STRUCTURE OF CHYMOPAPAIN AT 1.7-ANGSTROM RESOLUTION

The X-ray structure of chymopapain, a cysteine proteinase isolated fro m the latex of the fruits of Carica papaya L., has been determined by molecular replacement methods and refined to a conventional R factor o f 0.19 for all observed reflections in the range from 9.5 to 1.7 Angst rom resolution. The crystals used in this study contained a unique mol ecular species of chymopapain with two moles of thiomethyl attached to the two free cysteines per mole of enzyme. A comparison is made with the other known papaya proteinase X-ray structures: papain, caricain, and glycyl endopeptidase. Their backbone conformations are extremely s imilar except for two loop regions. Both regions are located at the su rface of the protein and far away of the active site cleft. In each X- ray structure the same water network was found at the interface betwee n the two domains of the enzyme. A close examination of the active sit e groove showed that the specificity restrictions dictated by the S2 s ubsite did not differ significantly among the four proteinases.