Photoinduced electron transfer facilitates tautomerization of the conserved signaling glutamine side chain in BLUF protein light sensors

The BLUF domain (sensor of blue light using flavin adenine dinucleotide) from a bacterial photoreceptor protein AppA undergoes a cascade of chemical transformations, including hydrogen bond rearrangements around the flavin adenine dinucleotide (FAD) chromophore, in response to light illumination. These transformations are initiated by photoinduced electron and proton transfer from a tyrosine residue to the photoexcited flavin which is assisted by a glutamine residue. According to the recent studies, the proton-coupled electron transfer leads to formation of a radical-pair intermediate Tyr•···FADH• and a tautomeric EE form of glutamine in the ground electronic state. This intermediate is a precursor of the light-induced state of the BLUF photoreceptor implicated in biological signaling. In order to describe evolution of the radical pair, we computed reaction pathways on the ground state potential energy surface employing quantum-chemical calculations in the DFT PBE0/cc-pVDZ approximation for a molecular cluster mimicking the chromophore containing pocket of the AppA BLUF protein. We found a minimum-energy pathway comprised of the following consecutive reaction steps: (1) rotation of the imidic group of the EE glutamine side chain around the Cγ-Cδ bond; (2) flip of the OεH group and formation of the ZE form of the glutamine side chain; and (3) biradical recombination via coupled proton and electron transfer, leading to the ZZ form of the glutamine side chain. The potential-energy barriers for stages 1-3 do not exceed 9 kcal/mol. Energy barrier 3 describing the ZE to ZZ glutamine tautomerization is significantly smaller in the BLUF model than in isolated glutamine, since tautomerization in BLUF is facilitated by electron transfer and radical recombination. Thus, our study shows that tautomerization of the conserved glutamine is coupled to the light-induced electron transfer process in BLUF and, thus, is a viable candidate for the photoactivation mechanism which at present is very much debated.