Papain
Papain is a cysteine protease (EC 3.4.22.2) hydrolase enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis).

# Structure
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-158.

# Function
The mechanism by which it breaks peptide bonds involves deprotonation of Cys-25 by His-159. Asn-158 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. In immunology, papain is known to cleave the Fc (crystallisable) portion of immunoglobulins (antibodies) from the Fab (antigen-binding) portion.

# Uses
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.  It is also the main ingredient in Stop Itch and Stop Itch Plus, a DermaTech Laboratories first aid cream popular in Australia.
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 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 to clean up dead tissue.
It can also be found as an ingredient in some toothpastes or mints as teeth-whitener. Its whitening effect in toothpastes and mints however is minimal, because the papain is present in low concentrations, and will be quickly diluted by saliva. It would take several months of using the whitening product to have noticeably whiter teeth.

# Immunoglobulins
An antibody digested by papain yields three fragments, two 50 kDa Fab fragments and one 50kDa Fc fragment.
The agents are not approved by the FDA, and according to the latest FDA position/policy papers these products are not legally marketed.

# Production
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 where 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.