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Pepsin is a protease, a digestive enzyme that degrades proteins in the stomach; the other important digestive enzymes are trypsin and chymotrypsin. It was the first animal enzyme to be discovered, by Theodor Schwann in 1836.

Pepsin is expressed as a pro-form, pepsinogen with an additional 44 amino acids, which are cleaved off outside the secreting cell, to avoid digestion of cellular proteins.

Pepsin is most active at pH 2-4. It is permanently inactivated above pH 6.

The name Pepsi is derived from pepsin. Pepsin is also used in the preparation of cheese.