Molecular Mechanism of Spectral Tuning by Chloride Binding in Monkey Green Sensitive Visual Pigment

The
visual pigments of the cones perceive red, green, and blue
colors. The monkey green (MG) pigment possesses a unique Cl– binding site; however, its relationship to the spectral tuning in
green pigments remains elusive. Recently, FTIR spectroscopy revealed
the characteristic structural modifications of the retinal binding
site by Cl– binding. Herein, we report the computational
structural modeling of MG pigments and quantum-chemical simulation
to investigate its spectral redshift and physicochemical relevance
when Cl– is present. Our protein structures reflect
the previously suggested structural changes. AlphaFold2 failed to
predict these structural changes. Excited-state calculations successfully
reproduced the experimental red-shifted absorption energies, corroborating
our protein structures. Electrostatic energy decomposition revealed
that the redshift results from the His197 protonation state and conformations
of Glu129, Ser202, and Ala308; however, Cl– itself
contributes to the blueshift. Site-directed mutagenesis supported
our analysis. These modeled structures may provide a valuable foundation
for studying cone pigments.