Proteases (peptidases) are with the ability to break hydrolytic peptide bonds in a protein- or polypeptide molecule. They exist in almost all tissues and cells of any organism. Generally, two types of proteases are distinguished, i.e. intracellular proteases which carry out regulatory tasks in cell compartments, thereby regulating the protein content of the cell. Newly synthesized protein functionality can be activated by cleaving off peptide fragments, for example. Signal proteases on the other hand cleave off peptides that act as signaling component, which leads to their transport to their designated place of action. Additionally, intracellular peptidases are involved in the processing of antigens.

In this context the cytoplasm and the in the nucleus are of crucial importance. The large peptidase-complex (of approximately 11 by 17 nanometers) recognizes proteins that have been labeled with a chain, which designates them for disassembly into peptides. This process assures destruction of faulty, damaged or simply superfluous proteins. The extracellular peptidase are found mainly in the intestines of higher organisms. They catalyse the hydrolytic cleavage of foods. They may possess highly specific functions in other extracellular fluids, for example in the clotting system of the blood, the complement system (involved in immune function) and the fibrinolytic system. Inhibitors of protease function are for example , iodacetate and .

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