Bioactivity of Visual Pigments with Sterically Modified Retinal Analogs

Studies of several retinal analogs and the rhodopsin pigments incorporating them, lead to further clarification of the structural requirement of the retinal chromophore in the photoactivation process of rhodopsin. The data with a pigment incorporating an acyclic retinal show that the cyclohexene moiety is not required for formation of a stable pigment, which, however, has a reduced photoactivity. Our data also show that extra methyl groups at 13- and 14-positions of 11-cis retinal reduce the rate of retinal binding, photoactivity, and stability of the pigments. These data, taken together, give rise to a clear picture about the binding environment of the retinal chromophore and how retinal interacts with and activates the protein following its photoisomerization. The opsin/11-cis retinal complex has evolved into an ideal system, which is capable of converting photoenergy into Meta-II formation with high efficiency, critical for visual transduction.