papainin

Papainin is a class of small cysteine protease inhibitors that were first characterized in the latex of certain plant species, most notably in the fruit of the papaya. These proteins exert their inhibitory activity against papain‑type cysteine proteases, enzymes that cleave peptide bonds through a cysteine residue in the active site. By binding to the catalytic site of the protease, papainin blocks substrate access and thereby reduces proteolytic activity.

Structurally, papainins are short polypeptides, typically ranging from 70 to 100 amino acids in length. They contain multiple disulfide bonds that confer stability to the protein in the harsh extracellular environment where proteases are often active. The inhibition mechanism involves a combination of steric hindrance and charge interactions that lock the protease in a non‑catalytic conformation. Sequence analysis shows that papainins share limited homology with the cystatin family, suggesting a distinct evolutionary origin for their mode of action.

Biologically, papainins are thought to protect plants against predation and tissue damage by limiting the activity of endogenous and exogenous proteases. In addition to their natural role, papainins have been explored as research tools in protease assays, as potential therapeutics to modulate digestive or inflammatory protease activity in human disease, and as additives in formulations that require protease inhibition.

Research into papainin variants continues to investigate their specificity spectrum, kinetic parameters, and potential for engineering more robust inhibitors for pharmaceutical and industrial applications.