Binding of More Than One Retinoid to Visual Opsins

• Published in: Biophysical journal Vol. 99; no. 7; pp. 2366 - 2373
• Main Authors: Makino, Clint L., Riley, Charles K., Looney, James, Crouch, Rosalie K., Okada, Tetsuji
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 06.10.2010; Biophysical Society; The Biophysical Society
• Subjects: Affinity; Animals; Binding; Binding sites; Biochemistry; Biophysics; Cattle; Chemical compounds; Crystallography, X-Ray; Effectiveness; Membranes; Microspectrophotometry; Norisoprenoids - chemistry; Norisoprenoids - metabolism; Protein Binding; Proteins; Retinal Rod Photoreceptor Cells - metabolism; Retinoids - chemistry; Retinoids - metabolism; Rhodopsin - chemistry; Rhodopsin - metabolism; Rod Cell Outer Segment - metabolism; Segments; Spectroscopy, Imaging, and Other Techniques; Uptakes; Urodela - metabolism; Visual; X-rays
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/j.bpj.2010.08.003

Visual opsins bind 11- cis retinal at an orthosteric site to form rhodopsins but increasing evidence suggests that at least some are capable of binding an additional retinoid(s) at a separate, allosteric site(s). Microspectrophotometric measurements on isolated, dark-adapted, salamander photoreceptors indicated that the truncated retinal analog, β-ionone, partitioned into the membranes of green-sensitive rods; however, in blue-sensitive rod outer segments, there was an enhanced uptake of four or more β-ionones per rhodopsin. X-ray crystallography revealed binding of one β-ionone to bovine green-sensitive rod rhodopsin. Cocrystallization only succeeded with extremely high concentrations of β-ionone and binding did not alter the structure of rhodopsin from the inactive state. Salamander green-sensitive rod rhodopsin is also expected to bind β-ionone at sufficiently high concentrations because the binding site is present on its surface. Therefore, both blue- and green-sensitive rod rhodopsins have at least one allosteric binding site for retinoid, but β-ionone binds to the latter type of rhodopsin with low affinity and low efficacy.