Papain is a cysteine hydrolase enzyme that is found naturally occurring in Papaya(Carica papaya).  Papain has various uses including cell isolation, protein structural studies, cleaving antibodies, and various others.  Papain is formed from a peptide that is 212 amino acids in length.  It consists of 25%  and 21% .The structure also consists of three  between cysteine residues. Another important residue to the structure is Cysteine 25 which has a sulfhydryl group that plays a major role in the active site activity[[1]].
 
Possible ligand