Retinal isomerization

Kobayashi T, Saito T, Ohtani H (2001) Real-time spectroscopy of transition states in bacteri-orhodopsin during retinal isomerization. Nature 414 531... 

Figure 7.3 Nornicotine catalyzes retinal isomerization in vitro...

A stack of about 1 000 disks in each rod cell contains the lightsensing protein rhodopsin,10 in which the chromophore 11-o i-retinal (from vitamin A) is attached to the protein opsin. When light is absorbed by rhodopsin, a series of rapid transformations releases all-frans-retinal. At this stage, the pigment is bleached (loses all color) and cannot respond to more light until retinal isomerizes back to the 11 -cis form and recombines with the protein. 

Post-ingestion precursor for RA-R agonist RA 11-cis-Retinal chromophore linked to protein opsin, in visual excitation isomerizes to all-fram-Retinal -George Wald (USA, retinal isomerization), Ragnar Granit (Finland/ Sweden) Haldan Hartline (USA) (Nobel Prize, Physiology/Medicine, 1967, vision)... 

Patel AB, Crocker E, Eilers M, Hirshfeld A, Sheves M, Smith SO. Coupling of retinal isomerization to the activation of rhodopsin. Proc. Natl. Acad. Sci. U.S.A. 2004 101 10048-10053. 

When bacteriorhodopsin was discovered in 1971, its similarity to the visual rhodopsin in the chromophore structure (retinal), the primary light-induced event (retinal isomerization) and the photocycle (short-wavelength intermediate formation) impelled the authors to suggest that this novel retinal protein is somehow involved in photoreception [1]. Such a suggestion seemed reasonable since halobacteria are known to change their direction of swimming in response to a light stimulus. 

I. Kawamura, S. Yamaguchi, H. Nishikawa, K. Tajima, M. Horigome, S. Tuzi, H. Saito, A. Naito, Change in local dynamics ofbacteriorhodopsin with retinal isomerization under pressure as studied by fast magic angle spinning NMR, Polymer J. 44 (2012) 863-867. 

Aharoni, A., Ottolenghi, M., Sheves, M. Retinal isomerization in bacteiimhodopsin is controlled by specific chnnnophore-protein interactions. A study with noncovalent artificial pigments. Biochemistry 40, 13310-13319 (2001)... 

Isin, B., Schulten, K., Tajkhorshid, E., Bahar, 1. Mechanism of signal propagation upon retinal isomerization insights from molecular dynamics simulations of rhodopsin restrained by normal modes. Biophys. J. 95, 789-803 (2008)... 

Our ability to see depends in part on an interconversion of cis and trans isomers that takes place in our eyes. A protein called opsin binds to cA-retinal (formed from vitamin A) in photoreceptor cells (called rod cells) in the retina to form rhodopsin. When rhodop-sin absorbs light, a double bond interconverts between the cis and trans configurations, triggering a nerve impulse that plays an important role in vision. fran -Retinal is then released from opsin. trawi-Retinal isomerizes back to cA-retinal and another cycle begins. To trigger the nerve impulse, a group of about 500 rod cells must register five to seven rhodopsin isomerizations per cell within a few tenths of a second. 

As one can see in this brief review, despite important advances in the studies of retinal and its models based on dynamics simulation methods achieved in the last years, there is still an ongoing discussion about the mechanism of retinal isomerization in rhodopsin proteins, which calls for further investigations. 

Send, R., Sundholm, D. (2007). Stairway to the conical intersection A computational study of the retinal isomerization. The Journal of Physical Chemistry A, 111(36), 8766-8773. 

Vitamin A has an important role in vision in the form of retinal, in which the alcohol group (-OH) is replaced by an aldehyde group (-CHO) by the f-carotene 15,15 -monooxygenase enzyme. As light enters the eye, 1,1,-cis-retinal isomerizes to 1,1-trans-retinal This trans-retinal stimulates a nerve through a series of steps, sending a signal to the visual center of the brain. After the information is sent, the 1,1-trans-retinal will be converted back to 1,1,-cis-retinal for further stimulants. 

Transition States in Bacteriorhodopsin During Retinal Isomerization. 

Haupts, U., Tittor, J., and Oesterhelt, D., Closing in on bacteriorhodopsin progress in understanding the molecule, Annu. Rev. Biophys. Biomol Struct., 28, 367-399, 1999 Kobayashi, T, Saito, T, and Ohtani, H., Real-time spectroscopy of transition states in bacteriorhodopsin during retinal isomerization, Nature, 414, 531-534,2001 Chizhov, L, Chernavskii, D.S., Engelhard, M., Mueller, K.H., Zubov, B.V., and Hess, B., Spectrally silent transitions in the bacteriorhodopsin photocycle, Biophys. J., 71, 2329-2345, 1996. 

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