Papain
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Papain is a cysteine protease (EC [3.4.22.2]) present in papaya (Carica papaya) which is useful in tenderizing meat and other proteins. It consists of 212 amino acids stabilised by 3 disulfide bridges. Its 3D structure consists of 2 distinct structural domains with a cleft between them. This cleft contains the active site, which contains a catalytic triad that has been likened to that of chymotrypsin. Its catalytic triad is made up of 3 amino acids - cysteine-25 (from which it gets its classification), histidine-159, and asparagine-175.
The mechanism by which it breaks peptide bonds involves deprotonation of Cys-25 by His-159. Asn-175 helps to orient the imidazole ring of His-159 to allow this deprotonation to take place. Cys-25 then performs a nucleophilic attack on the carbonyl carbon of a peptide backbone. This frees the amino terminal of the peptide, and forms a covalent acyl-enzyme intermediate. The enzyme is then deacylated by a water molecule, and releases the carboxy terminal portion of the peptide.
Its utility is in breaking down the tough meat fibers and has been utilized for thousands of years in its native South America. It is sold as a component in powdered meat tenderizer available in most supermarkets. Papain, in the form of a meat tenderizer such as Adolph's, made into a paste with water, is also a home remedy treatment for jellyfish, bee, yellow jacket(wasps) stings and possibly stingray wounds, breaking down the protein toxins in the venom.
Papain is used to dissociate cells in the first step of cell culture preparations. A 10 minute treatment of small tissue pieces (less than 1 mm cubed) will allow papain to begin breaking down of the extracellular matrix molecules holding the cells together. After 10 minutes, the tissue should be treated with a protease inhibitor solution to stop the protease action (if left untreated papain's activity will lead to complete lysis of the cells). The tissue must then be triturated (passed quickly up and down through a Pasteur pipette) in order to break up the pieces of tissue into a single cell suspension..
It is also used as an ingredient in various enzymatic debriding preparations, notably Accuzyme. These are used in the care of some chronic wounds in order to clean up dead tissue.
Papain is usually produced as a crude, dried material by collecting the latex from the fruit of the papaya tree. The latex is collected after scoring the neck of the fruit whereupon it may either dry on the fruit or drip into a container. This latex is then further dried. It is now classified as a dried, crude material. A purification step is necessary to remove contaminating substances. This purification consists of the solubilization and extraction of the active papain enzyme system through a government registered process. This purified papain may be supplied as powder or as liquid.
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