kathepsin

Kathepsin is not a widely recognized term in current biochemical nomenclature. It is most likely a misspelling or mistaken reference to cathepsins, a family of proteases that function primarily as lysosomal enzymes. Cathepsins are encoded by a set of CTS genes and include multiple members such as cathepsins B, D, L, K, and S, among others. They are categorized as cysteine proteases (and, in some cases, other classes such as aspartyl proteases) and are conserved across many species.

Biochemically, cathepsins are synthesized as inactive zymogens and are activated within the acidic environment of lysosomes or secretory compartments. They often require proteolytic processing to reach their active forms and rely on trafficking signals to reach lysosomes via mannose-6-phosphate receptors. Their optimum activity generally occurs at acidic pH, consistent with lysosomal localization, though some forms can be secreted and act in the extracellular space.

Functionally, cathepsins participate in intracellular protein turnover and degradation of misfolded or damaged proteins. They play roles in antigen processing for MHC class II presentation and in remodeling of the extracellular matrix. Cathepsin K, for example, is highly expressed in osteoclasts and is implicated in bone resorption; cathepsins S and L contribute to immune processing. Dysregulation of cathepsins has been linked to diseases such as cancer invasion and metastasis, osteoporosis, and inflammatory or neurodegenerative conditions.

Medical relevance and research interest focus on cathepsin inhibitors as potential therapeutics, including treatment strategies for osteoporosis and certain cancers. Mutations in CTSK cause pycnodysostosis, a bone disorder, while mutations in CTSC (cathepsin C) can cause Papillon-Lefèvre syndrome. See also cathepsin family and lysosomal proteases.