Fig. 1: Photocycle of bacterial phytochromes as derived from spectroscopic data28.
From: Ultrafast proton-coupled isomerization in the phototransformation of phytochrome
In Agp2, Pfr is the stable dark state. The biliverdin chromophore (BV) is bound to a cysteine residue, with a ZZZssa and ZZEssa configuration in Pr and Pfr, respectively. In both states, the chromophore is protonated at all four pyrrole nitrogens. The curved solid and dotted arrows refer to thermal and photochemical reactions, respectively. Upon photoexcitation, isomerization at the methine bridge between rings C and D occurs on a picosecond timescale, corresponding to a ZZZssa → ZZEssa and ZZEssa → ZZZssa conversion between Pr and Pfr, respectively. The last step in the reaction cascade from Pfr to Pr is associated with a structural change of the tongue segment in the PHY domain (depicted in Fig. 2), a phytochrome-specific building block of the photosensor. States highlighted by the white and grey backgrounds represent the BV chromophore in the ZZZssa and ZZEssa states, respectively. ms, millisecond; ps, picosecond; μs, microsecond.
