Meta-rhodopsin

Nikiforovich, G. V., and Marshall, G. R. (2003). Three-dimensional model for meta-II rhodopsin, an activated G-protein-coupled receptor. Biochemistry 42, 9110-9120. 

Resek JF, Farahbakhsh ZT, Hubbell WL, Khorana HG. Formation of the meta II photointermediate is accompanied by conformational changes in the cytoplasmic surface of rhodopsin. Biochemistry 1993 32 12,025-12,032. 

Fig. 6 Comparison of retinal protonated Schiff base chemical shifts in rhodopsin different photocycle states. The chemical structure of retinal chromophore in 11-cw (a) and all-trans configuration (b). Chemical shifts of the retinal atoms in ground state, and Batho-, Meta I, and Meta II intermediate states (c) and a schematic drawing of the retinal binding pocket containing all residues within 4 A to the retinal and Lys296 (d). The chemical shifts are adapted from the following references [146-148] (black) [14] (blue) [149-151] (green) and [17, 145] (red), (d) is adapted from [188] with permission from the Elsevier B.V...

Fig. 8 Two-dimensional DARR NMR spectra of retinal-EL2 interactions. Rows from the two-dimensional DARR NMR spectra of rhodopsin black) and Meta II red) are shown. The rhodopsin crystal structure gray) with the Meta II model orange) obtained from molecular dynamic simulations are shown in the middle of the figure. Adapted from [18] with permission from Nature Publishing Group...

Shichida, Y., Kandori, H., Okada, X., Yoshizawa, X., Nakashima, N., and Yoshihara, K., Differences in the photobleaching process between 7-cis- and 11-c/s-rhodopsins a unique interaction change between the chromophore and the protein during the lumi-meta 1 transition. Biochemistry, 30, 5918, 1991. 

Stoichiometry of the Protonation Changes Associated with Meta II Formation Kinetics of the Protonation Changes in Photolyzed Rhodopsin Site of Proton Uptake The Bleaching Intermediates of the Cone ... 

I to Meta II. It was shown that when Hght initiates events that lead to Schiff base deprotonation, the proton is transferred to its counterion, GIull3. The Meta I-Meta II transition is identified with this proton transfer. More recently, the x-ray structure of bovine rhodopsin has shown that another carbox-yhc acid, GIulSl, is also close to the Schiff base. Studies on an invertebrate pigment, retinochrome, suggest it can be in its anionic form so that it can, in some circumstances, act as a counterion. 

II formation for rhodopsin in ROS or digitonin at 15 to 25°C (see, e.g., Jager et al. ). An explanation for the complexity may he in the observation, as noted above, that there is a Meta-II-like species formed directly from lumi. In addition, Hofmann and coworkers > provided convincing evidence for two Meta IIs in series, with the transition to the second involving the picking up of a proton. 

Meta II takes minutes to decay. The lifetime of the active form of rhodopsin is not set by the lifetime of Meta II, for soon after it is formed, Meta II is inactivated by being phosphorylated and binding arrestin. Therefore, a discussion of the role of the lifetime of Meta II in controlling physiological processes in rods and cones is misleading, because the spectroscopic species is not the active species in vivo. In vivo, soon after it is formed, most Meta II is inactivated without its absorption spectrum changing. 

Several of the techniques used to measure the stoichiometry of the protonation changes were also used to measure the time course with which the proton is taken up during the photolysis of rhodopsin. With measurements taken in ROS membranes or rhodopsin in digitonin, it was found that proton uptake fairly well matched the Meta-I-to-Meta-II transition rate, although resolution and precision were limited. Bennett reported good time resolution data for proton uptake (bromocresol purple) and Meta II formation (absorbance at 365 nm), and found that they matched. Kaupp et al. used the pH indicator bromocresol purple to show that in sonicated ROS membranes, proton uptake and Meta II formation times were identical at 20°C. 

There is another way to potentiaUy foUow proton kinetics in intact ROS and that is to use the early receptor potential (ERP) or the early receptor current (ERG). The ERP is a very fast potential/current evoked from oriented photoreceptors with bright flashes (reviewed in Cone and Pak and Sullivan and Shukla ). At room temperature, two phases can be easily observed for the ERP, a somewhat faster (sub msec) corneal negative R1 and a bit slower (1 to 10 msec) potential of opposite sign, R2. For rhodopsin, Cone showed that the kinetics of appearance of R2 and Meta II were identical within experimental error (this correlation was also reported by Spalink and Stieve " for bovine retinas at 37°C). This finding, along with R2 s temperature dependence, led Cone to propose that R2 was due to charge movements associated with the Meta-I-to-Meta-II transition. (R1 was similarly linked to the RH to Meta I transitions.)... 

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