This gene encodes a lysosomal aspartyl protease composed of a protein dimer of disulfide-linked heavy and light chains, both produced from a single protein precursor. This proteinase, which is a member of the peptidase A1 family, has a specificity similar to but narrower than that of pepsin A. Transcription of this gene is initiated from several sites, including one that is a start site for an estrogen-regulated transcript. Mutations in this gene are involved in the pathogenesis of several diseases, including breast cancer and possibly Alzheimer disease.
Cathepsin-D is an aspartic protease that depends critically on protonation of its active site Asp residue and gets activated at pH 5 in endosome of hepatocytes where it degrades insulin. Along with Asp-protonation, lower pH also leads to conformational switch in cathepsin-D : the N terminal segment of the protease moves out of the active site as pH drops. 
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