Sw. Lin et al., MECHANISMS OF SPECTRAL TUNING IN BLUE CONE VISUAL PIGMENTS - VISIBLE AND RAMAN-SPECTROSCOPY OF BLUE-SHIFTED RHODOPSIN MUTANTS, The Journal of biological chemistry, 273(38), 1998, pp. 24583-24591
Spectral tuning by visual pigments involves the modulation of the phys
ical properties of the chromophore (Il-cis-retinal) by amino acid side
chains that compose the chromophore-binding pocket, We identified 12
amino acid residues in the human blue cone pigment that might induce t
he required green-to blue opsin shift. The simultaneous substitution o
f nine of these sites in rhodopsin (M86L, G90S, A117G, E122L, A124T, W
265Y, A292S, A295S, and A299C) shifted the absorption maximum from 500
to 438 nn, accounting for 2,830 cm(-1), or 80%, of the opsin shift be
tween rhodopsin and the blue cone pigment. Raman spectroscopy of mutan
t pigments shows that the dielectric character and architecture of the
chromophore-binding pocket are specifically altered, An increase in t
he number of dipolar side chains near the protonated Schiff base of re
tinal increases the ground-excited state energy gap via long range dip
ole-dipole Coulomb interaction. In addition, the W265Y substitution ca
uses a decrease in solvent polarizability near the chromophore ring st
ructure. Finally, two substitutions on transmembrane helix 3 (A117G an
d E122L) act in combination with the other substitutions to alter the
binding-pocket structure, resulting in stronger interaction of the pro
tonated Schiff base group with the surrounding dipolar groups and the
counterion, Taken together, these results identify the amino acid side
chains and the underlying physical mechanisms responsible for a major
ity of the opsin shift in blue visual pigments.