Hydrogen Bond Switching
among Flavin and Amino Acids
Determines the Nature of Proton-Coupled Electron Transfer in BLUF
Photoreceptors
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Abstract
BLUF domains are flavin-binding photoreceptors that can
be reversibly
switched from a dark-adapted state to a light-adapted state. Proton-coupled
electron transfer (PCET) from a conserved tyrosine to the flavin that
results in a neutral flavin semiquinone/tyrosyl radical pair constitutes
the photoactivation mechanism of BLUF domains. Whereas in the dark-adapted
state PCET occurs in a sequential fashion where electron transfer
precedes proton transfer, in the light-adapted state the same radical
pair is formed by a concerted mechanism. We propose that the altered
nature of the PCET process results from a hydrogen bond switch between
the flavin and its surrounding amino acids that preconfigures the
system for proton transfer. Hence, BLUF domains represent an attractive
biological model system to investigate and understand PCET in great
detail