Wald's Visual cycle

Wald’s Visual Cycle: Retinol is transported to the retina via the circulation, where it moves into retinal pigment epithelial cells. There, retinol is esterified to form a retinyl ester that can be stored. When needed, retinyl esters are broken apart (hydrolyzed) and isomerized to form 11-cis-retinol, which can be oxidized to form 11-cis-retinal. 11-cis-retinal can be shuttled to the rod cell, where it binds to a protein called opsin to form the visual pigment, rhodopsin (also known as visual purple). Absorption of a photon of lightcatalyzes the isomerization of 11-cis-retinal to all-trans-retinal and results in its release. Thisisomerization triggers a cascade of events, leading to the generation of an electrical signal to theoptic nerve. The nerve impulse generated by the optic nerve is conveyed to the brain where it can be interpreted as vision. Once released, all-trans-retinal is converted to all-trans-retinol, which can be transported across the interphotoreceptor matrix to the retinal epithelial cell to complete the visualcycle. Wald, Hubbard, and Brown, working with several organic chemists, were able to show that onecisisomer, the 11-cis isomer, was precursor to all visual pigments. This isomer is not only bent, it issterically hindered and twisted, which makes it particularly light sensitive and optimal as a visualpigment chromophore. A secondcis isomer, 9-cis, will form a light-sensitive pigment, but with lower light sensitivity and a different absorption maximum. No other isomer works at all.