Crystal structures of pro-cathepsin B and cathepsin B. (A) The crystal structure of pro-cathepsin B (PDB:1MIR) (Cygler et al., 1996). The 62-residue pro-peptide region (magenta) is folded along the surface of mature cathepsin B (cygans) and covers the active site cleft. (B) The crystal structure of cathepsin B (PDB:1HUC) (Musil et al., 1991). The occluding loop (108–122) is shown in magenta on the surface of cathepsin B (orange). Two cysteine residues from the occluding loop form a di-sulfide bridge. Residues Cys29 and His199 form the catalytic dyad, and two histidines, His110 and His111, are positioned within the active site cleft. These functional residues are represented as green ball-and-stick model on the molecular surface model of cathepsin B. The S1, S2, S3 and S2′ active pocket are highlighted with bold labeling. The figure is prepared with PYMOL using reported structures in PDB.