Rhodopsin regeneration is accelerated via noncovalent 11-cis retinal-opsin complex--a role of retinal binding pocket of opsin

The regeneration of bovine rhodopsin from its apoprotein opsin and the prosthetic group 11-cis retinal involves the formation of a retinylidene Schiff base with the epsilon-amino group of the active lysine residue of opsin. The pH dependence of a Schiff base formation in solution follows a typical bell-shaped profile because of the pH dependence of the formation and the following dehydration of a 1-aminoethanol intermediate. Unexpectedly, however, we find that the formation of rhodopsin from 11-cis retinal and opsin does not depend on pH over a wide pH range. These results are interpreted by the Matsumoto and Yoshizawa (Nature 258 1975] 523) model of rhodopsin regeneration in which the 11-cis retinal chromophore binds first to opsin through the beta-ionone ring, followed by the slow formation of the retinylidene Schiff base in a restricted space. We find the second-order rate constant of the rhodopsin formation is 6100+/-300 mol(-1) s(-1) at 25 degrees C over the pH range 5-10. The second-order rate constant is much greater than that of a model Schiff base in solution by a factor of more than 10(7). A previous report by Pajares and Rando (J Biol Chem 264 1989] 6804) suggests that the lysyl epsilon-NH(2) group of opsin is protonated when the beta-ionone ring binding site is unoccupied. The acceleration of the Schiff base formation in rhodopsin is explained by stabilization of the deprotonated form of the lysyl epsilon-NH(2) group which might be induced when the beta-ionone ring binding site is occupied through the noncovalent binding of 11-cis retinal to opsin at the initial stage of rhodopsin regeneration, followed by the proximity and orientation effect rendered by the formation of noncovalent 11-cis retinal-opsin complex.