Reductive and oxidative half-reactions of morphinone reductase from Pseudomonas putida M10: A kinetic and thermodynamic analysis

The reaction of morphinone reductase (MR) with the physiological reductant NADH and the oxidizing substrate codeinone has been studied by multiple and single wavelength stopped-flow spectroscopy. Reduction of the enzyme with NADH proceeds in two kinetically resolvable steps. In the first step, the oxidized enzyme forms a charge-transfer intermediate with NADH. The charge-transfer complex is characterized by an increase in absorbance at long wavelength (540 to 650 nm), and its rate of formation is dependent on substrate concentration and is controlled by a second-order rate constant of 4.8 x 105 M-1 s-1 at pH 7.0 and 5°C. In the second step, the enzyme- bound flavin is reduced to the dihydroflavin form. The rate of flavin reduction (23.4 s-1 at pH 7.0 and 5°C) is independent of substrate concentration and is observed as a monophasic decrease in absorbance at 462 nm. The oxidative half-reaction proceeds in three kinetically resolvable steps. The first is due to the formation of a reduced enzyme-codeinone charge-transfer complex and is observed at long wavelength (about 650 nm). The rate of charge-transfer complex formation is dependent on codeinone concentration and is controlled by a second-order rate constant of 11.5 x 103 M-1 s-1 at pH 7.0 and 5°C. The second step represents flavin reoxidation and is observed at 462 (absorption increase) and 650 nm (absorption decrease) and progresses with a rate (about 45 s-1) which is independent of codeinone concentration. The third step is observed as a further small increase in absorbance at 462 nm and proceeds with a rate of about 2.5 s-1. This step most likely represents hydrocodone release from the oxidized enzyme. Analysis of the temperature dependence of the reductive half-reaction has enabled calculation of the entropic and enthalpic contributions for charge-transfer formation, charge-transfer decay (yielding free enzyme and substrate), and electron transfer to the enzyme-bound FMN, and the construction of a partial energy profile for the reaction catalyzed by MR. The reaction scheme and redox properties of MR are compared with those described previously for the closely related flavoprotein, old yellow enzyme. Although common features are identified, there are notable differences in the kinetic and redox properties of the two enzymes.