= All-trans-retinal
\b[All-trans-retinal] is the geometrical form of <retinal> whose relevant chain double bonds have trans configurations. In rod vision it is formed from the bound <11-cis-retinal> chromophore after light absorption by <rhodopsin>. The change in geometry helps initiate receptor activation, while later processing of retinal participates in the <visual cycle>.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Page 739: 11-cis-to-all-trans photoisomerization, receptor change and chromophore dissociation.]{ref}
“All-trans” describes <cis-trans isomerism>. It does not mean that the entire molecule is a perfectly straight, rigid line, or that the double bonds have become single bonds. The carbon skeleton and elemental composition are preserved during the idealized isomerization; arrangement changes.
The visual system must subsequently restore a pigment-compatible retinal supply. After bleaching, the retinal-derived material undergoes biochemical processing involving the <photoreceptor> and <retinal pigment epithelium>; rebuilding rod <rhodopsin> requires <11-cis-retinal>, not merely any molecule labeled retinal. The <RPE65> knockout illustrates the dependence on chemical form: all-trans retinyl esters accumulated while 11-cis retinoids were absent and normal rod responses were lost.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract: accumulated all-trans retinyl esters and deficient 11-cis products.]{ref}
Retinyl esters are distinct chemical species from <all-trans-retinal>. The experiment demonstrates a disrupted conversion pathway; it should not be paraphrased as though the accumulated ester and the newly photoisomerized aldehyde were the same compound.
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