Prezentace aplikace PowerPoint

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MECHANISMS OF VISION
Jirí Wilhelm
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Rhodopsin
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One photon corresponds to 100 000 cGMP hydrolyzed
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Human retina contains about 100 milion rods and 3
milion cones. Maximum light absorption
of rhodopsin in rods is around 500 nm 3 classes
of cones absorb at 440 nm 530 nm 570 nm
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DAMPING OF THE RESPONSE
In daylight all rods are fully activated, thus in
order to distinguish contrast the response must
be dampened. Downregulation of signaling by G
proteins is generally produced by RGS proteins
(Regulators of G protein Signaling). In retina,
RGS proteins are represented by Phosducin. In
slight contrast to other RGS proteins Phosducin
preferes bg-subunits of the G protein over
a-subunit. Binding both bg- and a- subunits in
light-adapted rods blocks the formation of
abg complex and reduces the turnover of the
G-protein cycle. In the dark-adapted eye
Phosducin is inactive.
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Another damping protein
GARP, glutamic-acid-rich protein is contained in
rods but not cones receptors. PDE
phosphodiesterase ABCR Retinal ATP- binding ABC
transporter
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RESENSITIZATION OF VISUAL RESPONSE
The general step consists in G-protein
dephosphorylation. In retina it is regulated by
calcium and recoverin, a calcium-regulated
protein. The phosphatase dephosphorylating
rhodopsin is typical serine/ threonine
phosphatase. The dissociated all-trans-retinal
is reduced to all-trans-retinol, then it is
transfered to the neighboring retinal pigment
epithelial cells, where reisomerisation and
reoxidation recycles 11-cis-retinal. That
is returned to rod and cone outer segments, where
it recombines with opsin to form rhodopsin.