1 · Dark state · 11-cis retinal at rest
Before a photon arrives, the 11-cis retinal at the centre of rod rhodopsin is bent, and like a bent key it holds the protein in the off conformation.Retinal rod cell outer segments are densely packed with a membrane protein called rhodopsin — a G-protein coupled receptor (GPCR) with a special small molecule embedded in its centre: 11-cis retinal.This retinal is the aldehyde form of vitamin A (retinol). Before it is loaded into opsin, RPE cells use LRAT to esterify retinol into retinyl ester for storage, then RPE65 converts all-trans retinyl ester into 11-cis retinol, which is oxidised to 11-cis retinal and sent back to the rod. Until a photon arrives, the bent key holds the protein in the off conformation.
Key chemical geometry:
· The 11-cis bend makes the molecule fold into an S shape
· It fits exactly into the opsin internal cavity, like a key in a lock
· When the key is bent → the whole protein is in the off conformation
One rod cell has about 10⁸ rhodopsin molecules — it's the most sensitive sensor in the body, detecting single photons.
Vitamin A deficiency is rare in industrialised countries, but is the world's leading preventable cause of blindness — about 250,000-500,000 children in developing countries lose their sight to it annually (WHO). This is why international organisations list biannual high-dose vit A supplementation for 6-59 month children as a public health intervention.