Hydrogen Bond between a Tyrosine Residue and the C‑Ring Propionate Has a Direct Influence
on Conformation and Absorption of the Bilin Cofactor in Red/Green
Cyanobacteriochromes
posted on 2021-02-01, 22:03authored bySusanne Altmayer, Sascha Jähnigen, Lisa Köhler, Christian Wiebeler, Chen Song, Daniel Sebastiani, Jörg Matysik
Cyanobacteriochromes
(CBCRs) are photoreceptors of the phytochrome
superfamily showing remarkable variability in the wavelengths of the
first electronic transitionsometimes denoted as Q bandcompared
to canonical phytochromes. Both classes carry the same cofactor, a
bilin, but the molecular basis for the wide variation of their absorption
properties is still a matter of debate. The interaction between the
cofactor and the surrounding protein moiety has been proposed as a
possible tuning factor. Here, we address the impact of hydrogen-bonding
interaction between the covalently bound tetrapyrrole cofactor (phycocyanobilin,
PCB) and a conserved tyrosine residue (Y302) in the second GAF (cGMP-specific phosphodiesterase, adenylyl cyclases,
and FhlA) domain of the red-/green-switching CBCR AnPixJ
(AnPixJg2). In the wild type, AnPixJg2 shows absorption maxima of
648 and 543 nm for the dark-adapted (Pr) and photoproduct (Pg) states,
respectively. The Y302F mutation leads to the occurrence of an additional
absorption band at 687 nm, which is assigned to a new spectroscopically
identified sub-state called PIII. Similar spectral changes
result upon mutating the Y302F-homologue in another representative
red-/green-switching CBCR, Slr1393g3. Molecular dynamics simulations
on the dark-adapted state suggest that the removal of the hydrogen
bond leads to an additional PCB sub-state differing in its A- and D-ring
geometries. The origin of the Q band satellite in the dark-adapted
state is discussed.