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Papain is a cysteine protease, also known as papaya proteinase I,from the peptidase C1 family. Naturally found in the latex of the papaya fruit, one of the most common uses of papain is as a meat tenderizer because of its ability to hydrolyze esters and amides.^[1] Another common use is as a digestive aid. Papaya is commonly referenced as a preferred fruit for those suffering from gastroesophageal reflux disease due to its ability to help the the stomach with digestion of complex proteins.

History

Papain's enzymatic use was first discovered in 1873 by G.C. Roy. Roy published his results in the Calcutta Medical Journal in the article, "The Solvent Action of Papaya Juice on Nitrogenous Articles of Food."^[2] In 1879, papain was named officially by Wurtz and Bouchut, who managed to partially purify the product from the sap of papaya.^[2]. It wasn't until the mid-twentieth century that the complete purification and isolation of papain was achieved. In 1968, Drenth et al. determined the structure of papain by x-ray crystallography, making it the second enzyme whose structure was successfully determined by x-ray crystallography. Additionally, papain was the first cysteine protease to have its structure identified. ^[2] In 1984, Kamphuis et al. determined the geometry of the active site, and the three-dimensional structure was visualized to a 1.65 Angstrom solution.^[3] Today, studies continue on the stability of papain, involving changes in environmental conditions, in addition to testing of inhibitors such as phenylmethanesulfonylfluoride (PMSF), TLCK, TPCK, aplh2-macroglobulin, heavy metals, AEBSF, antipain, cystatin, E-64, leupeptin, sulfhydryl binding agents, carbonyl reagents, and alkylating agents.^[2]

Papain is a relatively simple enzyme. It consists of only one chain of 212 residues with three disulfide bonds.^[3] A modified cysteine residue with a sulhydryl group, , is necessary for the activity of the enzyme.^[4]

Papain will digest most protein substrates more extensively than the pancreatic proteases. Papain exhibits broad specificity, cleaving peptide bonds of basic amino acids, leucine, or glycine. It also hydrolyzes esters and amides. Papain exhibits a preference for an amino acid bearing a large hydrophobic side chain at the P2 position. It does not accept Val at the P1' position. 1