Activity coupling of Vibrio harveyi luciferase and flavin reductase (FRP): Oxygen as a probe

Bacterial luciferase: Molecular mechanisms and applications

Bacterial luciferase is a flavin-dependent monooxygenase which is remarkable for its distinctive feature in transforming chemical energy to photons of visible light. The bacterial luciferase catalyzes bioluminescent reaction using reduced flavin mononucleotide, long-chain aldehyde and oxygen to yield oxidized flavin, corresponding acid, water and light at λ_max around 490nm. The enzyme comprises of two non-identical α and β subunits, where α subunit is a catalytic center and β subunit is crucially required for maintaining catalytic function of the α subunit. The crystal structure with FMN bound and mutagenesis studies have assigned a number of amino acid residues that are important in coordinating critical reactions and stabilizing intermediates to attain optimum reaction efficiency. The enzyme achieves monooxygenation by generating C4a-hydroperoxyflavin intermediate that later changes its protonation status to become C4a-peroxyflavin, which is necessary for the nucleophilic attacking with aldehyde substrate. The decomposing of C4a-peroxyhemiacetal produces excited C4a-hydroxyflavin and acid product. The chemical basis regrading bioluminophore generation in Lux reaction remains an inconclusive issue. However, current data can, at least, demonstrate the involvement of electron transfer to create radical molecules which is the key step in this mechanism. Lux is a self-sufficient bioluminescent system in which all substrates can be recycled and produced by a group of enzymes from the lux operon. This makes Lux distinctively advantageous over other luciferases for reporter enzyme application. The progression of understanding of Lux catalysis is beneficial to improve light emitting efficiency in order to expand the robustness of Lux application.

Fre Is the Major Flavin Reductase Supporting Bioluminescence from Vibrio harveyi Luciferase in Escherichia coli

Unlike the vast majority of flavoenzymes, bacterial luciferase requires an exogenous source of reduced flavin mononucleotide for bioluminescence activity. Within bioluminescent bacterial cells, species-specific oxidoreductases are believed to provide reduced flavin for luciferase activity. The source of reduced flavin in Escherichia coli-expressing bioluminescence is not known. There are two candidate proteins potentially involved in this process in E. coli, a homolog of the Vibrio harveyi Frp oxidoreductase, NfsA, and a luxG type oxidoreductase, Fre. Using single gene knock-out strains, we show that deletion of fre decreased light output by greater than two orders of magnitude, yet had no effect on luciferase expression in E. coli. Purified Fre is capable of supporting bioluminescence in vitro with activity comparable to that with the endogenous V. harveyi reductase (Frp), using either FMN or riboflavin as substrate. In a pull-down experiment, we found that neither Fre nor Frp co-purify with luciferase. In contrast to prior work, we find no evidence for stable complex formation between luciferase and oxidoreductase. We conclude that in E. coli, an enzyme primarily responsible for riboflavin reduction (Fre) can also be utilized to support high levels of bioluminescence.