Molecular mechanism of the dark-state recovery in BLUF photoreceptors

Photoactivation of the bacterial photoreceptor BLUF is achieved by a unique tautomerization and rotation of the conserved Gln residue. To identify interactions controlling the lifetime of the high-energy ZZ-imide tautomer, we consider thermal dark-state recovery of the BLUF photoreceptor BlrB using MD and QM/MM calculations. We found that protonation/deprotonation of Gln51 by protonated His73 via Tyr9 was energetically favorable. The rate-limiting steps correspond to rearrangements of hydrogen bonds and rotation of the protonated Gln51 side chain. Notably, strengthening the hydrogen bond between protonated Gln51 and flavin 04 plays a central role in extending the lifetime of the photoactivated state. (C) 2017 Elsevier B.V. All rights reserved.