Molecular models predict light-induced glutamine tautomerization in BLUF photoreceptors

The recently discovered photoreceptor proteins containing BLUF (sensor of blue light using FAD) domains mediate physiological responses to blue light in bacteria and euglena. In BLUF domains, blue light activates the flavin chromophore yielding a signaling state characterized by a approximately 10 nm red-shifted absorption. We developed molecular models for the dark and light states of the BLUF domain of the Rhodobacter sphaeroides AppA protein, which are based on the crystal structures and quantum-mechanical simulations. According to these models, photon absorption by the flavin results in a tautomerization and 180 degree rotation of the Gln side chain that interacts with the flavin cofactor. This chemical modification of the Gln residue induces alterations in the hydrogen bond network in the core of the photoreceptor domain, which were observed in numerous spectroscopic experiments. The calculated electronic transition energies and vibrational frequencies of the proposed dark and light states are consistent with the optical and IR spectral changes observed during the photocycle. Light-induced isomerization of an amino acid residue instead of a chromophore represents a feature that has not been described previously in photoreceptors.