1. Chemical structures of the vitamin A derivatives that participate in the visual cycle. The chemical structure of retinol (vitamin A) is shown at the top. Retinol is converted to the aldehyde 11-cis-retinal in the retinal pigment epithelium. On transport to the photoreceptor outer segments, this 11-cis-retinal (called the chromophore) forms a covalent (Schiff-base) linkage to a lysine residue at position 296 in rod opsin to produce a light-sensitive molecule. The structure of that covalent bond is represented in the middle of this figure. A photon of light will convert the 11-cis-retinal to all-trans-retinal, creating metarhodopsin I. A proton is lost from metarhodopsin I to form metarhodopsin II, which is the form of photoactivated, deprotonated opsin that can activate transducin. The all-trans-retinal detaches from opsin at an exponential rate with a half-life of about 4 to 7 minutes The opsin molecule is then able to regenerate by incorporating a new molecule of 11-cis-retinal. The molecule of all-trans-retinal must travel back to the retinal pigment epithelium to be reconverted to 11-cis-retinal.
Source: Retinitis Pigmentosa and Stationary Night Blindness, The Online Metabolic and Molecular Bases of Inherited Disease
Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: Accessed: November 11, 2017
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