Peptidases are present in all living organisms, including viruses, bacteria, archaea, protists, fungi, plants and animals. There are 226 peptidase families assigned in the MEROPS database (Release 9.5, July 2011) and 57 clans, based on structural data (Barrett 2001 Rawlings et al. In the MEROPS database ( ), peptidases are classified further into families, according to their sequence similarity, and into clans, according to their structural similarity. Metallopeptidases, on the other hand, are those whose catalytic activity depends on the presence of a divalent metal ion bound within the active site. Peptidases are classified according to their catalytic type into aspartic, cysteine, glutamic, serine and threonine peptidases, according to the main, functional amino acid residue at the active site. Peptidases that cleave peptide bonds at the termini of polypeptide chains are called exopeptidases, while endopeptidases cleave peptide bonds within the polypeptide chain. Cleavage of peptide bonds can be general, leading to complete degradation of protein substrates into their constituent amino acids, or it can be specific, leading to selective protein cleavage for post-translational modification and processing. Proteases, also called peptidases or proteolytic enzymes, constitute a large group of enzymes that catalyse the hydrolysis of peptide bonds. Because there are such a large number and diversity of proteases in prokaryotes, yeasts, filamentous fungi and mushrooms, we can expect them to be a rich source of protease inhibitors as well. They are also valuable tools for simple and effective purification of proteases, using affinity chromatography. Furthermore, protease inhibitors are indispensable in protein purification procedures to prevent undesired proteolysis during heterologous expression or protein extraction. They can be involved in crop protection against plant pathogens and herbivorous pests as well as against abiotic stress such as drought. In medicine, protease inhibitors can be used as diagnostic or therapeutic agents for viral, bacterial, fungal and parasitic diseases as well as for treating cancer and immunological, neurodegenerative and cardiovascular diseases. Therefore, small molecules and proteins that inhibit proteases can be versatile tools in the fields of medicine, agriculture and biotechnology. Because of their essential roles, their proteolytic activity needs to be tightly regulated.
Proteolytic enzymes play essential metabolic and regulatory functions in many biological processes and also offer a wide range of biotechnological applications.
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