Bleaching process, rhodopsins

Absorption of a photon by the chromophore induces primary photoreaction, followed by conformational changes of protein, and eventually activates transducin. This is called the bleaching process because rhodopsin loses its color. To investigate the primary photoreaction processes in rhodopsin, two spectroscopic approaches have been applied low-temperature and time-resolved spectroscopies... 

Changes in Chromophore-Opsin Interaction in Bleaching Process of Rhodopsin... 

To further understand the molecular mechanism of hght absorption and G protein activation by rhodopsin, it is important to determine structural changes in the bleaching process of rhodopsin at atomic resolution. Recent success of the preparation of the rhodopsin crystal enables us to determine the crystal structure of the intermediates upon irradiation of the crystal at adequate temperature. ... 

Bleaching is reversed in the dark and the red-purple color of rhodopsin returns. This is thought to occur by the reduction of all-Pms-retinal to vitamin Ai (retinal), which diffuses from the rod into the pigment epithelium, where it is converted enzymatically to the 1 l-c isomer of vitamin At. The enzymatic isomerization is followed by diffusion back into the rod, oxidation to 11 -rfr-retinal, and combination with opsin to form rhodopsin. This process is shown schematically in Figure 12.5.[Pg.289]

FIGURE 28. 283-MHz 19F NMR spectra of isomers of 8-F-rhodopsin in CFIAPS before (lower) and after photoirradiation (upper) (a) 11-cis (pulse delay, D5 = 5.0 s, number of acquisitions, NA = 5200, line broadening, LB = 80 Hz) (b) 9-cis (D5 = 50 ms, NA = 160000, LB = 80 Hz). Disappearance of the excess 9-cis aldehyde was due to repeated formation and bleaching of pigment during the irradiation process. Reprinted with permission from Reference 48. Copyright (1996) American Chemical Society... 

The absorption of light by rhodopsin results in a change in the configuration of the retinaldehyde from the 11 -cis to the all- trans isomer, together with a conformational change in opsin. This results in both the release of retinaldehyde from the Schiff base and the initiation of a nerve impulse. The overall process is known as bleaching, because it results in the loss of the color of rhodopsin. 

The photopigment inside the discs is rhodopsin, a protein sensitive to light. When rhodopsin is hit by photons, a chemical process named (photoisomerization) is triggered The pigment photo-bleaches within milliseconds (see Sect. 4) and causes a change in the electrical potential of the receptor membrane. After complete... 

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