cysteinproteaser

Cysteine proteases are proteolytic enzymes in which a catalytic cysteine residue acts as a nucleophile to cleave peptide bonds. Most belong to the papain-like or related clans and share a catalytic mechanism that typically features a cysteine–histidine dyad, with an asparagine or aspartate residue stabilizing histidine in many enzymes. They often function best at acidic pH in lysosomes but can be secreted or active in other cellular compartments.

Major groups include papain-like cysteine proteases (the C1 family) which includes papain and mammalian cathepsins B, L, S, and K; other families include caspases (cysteine proteases that cleave after aspartate residues) and legumains. Papain-like proteases are found in animals, plants, and pathogens and can be secreted or lysosomal; caspases are key mediators of apoptosis and inflammatory signaling.

Biological roles vary: proteolysis in protein turnover, antigen processing, and tissue remodeling; proteases such as cathepsin K participate in bone matrix degradation; plant cysteine proteases contribute to seed germination and defense; pathogen-derived cysteine proteases facilitate invasion and nutrient acquisition.

Regulation occurs through zymogen forms, pH, and endogenous inhibitors. Cystatins and stefins are major families of cysteine protease inhibitors; dysregulation is linked to diseases including cancer, arthritis, and neurodegeneration. Inhibitors and selective inhibitors have been developed for research and potential therapies.