Bacterial luciferase 4a-hydroperoxyflavin intermediates: stabilization, isolation, and properties

The Sensitized Bioluminescence Mechanism of Bacterial Luciferase

After more than one-half century of investigations, the mechanism of bioluminescence from the FMNH₂ assisted oxygen oxidation of an aliphatic aldehyde on bacterial luciferase continues to resist elucidation. There are many types of luciferase from species of bioluminescent bacteria originating from both marine and terrestrial habitats. The luciferases all have close sequence homology, and in vitro, a highly efficient light generation is obtained from these natural metabolites as substrates. Sufficient exothermicity equivalent to the energy of a blue photon is available in the chemical oxidation of the aldehyde to the corresponding carboxylic acid, and a luciferase-bound FMNH-OOH is a key player. A high energy species, the source of the exothermicity, is unknown except that it is not a luciferin cyclic peroxide, a dioxetanone, as identified in the pathway of the firefly and the marine bioluminescence systems. Besides these natural substrates, variable bioluminescence properties are found using other reactants such as flavin analogs or aldehydes, but results also depend on the luciferase type. Some rationalization of the mechanism has resulted from spatial structure determination, NMR of intermediates and dynamic optical spectroscopy. The overall light path appears to fall into the sensitized class of chemiluminescence mechanism, distinct from the dioxetanone types.

Probing the Vibrio harveyi luciferase beta subunit functionality and the intersubunit domain by site-directed mutagenesis

While the critical role of the bacterial luciferase alpha subunit in catalysis has been amply documented, the beta subunit was only known to be involved in thermal stability and substrate binding. Two conserved histidyl residues at position 81 and 82 of the beta subunit of Vibrio harveyi luciferase were each mutated to an alanine, aspartate, or lysine to probe further the beta functionality. These mutations resulted in higher Km values for reduced riboflavin 5'-phosphate, less efficient oxidations of the aldehyde substrate, and decreased light-emitting activities. beta His82 appears to be significantly more critical than beta His81. For the beta His82-mutated luciferases, the maximal light intensities and total light outputs were reduced to 19-4% of that for the wild-type enzyme, and the values of Vmax/Km,flavin were decreased by 2-3 orders of magnitude. The reduced light emission activities for these mutated luciferases can be correlated to lower yields of the flavin 4a-hydroperoxide intermediate, reduced productions of the excited flavin emitter, and/or enhanced quenching of the emitter. The beta subunit and the conserved beta His82 in particular have thus been shown to be critical not only to flavin binding but also to catalytic characteristics of luciferase. The dimeric structure of luciferase is essential to its high catalytic efficiency. To characterize the intersubunit domain, three sets of single/double mutants were constructed, and the additivities of mutational effects were tested to screen for residues that could interact across the subunit interface.(ABSTRACT TRUNCATED AT 250 WORDS)

Lumazine protein and the excitation mechanism in bacterial bioluminescence

The spectral properties of lumazine protein and mixtures with the intermediates of the bacterial luciferase reaction, are reviewed. Measurements of fluorescence dynamics in particular have been employed with the aim of elucidating the mechanism by which lumazine protein functions in the bioluminescence of the bacteria of the type Photobacterium. The reaction of bacterial luciferase with its substrates produces bioluminescence emission with a spectral maximum at 496 nm. This spectrum is the same as the fluorescence of a luciferase flavin intermediate in the reaction, called the Fluorescent Transient. When lumazine protein is also present in the reaction; however, the bioluminescence emission now corresponds to the fluorescence of lumazine protein, which has a maximum at 475 nm. From measurements of the decay of fluorescence anisotropy of lumazine protein alone and in mixtures with the luciferase fluorescent transient, it is shown that a protein-protein complex is formed and that there is rapid energy transfer between the flavin on the luciferase and the lumazine derivative bound to its protein. An approximate calculation estimates the rate of this energy transfer to be faster than 10(9) s-1, and this would account for the efficient transfer of excitation from the flavin on the associated luciferase in the mixed protein bioluminescence reaction.

Dynamic fluorescence properties of bacterial luciferase intermediates

Three fluorescent species produced by the reaction of bacterial luciferase from Vibrio harveyi with its substrates have the same dynamic fluorescence properties, namely, a dominant fluorescence decay of lifetime of 10 ns and a rotational correlation time of 100 ns at 2 degrees C. These three species are the metastable intermediate formed with the two substrates FMNH2 and O2, both in its low-fluorescence form and in its high-fluorescence form following light irradiation, and the fluorescent transient formed on including the final substrate tetradecanal. For native luciferase, the rotational correlation time is 62 or 74 ns (2 degrees C) derived from the decay of the anisotropy of the intrinsic fluorescence at 340 nm or the fluorescence of bound 8-anilino-1-naphthalenesulfonic acid (470 nm), respectively. The steady-state anisotropy of the fluorescent intermediates is 0.34, and the fundamental anisotropy from a Perrin plot is 0.385. The high-fluorescence intermediate has a fluorescence maximum at 500 nm, and its emission spectrum is distinct from the bioluminescence spectrum. The fluorescence quantum yield is 0.3 but decreases on dilution with a quadratic dependence on protein concentration. This, and the large value of the rotational correlation time, would be explained by protein complex formation in the fluorescent intermediate states, but no increase in protein molecular weight is observed by gel filtration or ultracentrifugation. The results instead favor a proposal that, in these intermediate states, the luciferase undergoes a conformational change in which its axial ratio increases by 50%.