Binding of more than one retinoid to visual opsins.

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

SUBMITTER: Makino CL 

PROVIDER: S-EPMC3042582 | biostudies-literature | 2010 Oct

REPOSITORIES: biostudies-literature