How do opsin proteins control the spectral sensitivity of 11-cis-retinal?

[Drakkith]

Hey all. I had a question. What makes the different cone cells in your eye respond to different wavelengths of light? I know that light strikes Retinal and causes it to undergo photoisomerisation, which starts the chain that leads to you seeing something. Do the different cone cells contain different retinal that is more sensative to other wavelengths or something?

 

[Andy Resnick]

There are three different isomers of rhodopsin (in human vision):

http://www.kettering.edu/visitors/storydetail.jsp?storynum=174

http://neuro.cjb.net/content/19/24/10716.full

 

[Drakkith]

Ah, I see now. Thanks Andy.

 

[DavidMcC]

There is some confusion in the literature concerning "rhodopsin", and I think it is best to use the definition given here: http://en.wikipedia.org/wiki/Rhodopsin"
The point is not entirely trivial, as humans have no gene for synthesising the rhodopsin defined in the above link (we eat it as vitamin A, almost unchanged), but we do have genes for making the cone opsins covered in the other, broader definition in the post above.

 

[Vesalius]

There are supposed to be different pigments with different photo sensitivities present in each of the different types of cones present in the retina. Assuming that there are three types of cones each sensitive to one type of primary colour, that would give us three pigments- chlorolabe(green), cyanolabe (blue) and eythrolabe(red). None of them have been isolated in the lab of course, the only pigment that researchers can confidently state to play a role in colour vision is IODOPSIN. It is thought to work in a similar way to that of Rhodopsin. Again, it remains a mystery if colour is perceived at the retinal level or in the visual cortex.

 

[Ygggdrasil]

Despite the fact that the human eye contains three different opsin proteins that respond to different wavelengths of light, all of the opsin proteins in one's eyes uses the same pigment 11-cis-retinal. So if the three different opsin proteins all use the same pigment, how do they respond to different wavelengths?

Well, in free solution, 11-cis-retinal absorbs light of wavelength 440 nm in its protonated form and in its 365 nm deprotonated form, yet scientists have found opsins from different species with absorption maxima ranging from 360 nm to 560 nm. When retinal binds to the opsin proteins, it sits in a cavity at the center of the protein. Therefore, the protein has great control over the chemical environment surrounding retinal and can therefore alter its photophysical properties and change its spectral sensitivity.

Biochemists have identified a number of different amino acid positions within the opsin protein that are responsible for the spectral tuning of retinal (for a review, see S. Yokohama (2002) Molecular evolution of color vision in vertebrates. Gene 300: 69. ]doi:10.1016/S0378-1119(02)00845-4[/url]). Quantum mechanical and other computational chemistry studies are beginning to elucidate the physiochemical basis for these spectral changes (Altun, Yokoyama, and Morokuma. (2008) Quantum Mechanical/Molecular Mechanical Studies on Spectral Tuning Mechanisms of Visual Pigments and Other Photoactive Proteins. Photochem Photobiol. 84:845. http://dx.doi.org/10.1111/j.1751-1097.2008.00308.x PMC2575004).