Photopigment

Photopigment refers to a light-absorbing molecule that enables vision and other light-detecting processes. It is a chromoprotein composed of an opsin protein bound covalently to a light-sensitive chromophore, typically 11-cis-retinal derived from vitamin A. When a photon is absorbed, the chromophore photoisomerizes to all-trans-retinal, triggering a conformational change in the opsin and initiating a signal transduction cascade.

In the vertebrate retina, the primary photopigments are rhodopsin in rods and photopsins in cones. Rods, which enable vision in dim light, express rhodopsin and are highly sensitive. Cones express three opsins (S, M, L) with peak sensitivities corresponding to short, middle, and long wavelengths, enabling color discrimination in brighter light.

Phototransduction begins when an activated opsin stimulates a G protein (transducin), which activates a phosphodiesterase that lowers cyclic GMP levels. The fall in cGMP closes cGMP-gated cation channels, causing hyperpolarization of the photoreceptor and modulation of glutamate release onto bipolar cells.

Photopigments must be regenerated after activation. Retinal isomerase activity in the retinal pigment epithelium converts all-trans-retinal back to 11-cis-retinal, restoring the pigment for another cycle of photon detection.

Non-image forming photopigments, such as melanopsin in intrinsically photosensitive retinal ganglion cells, contribute to circadian entrainment and pupil responses. Spectral tuning of photopigments arises from variations in the opsin protein, shifting the absorption maxima to suit different lighting environments and species.