The binding mode of an E-64 analog to the active site of cathepsin B

Two binding modes of the isobutyl-NH-Eps-Leu-Pro inhibitor to cathepsin B have been proposed. Molecular docking using an empirical force field was carried out to distinguish between the two modes. The search began with manual docking, followed by random perturbations of the docking conformation and cycles of Monte Carlo minimization. Finally, molecular dynamics was carried out for the most favorable docking conformations. The present calculations predict that the isobutyl-NH-Eps-Leu-Pro inhibitor preferentially binds to the S' rather than the S subsites of cathepsin B. The S' binding mode prediction is supported by the X-ray crystal structure of cathepsin B bound to a closely related ethyl-O-Eps-Ile-Pro inhibitor, which was found to bind in the S'subsite with the C-terminal epoxy ring carbon making a covalent bond to the sulfur atom of Cys29. This agreement, in turn, validates our docking strategy. Furthermore, the calculations provide evidence that the dominant contribution to the total stabilization energy of the enzyme-inhibitor complex stems from the strong electrostatic interaction between the negatively charged C-terminal carboxylate group of the ligand and the positively charged imidazolium rings of His110 and His111. The latter are stabilized and held in an optimal orientation for interactions with the C-terminal end of the ligand through a salt bridge between the side chains of His110 and Asp22. By comparison with the crystal structure, some insight into the specificity of the epoxyldipeptide family towards cathepsin B inhibition has been extracted. Both the characteristics of the enzyme (e.g. subsite size and hydrophobicity) as well as the nature of the inhibitor influence the selectivity of an inhibitor towards an enzyme.