Studies on luciferase from Photobacterium phosphoreum. VIII. FMN-H2O2 initiated bioluminescence and the thermodynamics of the elementary steps of the luciferase reaction.:VIII. FMN-H<sub>2</sub>O<sub>2</sub> Initiated Bioluminescence and the Thermodynamics of the Elementary Steps of the Luciferase Reaction

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Bacterial luciferase from Photobacterium phosohoreum was found to produce biolumi-nescence on reaction with FMN and H2O2 in the presence of aldehyde. This lumi-nescence is presumably produced by the same X1 intermediate as that found in FMNH2-O2 initiated luminescence. From the ratio of the light intensities of the FMN-H2O2 initiated and FMNH2-O2 initiated reactions, we calculated the association constant of the reaction, luciferase+FMN+H2O2_??_X1, and estimated its temper-ature dependence. From these results we calculated the thermodynamic parameters of the reaction, luciferase+FMNH2+O 2_??_X1. We found that the free energy level of X1 is only 3.2 kcal below that of free FMN and H2O2. We also estimated the thermodynamic parameters of other steps of the luminescent reaction. The values obtained showed that the formation of X1 from luciferase, FMNH2 and O2 involves a positive entropy change, but the intermediate is in a state stabilized against decomposition. Results also suggest a considerable degree of electronic rearrangement on formation of the excited-state molecule from the X1-aldehyde complex.

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