[1] University of Maine</ref>.]]
This hydrolytic enzyme is able to break peptide bonds through the deprotonation of Cys-25 by His-159, with the help of Aspartate-125, which stabilizes the Histadine ring in order for this deprotonation to take place. The Cys-25 residue is then able to perform a nucleophilic attack on the carbonyl carbon of the peptide backbone, freeing the amino terminal of the peptide, forming a covalent intermediate. Next, the enzyme is deacylated by water, and the carboxy-terminal portion of the peptide is released.
History:
Papain was originally discovered during the colonial period in Congo. The native inhabitants discovered that wrapping their elephant meat in papaya leaves helped to tenderize the meat. While they did not know the direct cause, this was when the proteolytic enzyme was first discovered. The active binding site of this enzyme was first discovered by Drenth et al., through the crystallographic analysis of the enzymes structure.
Composition of Papain:
Papain consists primarily of carbon, nitrogen, sulfur, and oxygen. The can be view, as carbon is labled gray, oxygen red, nitrogen blue, and sulfur yellow. There are three that contribute to the structure of Papain. These bonds are indicated by their yellow color, as they link particular cysteine residues. Throughout this enzyme, there are specific charged residues, indicating the most polar portions of the molecule. These can be visualized, as the cationic residues are blue in color, while the anionic residues are red. Partially charged residues are simply lighter in color. It is interesting to note that nearly all of these charged residues are located on the outer portion of the molecule in order to interact with the surrounding environment.
Hydrophobicity and Hydrophilicity:
These charged residues contribute to the overall hydrophobicity (water hating) and hydrophilicity (water loving) portions of the enzyme, as the are a huge factor in determining protein folding. As mentioned before, the charged, hydrophilic, portions of the enzyme are primarily located on the out portion of the molecule. The polar amino acids are indicated by a purple color, while the hydrophobic residues are gray.
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