1991 – Koji Nakanishi

Why 11-cis retinal for vision?

The chromophores of all visual pigments, the rhodopsins, is the 11-cis form of retinal and analogs bound covalently to opsin, the receptor protein, via a protonated Schiff base linkage. Similarly, the pigments present in Halobacterium halobium, namely, bacteriorhodopsin (proton pump), halorhodopsin (chloride pump), and the two sensory rhodopsins (phototaxis receptors) contain the all-trans isomer of retinal bound to the apoprotein by a protonated Schiff base linkage. The chromophores are indeed uniquely designed to perform such vital functions. Why was retinal chosen as the chromophore of pigments, why is the chromophore a protonated Schiff base, and why 11-cis in some cases and all-trans in other cases? These aspects as well as new findings from bioorganic studies on retinal proteins will be presented and discussed.