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Papain, also known as papaya proteinase I, is a cysteine protease () enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis). It is the namesake member of the papain-like protease family. 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, known as the papain-like protease family, 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 papain precursor protein contains 345 amino acid residues, and consists of a signal sequence (1-18), a propeptide (19-133) and the mature peptide (134-345).

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Catalytic triad

A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, lipases and β-lactamases). An acid-base-nucleophile triad is a common motif for generating a nucleophilic residue for covalent catalysis. The residues form a charge-relay network to polarise and activate the nucleophile, which attacks the substrate, forming a covalent intermediate which is then hydrolysed to release the product and regenerate free enzyme.

Cysteine protease

Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal Chunder Roy in 1873, the first cysteine protease to be isolated and characterized was papain, obtained from Carica papaya. Cysteine proteases are commonly encountered in fruits including the papaya, pineapple, fig and kiwifruit. The proportion of protease tends to be higher when the fruit is unripe.

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The fragment antigen-binding region (Fab region) is a region on an antibody that binds to antigens. It is composed of one constant and one variable domain of each of the heavy and the light chain. The variable domain contains the paratope (the antigen-binding site), comprising a set of complementarity-determining regions, at the amino terminal end of the monomer. Each arm of the Y thus binds an epitope on the antigen. In an experimental setting, Fc and Fab fragments can be generated in the laboratory.

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