Firefly luciferase catalyzes the highly efficient emission of yellow-green light from substrate luciferin by a sequence of reactions that require Mg-ATP and molecular oxygen. We previously reported [Branchini, B. R., Magyar, R. A., Marcantonio, K. M., Newberry, K. J., Stroh, J. G., Hinz, L. K., and Murtiashaw, M. H. (1997) J. Biol. Chem. 272, 19359-19364] that 2-(4-benzoylphenyl)thiazole-4-carboxylic acid (BPTC), a firefly luciferin analogue, was a potent photoinactivation reagent for luciferase. We identified a luciferase peptide 244HHGF247, the degradation of which was directly correlated to the photooxidation process. We report here the construction and purification of wild-type and mutant luciferases H244F, H245F, H245A, and H245D. The results of photoinactivation and kinetic and bioluminescence studies with these proteins are consistent with His245 being the primary functional target of BPTC-catalyzed enzyme inactivation. The possibility that His245 is oxidized to aspartate during the photooxidation reaction was supported by the extremely low specific activity ( approximately 300-fold lower than WT) of the H245D mutant. Using the crystal structures of luciferase without substrates [Conti, E., Franks, N. P., and Brick, P. (1996) Structure 4, 287-298] and the functionally related phenylalanine-activating subunit of gramicidin synthetase 1 [Conti, E., Stachelhaus, T., Marahiel, M. A., and Brick, P. (1997) EMBO J. 16, 4174-4183] as a starting point, we have performed molecular-modeling studies and propose here a model for the luciferase active site with substrates luciferin and Mg-ATP bound. We have used this model to provide a structure-based interpretation of the role of 244HHGF247 in firefly bioluminescence.

Beetle luciferases (including those of the firefly) use the same luciferin substrate to naturally display light ranging in color from green (lambda(max) approximately 530 nm) to red (lambda(max) approximately 635 nm). In a recent communication, we reported (Branchini, B. R., Murtiashaw, M. H., Magyar, R. A., Portier, N. C., Ruggiero, M. C., and Stroh, J. G. (2002) J. Am. Chem. Soc. 124, 2112-2113) that the synthetic adenylate of firefly luciferin analogue D-5,5-dimethylluciferin was transformed into the emitter 5,5-dimethyloxyluciferin in bioluminescence reactions catalyzed by luciferases from Photinus pyralis and the click beetle Pyrophorus plagiophthalamus. 5,5-Dimethyloxyluciferin is constrained to exist in the keto form and fluoresces mainly in the red. However, bioluminescence spectra revealed that green light emission was produced by the firefly enzyme, and red light was observed with the click beetle protein. These results, augmented with steady-state kinetic studies, were taken as experimental support for mechanisms of firefly bioluminescence color that require only a single keto form of oxyluciferin. We report here the results of mutagenesis studies designed to determine the basis of the observed differences in bioluminescence color with the analogue adenylate. Mutants of P. pyralis luciferase putative active site residues Gly246 and Phe250, as well as corresponding click beetle residues Ala243 and Ser247 were constructed and characterized using bioluminescence emission spectroscopy and steady state kinetics with adenylate substrates. Based on an analysis of these and recently reported (Branchini, B. R., Southworth, T. L., Murtiashaw, M. H., Boije, H., and Fleet, S. E. (2003) Biochemistry 42, 10429-10436) data, we have developed an alternative mechanism of bioluminescence color. The basis of the mechanism is that luciferase modulates emission color by controlling the resonance-based charge delocalization of the anionic keto form of the oxyluciferin excited state.

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Beetle luciferases (including those of the firefly) use the same luciferin substrate to naturally display light ranging in color from green (lambda(max) similar 530 nm) to red (lambda(max) similar 635 nm). The original mechanism of bioluminescence color determination advanced by White and co-workers was based on the concept that the keto and enol tautomers of the emitter oxyluciferin produce red and green light, respectively. Alternatively, McCapra proposed that color variation is associated with conformations of the keto form of excited-state oxyluciferin. We have prepared the adenylate of D-5,5-dimethylluciferin and shown that it is transformed into the putative emitter 5,5-dimethyloxyluciferin in bioluminescence reactions catalyzed by luciferases from Photinus pyralis and the green-emitting click beetle. 5,5-Dimethyloxyluciferin is constrained to exist in the keto form and fluoresces in the red. However, bioluminescence spectra revealed that green light emission was produced by the firefly enzyme and red light was observed with the click beetle protein. These results, augmented with steady-state kinetic studies, may be taken as the first experimental support for McCapra's mechanism of firefly bioluminescence color or any other proposal that requires only a single keto form of oxyluciferin.