... reddish pigment that bleaches on exposure to light, which he called visual purple. Most rhodopsins contain retinal as the prosthetic group, but some have one of the other chromophores as shown below. For example freshwater fish have a rhodopsin containing 3,4-didehydroretinal, which has a red shifted UV absorption band. The opsins found in all organisms show strong homology for one another. All rhodopsins seem to be involved in light detection, with the notable exception of bacteriorhodopsin, which pumps protons using energy from light photons in order to generate ATP in anaerobic conditions i.e. is not a light sensing protein. Halobacteria do however have two sensory rhodopsins. Sensory rhodopsin I (archaeorhodopsin) has all trans retinal as the prosthetic group in its native state. It is photoisomerised by green-orange light (lmax = 587 nm) to the deprotonated 13-cis state (lmax = 370nm). ...

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