Photoisomerization in Visual Rhodopsins

HPLC analysis also revealed that the protonated Schiff base of all-traws-retinal in solution is isomerized predominantly into the 11-cis form (82% 11-cis, 12% 9-cis, and 6% 13-ds in methanol) [23]. The 11-cis form as a photoproduct is the nature of retinochrome, not those of archaeal rhodopsins. This suggests that the protein environment of retinochrome serves as the intrinsic property of the photoisomerization of the retinal chromophore. In contrast, it seems that the protein environment of archaeal rhodopsins forces the reaction pathway of the isomerization to change into the 13-cis form. In this regard, it is interesting that the quantum yield of bacteriorhodopsin (0.64) is 4—5 times higher than that in solution (-0.15) [21,23], The altered excited state reaction pathways in archaeal rhodopsins never reduce the efficiency. Rather, archaeal rhodopsins discover the reaction pathway from the all-trans to 13-cis form efficiently. Consequently, the system of efficient isomerization reaction is achieved as well as in visual rhodopsins. Structural and spectroscopic studies on archaeal rhodopsins are also reviewed in Section 4.3. 

---

Figure 4.3 shows photochemical reactions in visual (Fig. 4.3A) and archaeal (Fig. 4.3B) rhodopsins. In visual rhodopsins, the 11-as-retinal is isomerized into the a -trans form. The selectivity is 100%, and the quantum yield is 0.67 for bovine rhodopsin [20]. In archaeal rhodopsins, the all-trans-retinal is isomerized into the 13-cis form. The selectivity is 100%, and the quantum yield is 0.64 for bacteriorhodopsin [21]. Squid and octopus possess a photoisomerase called retino-chrome, which supplies the 11-ris-retinal for their rhodopsins through the specific photoreaction. Retinochrome possesses all-trans-retinal as the chromophore, and the all-trans-retinal is isomerized into the 11-cis form with a selectivity of 100% [22]. Thus, the photoproduct is different between archaeal rhodopsins and retinochrome, the aU-trans form being converted into the 13-cis and 11-cis forms, respectively. This fact implies that protein environment determines the reaction pathways of photoisomerization in their excited states. 

This chapter has gathered together the current understanding of retinal photoisomerization in visual and archaeal rhodopsins mainly from the experimental point of view. Extensive studies by means of ultrafast spectroscopy of visual and archaeal rhodopsins have provided an answer to the question, What is the primary reaction in vision We now know that it is isomerization from 11-cis to all-trans form in visual rhodopsins and from all-trans to 13-cis form in archaeal rho-dopsin. Femtosecond spectroscopy of visual and archaeal rhodopsins eventually captured their excited states and, as a consequence, we now know that this unique photochemistry takes place in our eyes and in archaea. Such unique reactions are facilitated in the protein environment, and recent structural determinations have further improved our understanding on the basis of structure. In parallel, vibrational analysis of primary intermediates, such as resonance Raman and infrared spectroscopies, have provided insight into the isomerization mechanism. 

The effect of receptor stimulation is thus to catalyze a reaction cycle. This leads to considerable amplification of the initial signal. For example, in the process of visual excitation, the photoisomerization of one rhodopsin molecule leads to the activation of approximately 500 to 1000 transdudn (Gt) molecules, each of which in turn catalyzes the hydrolysis of many hundreds of cyclic guanosine monophosphate (cGMP) molecules by phosphodiesterase. Amplification in the adenylate cyclase cascade is less but still substantial each ligand-bound P-adrenoceptor activates approximately 10 to 20 Gs molecules, each of which in turn catalyzes the production of hundreds of cyclic adenosine monophosphate (cAMP) molecules by adenylate cyclase. 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

Retinol A. can be enzymically formed from retinoic acid. B. is transported from the intestine to the liver in chylomicrons. C. is the light-absorbing portion of rhodopsin. D. is phosphorylated and dephosphorylated during the visual cycle. E. mediates most of the actions of the retinoids. Correct answer = B. Retinyf esters are incorporated into chylomicrons. Retinoic acid cannot be reduced to retinol. Retinal, the aldehyde form of retinol, is the chromophore for rhodopsin. Retinal is photoisomerized during the visual cycle. Retinoic acid, not retinol, is the most important retinoid. 

Photoisomerization.—Birge and Hubbard analyse the molecular dynamics of cis-trans isomerization in the visual pigment rhodopsin using INDO-CISD molecular orbital theory and semiempirical molecular dynamic theory. The analysis predicts that the excited-state species is trapped during isomerization in an activated complex that has a lifetime of 0.5ps. This activated species oscillates between two components which preferentially decay to form isomerized product (bathorhodopsin) or unisomerized 11-cw-chromophore (rhodopsin) within 1.9—2.3ps. The authors further conclude that the chromophore in bathorhodopsin has a distorted all-rraw-geometry and is the most realistic model for the first intermediate in the bleaching cycle of rhodopsin. 

---