Red, Orange, Green: Light- and Temperature-Dependent Color Tuning in a Cyanobacteriochrome

March 24, 2020


Red, Orange, Green: Light- and Temperature-Dependent Color Tuning in a Cyanobacteriochrome


David Buhrke, Giovanni Battocchio, Svea Wilkening, Matthew Blain-Hartung, Tobias Baumann, Franz-Josef Schmitt, Thomas Friedrich, Maria-Andrea Mroginski, Peter Hildebrandt






Cyanobacteriochromes (CBCRs) are photoreceptor proteins that photoconvert between two parent states and thereby regulate various biological processes. An intriguing property is their variable ultraviolet–visible (UV–vis) absorption that covers the entire spectral range from the far-red to the near-UV region and thus makes CBCRs promising candidates for optogenetic applications. Here, we have studied Slr1393, a CBCR that photoswitches between red- and green-absorbing states (Pr and Pg, respectively). Using UV–vis absorption, fluorescence, and resonance Raman (RR) spectroscopy, a further orange-absorbing state O600 that is in thermal equilibrium with Pr was identified. The different absorption properties of the three states were attributed to the different lengths of the conjugated π-electron system of the phycocyanobilin chromophore. In agreement with available crystal structures and supported by quantum mechanics/molecular mechanics (QM/MM) calculations, the most extended conjugation holds for Pr whereas it is substantially reduced in Pg. Here, the two outer pyrrole rings D and A are twisted out of the plane defined by inner pyrrole rings B and C. For the O600 state, the comparison of the experimental RR spectra with QM/MM-calculated spectra indicates a partially distorted ZZZssa geometry in which ring A is twisted while ring D and the adjacent methine bridge display essentially the same geometry as Pr. The quantitative analysis of temperature-dependent spectra yields an enthalpy barrier of ∼30 kJ/mol for the transition from Pr to O600. This reaction is associated with the movement of a conserved tryptophan residue from the chromophore binding pocket to a solvent-exposed position.




Circular dichroism, Protein structure, Biochemistry