Retinaldehyde is the prosthetic group of light-sensitive opsin proteins and is essential for initiating vision.
Visual pigments and their formation
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Retinaldehyde forms rhodopsin in rods and iodopsin in cones. Each cone contains one type of opsin and is sensitive to one color.
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In retinal pigment epithelium, all-trans-retinol is isomerized to 11-cis-retinol and then oxidized to 11-cis-retinaldehyde.
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11-cis-retinaldehyde reacts with a lysine residue in opsin to form rhodopsin.
Response to light
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Light absorption isomerizes retinaldehyde from 11-cis to all-trans and changes the conformation of opsin.
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Bathorhodopsin, the initial excited form, appears within picoseconds. Subsequent conformational changes produce metarhodopsin II, which initiates a guanine nucleotide amplification cascade and a nerve impulse.
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The final hydrolysis step releases all-trans-retinaldehyde and opsin.
Vitamin A deficiency
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Initiation of the visual cycle depends on the availability of 11-cis-retinaldehyde and therefore vitamin A.
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Deficiency impairs adaptation to darkness and the ability to see in poor light.