Difference in the Charge-Separation Energetics between Distinct Conformers in the PixD Photoreceptor

Blue light using flavin (BLUF) domain proteins are photoreceptors in various organisms. The PixD BLUF domain can adopt two conformations, W91(out) and W91(in), with Trp91 either proximal or distal to flavin (FMN). Using a quantum mechanical/molecular mechanical/polarizable continuum model approach, the energetics of charge-separated and biradical states in the two conformations were investigated. In the W91(out) conformation, the charge-separated state (FMN center dot-) is more stable than the photoexcited state (FMN*), whereas it is less stable due to an electrostatic repulsive interaction with the Ser28 side chain in the W91(in) conformation. This leads to a lower activation energy for the charge separation in the W91(out) conformation, resulting in a faster charge separation compared to that in the W91(in) conformation. In the W91(out) conformation, the radical state (FMNH center dot) is more stable than FMN center dot- and forms from FMN center dot-, leading to reorientation of the Gln50 side chain adjacent to FMN and formation of a hydrogen bond between Gln50 and FMN. Subsequently, a signaling state forms through charge recombination. In contrast, in the W91(in) conformation, FMN center dot- cannot proceed further, returning to the dark-adapted state, as FMNH center dot is less stable. Thus, formation of the signaling state exclusively occurs in the W91(out) conformation.

Automated summary

This study investigates the energetics of charge-separated and biradical states in the PixD BLUF domain using a quantum mechanical/molecular mechanical/polarizable continuum model approach. The results show that charge separation is more stable and faster in the W91(out) conformation, leading to the formation of a signaling state through hydrogen bonding and side chain rearrangement. In contrast, the W91(in) conformation exhibits less stability in charge separation and cannot proceed to the signaling state.