A protease (also called a peptidase or proteinase) is any enzyme that performs proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in a polypeptide chain. Proteases have evolved multiple times, and different classes of protease can perform the same reaction by completely different catalytic mechanisms. Proteases can be found in animals, plants, fungi, bacteria, archaea and viruses.
These enzymes are involved in a multitude of physiological reactions from simple digestion of food proteins to highly regulated cascades. Proteases can either break specific peptide bonds, depending on the amino acid sequence of a protein, or break down a complete peptide to amino acids . The activity can be a destructive change (abolishing a protein's function or digesting it to its principal components), it can be an activation of a function, or it can be a signal in a signalling pathway.