Cproteinases

Cproteinases, commonly referred to as cysteine proteases, are a diverse class of proteolytic enzymes that catalyze the hydrolysis of peptide bonds using a catalytic cysteine residue. In most cysteine proteases, the active site features a catalytic dyad or triad in which the cysteine thiolate acts as the nucleophile and is activated by a histidine residue; an asparagine or other residue often stabilizes the histidine through hydrogen bonding.

They are grouped largely into families; the largest is the papain-like family (C1), which includes papain from papaya and numerous mammalian cathepsins (B, L, S, K), characterized by broad substrate specificity and activity across acidic to neutral pH. Other cysteine proteases include calpains (Ca2+-dependent cytosolic proteases) and legumains (asparaginyl endopeptidases; also called cysteine proteases with a distinct specificity).

Localization and roles: Lysosomal cathepsins participate in protein turnover and antigen processing; calpains modulate cytoskeletal dynamics and signaling; plant cysteine proteases contribute to germination and defense. In pathology, dysregulated cysteine protease activity is linked to cancer invasion, inflammatory diseases, arthritis, and neurodegeneration. Some cysteine proteases from parasites, such as cruzipain from Trypanosoma cruzi, are important for pathogen biology and are therapeutic targets.

Regulation and inhibition: activity is tightly controlled by pH, zymogen activation, and endogenous inhibitors called cystatins; synthetic inhibitors and small molecules are explored for therapy and research.

Industrial use: plant cysteine proteases (papain, bromelain) are used in food processing, meat tenderizing, and various biotechnological applications.