Papain Inhibitor peptide,(C19H29N7O6),a peptide with the sequence H-Gly-Gly-Tyr-Arg-OH,MW= 451.4,this peptide Inhibits the peptidase activity of Papain.Papain,also known as papaya proteinase I,is a cysteine protease enzyme present in papaya and mountain papaya. The mechanism by which it breaks peptide bonds involves deprotonation of Cys-25 by His-159. Asparagine-175 helps to orient the imidazole ring of His-159 to allow this deprotonation to take place. Cys-25 then performs a nucleophilic attack on the carbonyl carbon of a peptide backbone. This frees the amino terminal of the peptide,and forms a covalent acyl-enzyme intermediate. The enzyme is then 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) portion¹. Papain prefers to cleave at: (hydrophobic) -(Arg or Lys)- cleaves here -(not Val). Hydrophobic is Ala,Val,Leu,Ile,Phe,Trp,or Tyr². Papain can be used to dissociate cells in the first step of cell culture preparations. 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 can also be found as an ingredient in some toothpastes or mints as teeth-whitener.

References:
1. Rawlings ND,Barrett AJ (1994). "Families of cysteine peptidases". Meth. Enzymol. 244: 461–486.
2. Lopes MC,Mascarini RC,da Silva BM,Flório FM,Basting RT (2007). "Effect of a papain-based gel for chemomechanical caries removal on dentin shear bond strength". J Dent Child (Chic) 74 (2): 93–7.