Two other Cys residues at positions 109 and 114 on the amino acid chain reside close to teach other in three dimensional space, however the distance between their sulfur atoms is 3.53 Å which is too large for the formation of a proper disulfide bond. The structure also has four hydrogen bonds between the Asp residues of the active site and the surrounding residues. A distinguishing factor of Cathepsin E in comparison with the structure of Cathepsin D and BACE1 can be seen at the formation of an extra hydrogen bond between the Asp 96 and Ser 99 residues, and absence of a hydrogen bond with Leu/Met at Asp 281.[1]