| Abstract: | Interactive computer graphics was used as a tool in studying the cleavage mechanism of the model substrate Z-Phe-Phe-Leu-Trp by the zinc endopeptidase thermolysin. Two Michaelis complexes and three binding orientations of the tetrahedral intermediate to the crystal structure of thermolysin were investigated. Our results indicate that a Michaelis complex, which does not involve coordination of the scissile peptide to the zinc, is consistent with available experimental data and the most plausible of the two complexes. A tetrahedral intermediate complex wherein the two oxygens of the hydrated scissile peptide straddle the zinc in a bidentate fashion results in the most favorable interactions with the active site. The preferred tetrahedral intermediate and Michaelis complex provide a rationalization for the published substrate data. A trajectory for proceeding from the Michaelis complex to the tetrahedral intermediate is proposed. This trajectory involves a simultaneous activation of the zinc-bound water molecule concurrent with attack on the scissile peptide. A detailed ordered product release mechanism is also presented. These studies suggest some modifications and a number of extensions to the mechanism proposed earlier [Kester, W. R., & Matthews, B. W. (1977) Biochemistry 16, 2506; Holmes, M. A., & Matthews, B. W. (1981) Biochemistry 20, 6912]. The binding mode of the thermolysin inhibitor N-(1-carboxy-3-phenylpropyl)-L-leucyl-L-tryptophan [Monzingo, A. F., & Matthews, B. W. (1984) Biochemistry (preceding paper in this issue)] is compared with that of the preferred tetrahedral intermediate, providing insight into this inhibitor design. |
