Binding of more than one retinoid to visual opsins

Biophys J. 2010 Oct 6;99(7):2366-73. 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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cattle
  • Crystallography, X-Ray
  • Microspectrophotometry
  • Norisoprenoids / chemistry
  • Norisoprenoids / metabolism
  • Protein Binding
  • Retinal Rod Photoreceptor Cells / metabolism*
  • Retinoids / chemistry
  • Retinoids / metabolism*
  • Rhodopsin / chemistry
  • Rhodopsin / metabolism*
  • Rod Cell Outer Segment / metabolism
  • Urodela / metabolism


  • Norisoprenoids
  • Retinoids
  • Rhodopsin
  • beta-ionone