Papain, also known as papaya proteinase I CAS 9001 - 73 - 4, is a cysteine protease (EC 126.96.36.199) enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis).
It has wide ranging commercial applications in the leather, cosmetic, textiles, detergents, food and pharmaceutical industries. In the food industry, papain is used as an active ingredient in many commercial meat tenderizers.
Papain belongs to a family of related proteins with a wide variety of activities, including endopeptidases, aminopeptidases, dipeptidyl peptidases and enzymes with both exo - and endopeptidase activity. Members of the papain family are widespread, found in baculoviruses, eubacteria, yeast, and practically all protozoa, plants and mammals. The proteins are typically lysosomal or secreted, and proteolytic cleavage of the propeptide is required for enzyme activation, although bleomycin hydrolase is cytosolic in fungi and mammals. Papain - like cysteine proteinases are essentially synthesised as inactive proenzymes (zymogens) with N - terminal propeptide regions. The activation process of these enzymes includes the removal of propeptide regions, which serve a variety of functions in vivo and in vitro. The pro - region is required for the proper folding of the newly synthesised enzyme, the inactivation of the peptidase domain and stabilisation of the enzyme against denaturing at neutral to alkaline pH conditions. Amino acid residues within the pro - region mediate their membrane association, and play a role in the transport of the proenzyme to lysosomes. Among the most notable features of propeptides is their ability to inhibit the activity of their cognate enzymes and that certain propeptides exhibit high selectivity for inhibition of the peptidases from which they originate.
The mechanism by which papain breaks peptide bonds involves the use of a catalytic dyad with a deprotonated cysteine.  A nearby Asn - 175 helps to orient the imidazole ring of His - 159 to allow it to deprotonate the catalytic Cys - 25. This cysteine then performs a nucleophilic attack on the carbonyl carbon of a peptide backbone. This forms a covalent acyl - enzyme intermediate and frees the amino terminus of the peptide. The enzyme is 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.