Conserved tyrosine in phytochromes controls the photodynamics through steric demand and hydrogen bonding capabilities

Using ultrafast spectroscopy and site-specific mutagenesis, we demonstrate the central role of a conserved tyrosine within the chromophore binding pocket in the forward (Pr → Pfr) photoconversion of phytochromes. Taking GAF1 of the knotless phytochrome All2699g1 from Nostoc as representative member of phytochromes, it was found that the mutations have no influence on the early (100 ps). Thus, the steric demand, position and H-bonding capabilities of the identified tyrosine control the chromophore photoisomerization while leaving the excited state chromophore dynamics unaffected. In effect, this residue operates as an isomerization-steric-gate that tunes the excited state lifetime and the photoreaction efficiency by modulating the available space of the chromophore and by stabilizing the primary intermediate Lumi-R. Understanding the role of such a conserved structural element sheds light on a key aspect of phytochrome functionality and provides a basis for rational design of optimized photoreceptors for biotechnological applications. [Display omitted] •Mutagenesis and ultrafast spectroscopy address the role of a highly conserved tyrosine in phytochrome photoreception.•The mutations do not affect the excited state chromophore dynamics, but drastically impact the longer excited state decay dynamics.•Direct evidence that the conserved tyrosine residue acts as a photoisomerization steric gate controlled by the protein.•The new mechanistic insight puts the focus onto the dynamic aspects of the protein environment.•The dynamic role of the protein provides a principal direction for engineering optimized photoreceptors for biotechnology.