Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Substitution reactions at carbonyl groups
Papain

Papain from the milky juice of the melon tree (papaya) belongs to a family that includes plant Enzymes (such as bromelain from pineapple and ficin from fig fruit) as well as certain enzymes of bacterial origin. Apparently, the Active Site of another bacterial proteinase, Clostripain from Clostridium histolyticum, is similar to that of papain, although in its Specificity this enzyme resembles Trypsin [40].

In papain, the nucleophile involved in the catalytic process appears to be the —SH group [41]. X-Ray Diffraction structural studies have shown that the —SH group is adjacent to an imidazole group, which could act as a proton donor for the leaving group [42]. Thus, despite certain differences, There are many common features in the MECHANISM OF ACTION of protein-hydrolyzing enzymes across all these families.

The inactive form of papain, which can be regarded as a kind of proenzyme, is activated by thiols. Apparently, propapain is a structural isomer of papain that undergoes intramolecular thiol-disulfide exchange, leading to The formation of the active —SH group [43].



Last update: 06/08/2026

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