National Research Council of Canada. NRC Biotechnology Research Institute
The ability of the lysosomal cysteine protease cathepsin B to function as a peptidyldipeptidase (removing C-terminal dipeptides) has been attributed to the presence of two histidine residues (His110 and His111) present in the occluding loop, an extra peptide segment located in the primed side of the active-site cleft. Whereas His111 is unpaired, His110 is present as an ion pair with Asp22 on the main body of the protease. This ion pair appears to act as a latch to hold the loop in a closed position. The exopeptidase activity of cathepsin B, examined using quenched fluorescence substrates, was shown to have a 20-fold preference for aromatic side chains in the P′3 position relative to glutamic acid as the least favourable residue. Site-directed mutagenesis demonstrated that His111 makes a positive 10-fold contribution to the exopeptidase activity, whereas His110 is critical for this action with the Asp22—His110 ion pair stabilizing the electrostatic interaction by a maximum of 13.9kJ/mol (3.3kcal/mol). These studies showed that cathepsin B is optimized to act as an exopeptidase, cleaving dipeptides from protein substrates in a successive manner, because of its relaxed specificity in P′3 and its other subsites.
Biochemical Journal361, no. 3 (1 February 2002): 613–619.