Photopsin
Photopsins
are the
photoreceptor
pigments
found in the
cone cells
of the
retina
that are the basis of
color vision
. Photopsins are very close analogs of the
visual purple
rhodopsin
that is used in dark vision. Photopsins consist of a
protein
called
opsin
and a bound
chromophor
, the
retinal
. Different opsins differ in a few
amino acids
and absorb light at different
wavelengths
as retinal-bound pigments. Opsins are
G protein-coupled receptors
.
Isomerization
of 11-
cis
-retinal into 11-
trans
-retinal by
light
induces a conformational change in the protein that activates the associated
G protein
and triggers a
second messenger
cascade. In
humans
there are three different opsins that form the pigments photopsin I, II, and II. They are called
porphyropsin
,
iodopsin
and
cyanopsin
, respectively . These photopsins have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet light (photopsin III).
George Wald
got the
1967
Nobel Prize in Physiology or Medicine
for his experiments in the
1950s
that showed the difference in absorbance by these photopsins (see image).
See also
Rhodopsins
, the pigment for
monochromatic
vision in the dark.
External links
Rhodopsin and the eye
, an excellent summary with pictures.
Advanced Light And Vision Concepts
, an extensive overview.
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