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Publication Abstract from PubMed

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, beta-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 beta-ionones per rhodopsin. X-ray crystallography revealed binding of one beta-ionone to bovine green-sensitive rod rhodopsin. Cocrystallization only succeeded with extremely high concentrations of beta-ionone and binding did not alter the structure of rhodopsin from the inactive state. Salamander green-sensitive rod rhodopsin is also expected to bind beta-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 beta-ionone binds to the latter type of rhodopsin with low affinity and low efficacy.

Binding of more than one retinoid to visual opsins.,Makino CL, Riley CK, Looney J, Crouch RK, Okada T Biophys J. 2010 Oct 6;99(7):2366-73. PMID:20923672^[3]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.