Ultrafast, remote-controlled protonation reaction enables structural changes in a phytochrome.

In photoactive proteins, coupling between the chromophore and protein matrix is exquisitely tuned. Proton transfer reactions can mediate this coupling, as in proton-coupled electron transfer and excited-state proton transfer. Additional mechanisms involving proton dislocations may exist but remain undiscovered. Here, we present a femtosecond crystallographic movie of the phytochrome from Deinococcus radiodurans. The structures reveal a space-conserving mechanism for rotation of the D-ring in the excited state. We observe rearrangement of a conserved hydrogen bond network within 300 fs, which precedes the isomerization reaction of the chromophore. Aided by molecular modeling and independently confirmed by femtosecond infrared spectroscopy, we attribute these changes to a protonation shift of the strictly conserved histidine-260. Although this histidine lies close to the photoexcited π-orbitals of the chromophore, it is not directly part of them. We propose that this "remote-controlled" proton transfer relays photoexcitation near-instantaneously to the protein matrix. This mechanism may be widely used to transduce cofactor signals to their hosting enzymes.