protective protein/cathepsin C

abbr.: PPCA; a lysosomal multifunctional protein that has separate protective properties (for lysosomal β-galactosidase and neuraminidase) and catalytic properties (being a serine protease). The gene at 20q13.1 encodes a glycoprotein of 480 residues, which is processed to form a 32 kDa polypeptide joined by a disulfide bridge to another of 20 kDa. A serine on the heavy chain, and a histidine and an aspartate on the light chain constitute the catalytic triad. It is active as cathepsin A at acid pH, and as an amidase (e.g. for substance P, endothelin I, and neurokinin) and an esterase at neutral pH. At least a dozen point mutations in the gene are known to cause galactosialidosis, in which there is a triple enzyme deficiency.

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