Capturing enzyme structure prior to reaction initiation: tropinone reductase-II-substrate complexes.

To understand the catalytic mechanism of an enzyme, it is crucial to determine the crystallographic structures corresponding to the individual reaction steps. Here, we report two crystal structures of enzyme-substrate complexes prior to reaction initiation: tropinone reductase-II (TR-II)-NADPH and TR-II-NADPH-tropinone complexes, determined from the identical crystals. A combination of two kinetic crystallographic techniques, a continuous flow of the substrates and Laue diffraction measurements, enabled us to capture the transit structures prior to the reaction proceeding.