Structure of chymopapain at 1.7 A resolution

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Chymopapain is a cysteine proteinase from Carica papaya L. latex.
• Papaya latex contains several related proteinases, including papain, caricain, and glycyl endopeptidase.
• Understanding the structural basis of chymopapain function is crucial for comparative enzymology.

Purpose of the Study:

• To determine the X-ray structure of chymopapain.
• To compare the structure of chymopapain with other known papaya proteinases.
• To investigate structural similarities and differences, particularly in active site regions and loop conformations.

Main Methods:

• X-ray crystallography
• Molecular replacement methods
• Refinement of the protein structure to 1.7 A resolution

Main Results:

• The X-ray structure of chymopapain was determined, showing two moles of thiomethyl attached to free cysteines.
• Chymopapain backbone conformation is highly similar to papain, caricain, and glycyl endopeptidase.
• Differences were localized to two surface loop regions, distant from the active site.
• A conserved water network was observed between the enzyme's two domains.
• The S2 subsite of the active site groove showed no significant differences in specificity restrictions among the four proteinases.

Conclusions:

• Chymopapain shares a conserved overall structure with other papaya proteinases.
• Structural variations are primarily in non-critical surface regions.
• The active site and substrate-binding pocket exhibit conserved features, suggesting similar catalytic mechanisms and substrate specificities.