Cathepsin X; the only papain-like lysosomal cysteine peptidase exhibiting carboxymonopeptidase activity. It can also act as a carboxydipeptidase, like cathepsin B, but has been shown to preferentially cleave substrates through a monopeptidyl carboxypeptidase pathway. The propeptide region of cathepsin X, the shortest among papain-like peptidases, is covalently attached to the active site cysteine in the inactive form of the enzyme. Little is known about the biological function of cathepsin X. Some studies point to a role in early tumorigenesis. A more recent study indicates that cathepsin X expression is restricted to immune cells suggesting a role in phagocytosis and the regulation of the immune response.
Comment:The catalytic residues of C1 family peptidases are Cys and His, forming a catalytic dyad. Two other residues play an important role in catalysis: a Gln preceding the catalytic Cys, believed to help in the formation of the oxyanion hole; and an Asn residue which orients the imidazolium ring of the catalytic His.