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

Area of Science:

• Biochemistry
• Photochemistry
• Molecular Biology

Background:

• Blue light using flavin (BLUF) domain proteins are crucial photoreceptors across diverse organisms.
• The PixD BLUF domain exhibits two distinct conformations: W91_out and W91_in, differing in the proximity of Tryptophan 91 (Trp91) to the flavin mononucleotide (FMN) chromophore.

Purpose of the Study:

• To investigate the energetics of charge-separated and biradical states within the two PixD BLUF domain conformations.
• To elucidate the conformational dynamics governing signal transduction in BLUF proteins.

Main Methods:

• Employed a quantum mechanical/molecular mechanical/polarizable continuum model (QM/MM/PCM) approach.
• Analyzed the stability and energetic landscapes of photoexcited, charge-separated, and radical states.

Main Results:

• In the W91_out conformation, the charge-separated state (FMN^•-) is more stable than the photoexcited state (FMN*).
• In the W91_in conformation, electrostatic repulsion from Ser28 destabilizes FMN^•-, hindering charge separation.
• The W91_out conformation facilitates formation of the radical state (FMNH^•) and subsequent signaling state via charge recombination, while W91_in leads to reversion to the dark-adapted state.

Conclusions:

• The W91_out conformation is essential for initiating the signaling cascade in PixD BLUF proteins.
• Conformational dynamics, specifically the W91 position, dictate the functional outcome of light activation in BLUF photoreceptors.