The three-dimensional structures of all papaya proteinases are
very similar to that of papain "stricto sensu". The
elements of secondary structure, the conformation around the
active site, and the asymmetrical environment of the aromatic
residue, were found to be almost identical.
The secondary structure in combination with the presence
of four disulfide bonds and a multitude of hydrogen-hydrophobic
and electrostatic interactions makes these proteolytic enzymes
particularly resistant to denaturation by heat and chaotropic
For reasons not yet fully understood, the chymopapain fraction
is much more resistant to acid denaturation than the three
other proteinases. This makes the refined papain from ENZYMASE
efficient in wide range of pH.