Enzymes that break down proteins by proteolysis of peptides are termed peptidases, proteinases or proteases. The proteolysis is a digestion process that removes amino acids from a polypeptide chain. Proteases are ubiquitous in most organisms. Their physiological roles include reactions in apoptosis pathways, the blood-clotting cascade, and the complement system but also in food digestion and many others. By digesting other proteins proteases control the lifetime of proteins. Thereby proteases are important for the recycling of peptides. Peptidases also form larger complexes with other proteins that recognize ubiquitinylated proteins which undergo digestion after recognition. Proteases also play a role in the pathology of certain bacterial infections. Bacterial infections may destroy cell structure by digestion of a host’s proteins. Proteases are sub-divided into six distinct groups by the amino acid that they cleave. Examples of amino acids that are utilized by proteases are serine, cysteine, threonine and glutamic-acid.