MECHANISMS OF VISION

Jiří Wilhelm

Rhodopsin

One photon corresponds to 100 000 cGMP hydrolyzed

Human retina contains about100 milion rods and3 milion cones.Maximum light absorption ofrhodopsin in rods is around500 nm3 classes of cones absorb at440 nm530 nm570 nm

DAMPING OF THE RESPONSE

In daylight all rods are fully activated, thus in order to distinguishcontrast the response must be dampened.

Downregulation of signaling by G proteins is generally producedby 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-richprotein is contained in rodsbut not cones receptors.

PDE – phosphodiesteraseABCR – Retinal ATP-binding ABC transporter

RESENSITIZATION OF VISUAL RESPONSE

The general step consists in G-protein dephosphorylation. In retinait 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, thenit is transfered to the neighboring retinal pigment epithelial cells,where reisomerisation and reoxidation recycles 11-cis-retinal. That isreturned to rod and cone outer segments, where it recombines withopsin to form rhodopsin.