Bacteriorhodopsin photoisomerization

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

Bacteriorhodopsin (bR) is a transmembrane protein located in the cell membrane of purple bacteria and contains in its ground state an all-trans retinal chromophore that absorbs at 570 nm. After illumination, the chromophore isomerizes, and a proton is pumped in five consecutive steps from the cytoplasm to the extracellular side of the membrane. The resulting pH gradient is then used to synthesize ATP. In the first proton-transfer step, the proton located at the retinal chromophore Schiff base is transferred to a nearby aspartate residue (Figure 14-6). Our studies on this first proton-transfer step in bacteriorhodopsin (bR) after photoisomerization [90,91,92]... 

Dioumaev, A. K., Brown, L. S., Needleman, R., and Lanyi, J. K. (1998). Partitioning of free energy gain between the photoisomerized retinal and the protein in bacteriorhodopsin. Biochemistry 37, 9889-9893. 

Bacteriorhodopsin translocates one H ion per photon which causes the aW-trans — 13-c 5 photoisomerization of chromophore, retinal. At least two H -acceptor groups are shown to be directly involved in the H transfer by bacteriorhodopsin, namely, (a) the Schiff base forming a link between the retinal and the e-amino group of Lys-216, and (b) the Asp-96 carboxylic group. The involvement of the Schiff base is confirmed by many independent pieces of evidence (e.g., the electrogenic H transfer disappears at a pH below 3.5, i.e., below the pK value of the Schiff base in the M-intermediate of bacteriorhodopsin photocycle reviewed in ref. [7]). As to Asp-96, its participation in the H transfer relay was recently demonstrated by site-directed mutagenesis studies [13-19]. 

Intramolecular heavy atom effects influence the photoisomerization derivatives of 5,5-diphenyl-1,3-cyclohexadiene The homogeneous acid catalysis of the photoisomerization of trans-3-(2-hydroxy-benzylidene)-4,5-dihydrofuran-2(3H)-one and model mechanisms for isomerization of carbocyanines have both been analyzed. The process of photoisomerization of the biologically important rhodopsin and bacteriorhodopsin has been examined by a theoretical ab initio study of retinal analogues. ... 

Song, L., El Sayed, M. A., Lanyi, J. K., Protein Catalysis of the Retinal Subpicosecond Photoisomerization in the Primary Process of Bacteriorhodopsin Photosynthesis, Science 1993, 261, 891 894. 

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. 

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. 

Bacteriorhodopsin is the sole membrane protein of seven a-helical transmembrane chains present in the purple membrane of Halobacterium salinarum. This is active as a light-driven proton pump through the photoisomerization of retinal (Fig. 2) from the aW-trans, 15-anti to the 13-cis, 15-anti form covalently linked to Lys216 (helix G) of a single-chain polypeptide of 248 amino acid... 

Bacteriorhodopsin is the quintessential transmembrane ion pun ). It consists of a small, seven-helix protein where proton transport across the membrane is driven by photoisomerization of retinal from the all trans to the 13-cis,l5-anti configuration. A number of high-resolution crystal structures of the protein and its photointermediates have been used to propose several competing mechanisms describing proton translocation to fhe extracellular surface. Unresolved issues include understanding how conformational changes couple to proton transfer and the role played by water molecules in the proton transfer process. ... 

The kinetics of the photoisomerization of bilirubin has been studied because of the relevance to phototherapy. The fluorescence of bilirubin increases on binding to human serum albumin. This and other primary photoprocesses have been investigated by picosecond spectroscopy. Karvaly has put forward a new photochemical mechanism for energy conversion in bacteriorhodopsin. An extensive review of the photophysics of light transduction in rhodopsin and bacteriorhodopsin has been made by Birge. The dynamics of cis-trans isomerization in rhodopsin has been analysed by INDO-CISD molecular orbital theory. Similar calculations on polyenes and cyanine dyes have also been reported. A new picosecond resonance Raman technique shows that a distorted... 

Photoisomerization in photoreceptor proteins is significantly different from that in solution. For example, the all-trans retinal in bacteriorhodopsin (Figure 6.1) experiences the bond-specific photoisomerization reaction only at the C13=C14 bond. 

Figure 4.4 (a) Schematic representation of the three-dimensional structure of bacteriorhodopsin (bR). (b) Photoisomerization of all-trans to 13-cis retinal in bR. 

WT Pollard, CH Brito Cruz, CV Shank, RA Mathies. Direct observation of the excited-state cis-trans photoisomerization of bacteriorhodopsin—Multilevel line-shape theory for femtosecond dynamic hole burning and its application. J Chem Phys 90 199-208, 1989. 

Bacteriorhodopsin is an intramembrane protein, which uses adsorbed light energy to transfer a proton through the membrane. The microscopic mechanism of the proton pumping is based on the set of isomerization processes initiated by the light adsorption. Upon dehydration, photoisomerization of bacteriorhodopsin reduces [480-484] and proton pumping stops below 60% relative humidity [483—486]. The above examples show... 

---