Cathepsin E is an intracellular, non-lysosomal aspartic protease expressed in a variety of cells and tissues. The protease has proposed physiological roles in antigen presentation by the MHC class II system, in the biogenesis of the vasoconstrictor peptide endothelin, and in neurodegeneration associated with brain ischemia and aging. Cathepsin E is the only A1 aspartic protease that exists as a homodimer with a disulfide bridge linking the two monomers. Like many other aspartic proteases, it is synthesized as a zymogen which is catalytically inactive towards its natural substrates at neutral pH and which auto-activates in an acidic environment. The overall structure follows the general fold of aspartic proteases of the A1 family, it is composed of two structurally similar beta barrel lobes, each lobe contributing an aspartic acid residue to form a catalytic dyad that acts to cleave the substrate peptide bond. The catalytic Asp residues are contained in an Asp-Thr-Gly-Ser/thr motif in both N- and C-terminal lobes of the enzyme. The aspartic acid residues act together to allow a water molecule to attack the peptide bond. One aspartic acid residue (in its deprotonated form) activates the attacking water molecule, whereas the other aspartic acid residue (in its protonated form) polarizes the peptide carbonyl, increasing its susceptibility to attack. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).