining of the stomach. (Compare this to the pH of your mouth's saliva, which is around 7.) Enzymes are like most other proteins, with one exception. What distinguishes enzymes from other proteins