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.
Matsumoto
Hiroyuki
H
Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA. hiroyuki-matsumoto@ouhsc.edu
Yoshizawa
Tôru
T
D000818:Animals; D001665:Binding Sites; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D012172:Retinaldehyde / Q000737:chemistry* / Q000378:metabolism; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism; D017299:Rod Opsins / Q000737:chemistry* / Q000378:metabolism; D012545:Schiff Bases; D013053:Spectrophotometry
D017299:Rod Opsins; D012545:Schiff Bases; D012243:Rhodopsin; D012172:Retinaldehyde