The chromophore binding space of opsin

Abstract

THE rate of the reaction between opsin and 11-cis retinal, that gives the visual pigment rhodopsin, is measured in the presence and absence of structural analogues of 11-cis retinal, the polyene side chain of which is varied systematically. A decrease in regeneration rate can be interpreted as competition with 11-cis retinal for the binding site on the protein. This approach seems to provide information on the molecular dimensions of the chromophore binding space of opsin. Rotmans noticed that all-trans retinal does not inhibit the reaction between opsin and 11-cis retinal leading to rhodopsin¹. More recently Mastumoto and Yoshizawa² have reported that β-ionone, a compound carrying the six-membered ring configuration of retinal but with a shorter side chain, does inhibit the rate of this reaction. The surprising discrepancy between these two observations has led us to study more systematically the effects on the regeneration rate of a number of 11-cis retinal analogues, in which either the polyene side chain or the aldehyde group has been modified. We report here results that suggest that opsin possesses a specific chromophore binding space, one dimension of which is determined by the longest axis of 11-cis retinal, measured from the hydrophobic binding site onwards.