Propensity for Proton Relay and Electrostatic Impact of Protein Reorganization in Slr1694 BLUF Photoreceptor

Photoreceptor proteins play a vital role in a wide range of light-regulated processes. The formation of the light-adapted state of blue light using flavin (BLUF) photoreceptors is thought to involve rearrangements of hydrogen-bonding networks upon photoexcitation. Free energy simulations with partial charges corresponding to relevant ground and excited states of the Slr1694 BLUF domain characterize conformations prior to and following photoexcitation. The simulations indicate that Trp91 is thermodynamically favored to be in the active site, although it is also able to sample conformations outside the active site. For experimentally observed conformations of Trp91, Gln50 is thermodynamically favored to be oriented for a proton relay bridging Tyr8 and the flavin. When Trp91 is rotated such that it can donate a hydrogen bond to Gln50, as observed in other BLUF domains, the proton relay is not thermodynamically favored in the ground state, providing a possible explanation for the relatively fast photocycle of the Slr1694 BLUF domain. Photoexcitation to the locally excited (LE) state of the flavin induces the formation of the proton relay if it is not already formed. Electrostatically embedded time-dependent density functional theory calculations indicate that the proton relay reduces the energy gap between the LE state and the charge-transfer (CT) state associated with electron transfer from Tyr8 to the flavin. Although the CT state is higher in energy than the LE state prior to photoexcitation, the protein environment can reorganize in a manner that stabilizes the CT state so that it is lower than the LE state, enabling the LE to CT state transition. An electrostatic analysis identifies motions of individual residues, such as Arg65, that stabilize electron transfer from Tyr8 to the flavin. These conformational changes facilitate the critical proton-coupled electron transfer reaction in the BLUF photocycle.