Abstract
Blue Light Using Flavin (BLUF) domains are increasingly being adopted for use in optogenetic constructs. Despite this, much remains to be resolved on the mechanism of their activation. The advent of unnatural amino acid mutagenesis opens up a new toolbox for the study of protein structural dynamics. The tryptophan analogue, 7-aza-Trp (7AW) was incorporated in the BLUF domain of the Activation of Photopigment and pucA (AppA) photoreceptor in order to investigate the functional dynamics of the crucial W104 residue during photoactivation of the protein. The 7-aza modification to Trp makes selective excitation possible using 310 nm excitation and 380 nm emission, separating the signals of interest from other Trp and Tyr residues. We used Förster energy transfer (FRET) between 7AW and the flavin to estimate the distance between Trp and flavin in both the light- and dark-adapted states in solution. Nanosecond fluorescence anisotropy decay and picosecond fluorescence lifetime measurements for the flavin revealed a rather dynamic picture for the tryptophan residue. In the dark-adapted state, the major population of W104 is pointing away from the flavin and can move freely, in contrast to previous results reported in the literature. Upon blue-light excitation, the dominant tryptophan population is reorganized, moves closer to the flavin occupying a rigidly bound state participating in the hydrogen-bond network around the flavin molecule.
Highlights
Flavins are found in more than 370 enzymes[1] but only a few of them are photoactive[2,3]
The Förster energy transfer (FRET) efficiency-which is defined as the fraction of energy absorbed by the donor that is subsequently transferred to the acceptor depends on the distance between the donor and the acceptor[33,34] and can be calculated by Eq (1)
Where E is the FRET efficiency, R is the distance between the donor and the acceptor fluorophores and R0 is the distance between the two fluorophores in the case of 50% transfer efficiency
Summary
Flavins are found in more than 370 enzymes[1] but only a few of them are photoactive[2,3]. Three major families of photoreceptors which utilize flavin as a cofactor and whose functions are triggered by absorption of light are the photolyase/cryptochromes, the light oxygen voltage (LOV) domains and the blue light sensors using flavin (BLUF) proteins. In BLUF domains, blue light excitation results in a signalling state (light-adapted state) that is characterized by a reorganization of the hydrogen bond network around FAD and the Tyr-Gln-Trp (Met) tetrad (Fig. 1) This is revealed by a characteristic 10–15 nm red-shift of the first π → π* transition and a 20 cm−1 downshift of the flavin C4=O stretching vibration compared to the dark-adapted state[8,9]. Tryptophan fluorescence spectroscopic studies have suggested that Trp does not become fully solvent exposed as suggested by some crystal structures but were conflicting regarding the conformation of the Trp[18,30]
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