Influence of the cofactor structure on the photophysical processes initiating signal transduction in a phototropin-derived LOV domain

Adaptive Laboratory Evolution of Flavin Functionality Identifies Dihydrolipoyl Dehydrogenase as One of the Critical Points for the Activity of 7,8-Didemethyl-Riboflavin as a Surrogate for Riboflavin in Escherichia coli

Riboflavin analogs lacking one methyl group (7α or 8α) can still serve as a surrogate for riboflavin in riboflavin-deficient microorganisms or animals. The absence of both methyl groups at once completely abolishes this substitution capability. To elucidate the molecular mechanisms behind this phenomenon, we performed an adaptive laboratory evolution experiment (in triplicate) on an E. coli strain auxotrophic for riboflavin. As a result, the riboflavin requirement of the E. coli strain was reduced ~10-fold in the presence of 7,8-didemethyl-riboflavin. The whole genome sequencing of E. coli strains isolated from three experiments revealed two mutation hotspots: lpdA coding for the flavoenzyme dihydrolipoyl dehydrogenase (LpdA), and ompF coding for the major outer membrane protein. In order to investigate the essentiality of flavin's methyl groups to LpdA, the wild type and mutant variants of lpdA were cloned. At least two lpdA mutants increased the fitness of E. coli, and when 7,8-didemethyl-flavin was added to the growth medium, the increase was significant. To the best of our knowledge, an adaptive laboratory evolution experiment running in triplicate as a tool for the identification of mutation hotspots in the genome of microorganisms exposed to metabolic stress challenges is described here for the first time.

Lightening flavin by amination for fluorescent sensing

Monitoring of reactive oxygen species (ROS), such as O2˙-, etc., in organisms is of great significance, not only for their essential role in biological processes, but their excessive production may also result in many diseases. Flavin (FL) is a fluorophore that naturally exists in flavoenzymes, and its fluorescent emission (FE) becomes negligible when reduced. This enables the application of FL derivatives as fluorescent sensors for ROS. We presented a theoretical investigation to address the impact of amino substitution on the photophysical properties of aminoflavins (AmFLs). Resulting from the interplay of electronic and positional effects, amination at C8 enhances the electronic coupling between the ground state and the first singlet excited state by enlarging the adiabatic energy change of the electronic transitions and the emission transition dipole moments, weakens the vibronic coupling by decreasing the contribution of isoalloxazine to the frontier molecular orbitals, redshifts the absorption band, and enhances the fluorescent emission drastically in 8AmFL. The theoretically estimated fluorescent emission intensity of 8AmFL is ∼40 times that of FL, suggesting its potential application as a fluorescent sensor.

Extreme dependence of Chloroflexus aggregans LOV domain thermo- and photostability on the bound flavin species

Light-oxygen-voltage (LOV) domains are common photosensory modules that found many applications in fluorescence microscopy and optogenetics. Here, we show that the Chloroflexus aggregans LOV domain can bind different flavin species (lumichrome, LC; riboflavin, RF; flavin mononucleotide, FMN; flavin adenine dinucleotide, FAD) during heterologous expression and that its physicochemical properties depend strongly on the nature of the bound flavin. We show that whereas the dissociation constants for different chromophores are similar, the melting temperature of the protein reconstituted with single flavin species varies from ~ 60 °C for LC to ~ 81 °C for FMN, and photobleaching half-times vary almost 100-fold. These observations serve as a caution for future studies of LOV domains in non-native conditions yet raise the possibility of fine-tuning various properties of LOV-based fluorescent probes and optogenetic tools by manipulating the chromophore composition.