MECHANISMS OF VISION Jiří Wilhelm

Rhodopsin

One photon corresponds to cGMP hydrolyzed

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

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  -subunits of the G protein over  -subunit. Binding both  - and  - subunits in light-adapted rods blocks the formation of  complex and reduces the turnover of the G-protein cycle. In the dark-adapted eye Phosducin is inactive.

Another damping protein GARP, glutamic-acid-rich protein is contained in rods but not cones receptors. PDE – phosphodiesterase ABCR – Retinal ATP- binding ABC transporter

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.