Primary intermediates of photobleaching of rhodopsin

On absorption of light, rhodopsin is converted to a physiologically active intermediate, metarhodopsin II, at which state a binding site of G-protein (transducin) is exposed, through several thermolabile intermediates. The present article describes light-induced configurational and conformational changes of the chromophore of rhodopsin and the following thermal change of the chromophore-opsin interaction with a focus on the primary intermediates formed in the bleaching process. Rhodopsin is converted to photorhodopsin through an excited state of rhodopsin. Photorhodopsin has an absorption maximum at wavelengths longer than 560 nm indicating that it has a twisted all-frans retinylidene protonated Schiff base as its chromophore. A small rearrangement of some amino acid residues near the chromophore may induce the conversion from photorhodopsin to bathorhodopsin. In the conversion process from bathorhodopsin to lumirhodopsin, change in interaction between β-ionone ring region of the chromophore and its surrounding opsin was suggested from experimental results with several spectroscopic techniques. Cattle and squid hypsorhodopsins, which were found by irradiation at liquid helium temperature, were not detected by excitation of rhodopsin with 532 nm laser pulse at room temperature. However, squid hypsorhodopsin was observed by irradiation of rhodopsin with ultraviolet light (347 nm) or by irradiation of rhodopsin with an intense visible light (532 nm).