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Molecular Control
of Visual Pigment Properties
Ebrey, Thomas1
and Takahashi, Yusuke1
University of Washington, Seattle, WA1
Abstract-
Visual pigments have three important physiological properties. First,
their spectra must be modulated so that the chromophore, a Schiff base
of retinal, absorbs at appropriate wavelengths. Second photoisomerization,
which initiates visual excitation, should occur with high quantum efficiency.
Third, that isomerization should have a low probability occurring in
the dark, so that the system has low noise. It has been shown that vertebrate
visual pigments fall into five distinct families. In at least four of
the families the retinal is attached to the protein as a protonated
Schiff base. Moreover, the pK of the Schiff base should be above 9 so
that the Schiff base is always protonated in the dark. Protonation is
necessary so that a Schiff base of retinal can absorb "in
the visible". Next, plausible changes
to amino acids mostly in the binding site of retinal are shown to be
able to regulate the absorption spectra in the Short Wavelength Sensitive
2 (SWS2) class of visual pigments; their spectra are closest to that
of a model protonated Schiff base of retinal, 440 nm. Plausible chains
of amino acid changes are then used to "walk"
to the spectra of the Rhodopsin 2 (RH2) pigments, then the RH1 pigments,
and finally the Mid and Long Wavelength Sensitive (M/LWS) pigments.
With regard to the second property, photoisomerization, an examination
of the photointermediates of some pigments strongly suggests that the
beta-ionone ring cannot move early in the bleaching sequence, throwing
doubt upon recent experiments which suggest that it does move. Most
interestingly, it appears that modulating the absorption spectrum with
polar groups along the retinal does not alter significantly the quantum
efficiency for photoisomerization. Finally with respect to the third
property, thermal noise, Barlow, Birge and co-workers have pointed out
that the pK of the Schiff base of rhodopsin is much higher than the
pK required just to make sure the Schiff base stays protonated. They
have suggested that this high pK may be related to the low thermal rate
of isomerization of rhodopsin. We will discuss a mechanism for how this
might occur, based on our studies of the controlling factors in the
thermal isomerization of bacteriorhodopsin.
Keywords: visual
pigment, photoisomerization, retinal, Schiff base
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