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. Papain is a relatively heat-resistant enzyme, with an optimal temperature range of 60 to 70 °C. Papain prefers to cleave after an arginine or lysine preceded by a hydrophobic unit (Ala, Val, Leu, Ile, Phe, Trp, Tyr) and not followed by a valine. Uses Papain breaks down tough meat fibres, and has been used since before European contact to tenderise meat eaten in its native South America. Meat tenderisers in powder form with papain as an active component are widely sold and the culinary use of papaya peel has featured in research papers. Papain can be used to dissociate cells in the first step of cell culture preparations. A ten-minute treatment of small tissue pieces (less than 1 mm3) will allow papain to begin cleaving the extracellular matrix molecules holding the cells together. After ten minutes, the tissue should be treated with a protease inhibitor solution to stop the protease action. Left untreated, papain activity will lead to complete lysis of the cells. The tissue must then be triturated (passed quickly up and down through a Pasteur pipette) 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. Papain is added to some toothpastes and mint sweets as a tooth whitener. Its whitening effect is minimal, because the papain is present in low concentrations and is quickly diluted by saliva. It would take several months of use to have a noticeable effect. Papain is the main ingredient of Papacarie, a gel used for chemomechanical dental caries removal. It does not require drilling and does not interfere in the bond strength of restorative materials to dentin. Papain has been known to interfere with