A protease (also called a peptidase or proteinase) is an enzyme that catalyzes (increases the rate of) proteolysis, the breakdown of proteins into smaller polypeptides, or single amino acids. They do this by cleaving the peptide bonds within proteins by hydrolysis, a reaction where the water breaks bonds. Proteases are involved in many biological functions, including digestion of ingested proteins, protein catabolism (the breakdown of old proteins), and cell signaling. Without additional helping mechanisms, proteolysis would be very slow, taking hundreds of years. Proteases can be found in all forms of life and viruses. They have independently evolved multiple times, and different classes of protease can perform the same reaction by completely different catalytic mechanisms.