Cathepsins

Cathepsins are involved in numerous normal and pathological processes. Cathepsin A is a member of the serine carboxypeptidase family. It is a multifunctional enzyme that possesses deaminidase and esterase activities at neutral pH and carboxypeptidase activity at acidic pH. Also known as a protective protein, its association with beta-galactosidase (beta-gal) and neuraminidase is essential for beta-gal stability and neuraminidase activation in lysosomes. Inherited cathepsin A deficiency causes the lysosomal storage disorder galactosialidosis. DPPI (cathepsin C) is a cysteine protease that sequentially removes dipeptides from the free N-termini of proteins and peptides. DPPI plays a role in lysosomal degradation. It also functions as a key enzyme in the activation of granule serine proteases in cytotoxic T lymphocytes and natural killer cells (granzymes A and B), mast cells (tryptase and chymase), and neutrophils (cathepsin G and elastase) by removing their N-terminal activation dipeptides. The lysosomal aspartic protease, cathepsin D, is essential for proteolysis of proteins regulating cell growth and tissue homeostasis. Cathepsin D is involved in breast and prostate cancer. Cathepsin X (also known as cathepsin Z and P) is a cysteine protease in the papain family. It is ubiquitously expressed in human tissues and conserved in many species. The nematode enzyme is apparently involved in molting of third stage larvae.