A protease (also known as peptidase or proteinase) is an enzyme that catalyzes the process of proteolysis, during which the peptide bonds are cleaved and proteins are broken down into smaller polypeptides or single amino acids. Proteases are involved in many biological functions, including digestion of ingested proteins, protein catabolism, as well as cell signaling.

Proteases can be classified based on their optimum pH in which they are active.

• Acid proteases have an activity optimum at pH 2.0-3.5. Examples are pepsin, chymosin and rennin.
• Neutral proteases, such as trypsin, chymotrypsin, and papain, are most active at pH 6-8.
• Basic proteases (also called alkaline proteases) work best at pH >8, for example, subtilisin BPN and leucine aminopeptidase.