= Rhodopsin
\b[Rhodopsin] is the light-sensitive <G protein-coupled receptor> of vertebrate rod <photoreceptors>. It consists of the <protein> <opsin> bound to the light-absorbing molecule <11-cis-retinal>. Absorbing a <photon> changes the retinal configuration toward <all-trans-retinal>, initiating structural changes that activate the signaling partner <transducin>. The <visual cycle> supplies fresh <11-cis-retinal> so the pigment can be regenerated.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Page 739, introduction: retinal photoisomerization, G-protein activation and pigment regeneration.]{ref}
The 2000 bovine <rhodopsin> structure, obtained by <X-ray crystallography> at $2.8$ angstrom resolution, showed seven membrane-spanning <alpha helices> surrounding the bound chromophore. The dark-state retinal helps stabilize an inactive receptor configuration. The structure made a molecular question visible: how can a small rearrangement of a bound light absorber alter the intracellular surface that engages another <protein>?<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Abstract, structure-determination section and Figures 2–3.]{ref}
The distinction between <opsin> and functional pigment matters experimentally. In mice lacking <RPE65>, investigators detected rod <opsin> but no normal <rhodopsin>, together with loss of rod responses on <electroretinography>. Having the receptor chain is insufficient when the required retinal supply is disrupted.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract: retinoid composition, rod opsin, absent rhodopsin and rod electroretinographic function in Rpe65-deficient mice.]{ref}
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