Molecular dynamics simulations of bovine cathepsin B and its complex with CA074

To promote our better understanding of the dynamic stability of the bovine cathepsin B structure. which is characterized by an extra disulfide bond at Cys148-Cys252 from the other species, and of the binding stability of CA07 4 (a cathepsin B-specific inhibitor), molecular dynamics (MD) simulations w ere performed for the enzyme and its CA074 complex, assuming a system in aq ueous solution at 300 K. The MD simulation covering 400 ps indicated that t he existence of a Cys148-Cys252 disulfide bond increases the conformational flexibility of the occluding loop, although the conformational stability o f the overall structure is little affected. The structural characteristics of the complex elucidated by X-ray analysis were suggested to be also intri nsic and stable in the dynamic state: the hydrogen bonding/electrostatic in teractions between the main and side chains of CA074 and the Sn and Sn' sub sites of the enzyme were maintained throughout the MD simulation. Furthermo re, the simulation made clear that the binding of CA074 significantly restr icted the conformational flexibility of the substrate binding region, espec ially the occluding loop, of cathepsin B, Statistical analyses during thr s imulation suggest that the selectivity of CA074 for cathepsin B stems from the tight P1'-S1' and P2'-S2' interactions, assisted in particular by doubl e hydrogen bonds between the carboxyl two oxygens of the CA074 C-terminus a nd the imidazole NH groups of His110 and His111 residues.