Bacterio-opsin

Isralewitz et eil., 1997] Isralewitz, B., Izrailev, S., and Schulten, K. Binding pathway of retinal to bacterio-opsin A prediction by molecular dynamics simulations. Biophys. J. 73 (1997) 2972-2979... 

The 13-c/j retinal-chromophore in dark-adapted bacteriorhodopsin exhibits a very different photocycle, whose predominant intermediate has an absorption maximum at 610 nm [199], and which contains no intermediate [202,238] analogous to M. The 610 nm intermediate will decay to either the 13-c/s chromophore or the dW-trans form, the latter pathway being responsible for the phenomenon of light-adaptation [199]. This pathway does not explain, however, why monomeric bacteriorhodopsin shows poor light-adaptation [168,239]. The chromophore in the 13-c/s configuration is not associated with proton translocation [240]. Indeed, reconstitution of bacterio-opsin with 13-demethyl retinal, which traps the retinal moiety in the 13-c/s configuration, results [241] in a non-transporting photocycle. 

The molecular weight of halorhodopsin is estimated from SDS gel patterns of halo-opsin labeled with H-labeled retinal [94] and of purified halorhodopsin [95,96], and it appears to be slightly lower than that of bacteriorhodopsin, i.e., about 25000. Differences in the cleavage patterns from limited proteolysis, as well as lack of immunological crossreactivity, suggest that halo-opsin and bacterio-opsin are very different proteins, however [95]. 

Molecular motions of intermediate frequency, with correlation times of 10 5 s, which can interfere with the proton decoupling frequency (ca 50 kHz), can be very easily detected when certain 13C NMR signals from both the DD-MAS and the CP-MAS NMR spectra of [3-13C]Ala-bR are simultaneously suppressed (blanked area b), as illustrated in Fig. 15A. Such motion was first recognized when the 13C NMR signals of the C terminus were almost completely suppressed both in the CP-MAS and DD-MAS NMR spectra, when the temperature was lowered to between -40 and -110°C.116 This kind of peak suppression is most pronounced at ambient temperature for the whole range of transmembrane a-helices and a part of the loop in the [3-13C] Ala-labeled bleached bacterio-opsin (bO) where retinal was removed from bR (see Fig. 16), as viewed from the suppression of Ala 39, 53, and 84 (B and C helices), Ala 215 (G-helix), the E-F loop (Ala 160), and the F-G loop (Ala 196).98 A similar type of peak suppression... 

Fig. 16. I3C NMR spectra of bacterio-opsin (bO) (black traces) as compared with those of bacteriorhodopsin (bR) (gray traces), recorded by the DD-MAS (A) and CP-M AS (B) NMR methods.98 Reproduced with permission from the Japanese Biochemical Society.

Braiman MS, Stem LJ, Chao BH, Khorana HG (1987) Stmcture-function studies on bacteri-orhodopsin. IV. Purification and renaturation of bacterio-opsin polypeptide expressed in Escherichia coli. J Biol Chem 262 9271-9276... 

It is expected, however, that the backbone dynamics could be substantially modified when such a 2D lattice assembly is distorted or disrupted as in bacterio-opsin (bO) prepared from either an hydroxylamine-treated bR or retinal-deficient ElOOl strain in which retinal-helix interactions is absent owing to lack of retinal. The resultant protein dynamic change caused by the removed retinal through the modified helix-helix interaction resulted in the preferentially suppressed NMR signals of [3-Ala-labeled bO at the loops and transmembrane a-helices near to the membrane surface. At the same time, the NMR spectrum of [l- C]Val-labeled bO was also partly suppressed especially in the region of the loops at lower frequency by acquisition of the fluctuation motions. 

S. Yamaguchi, S. Tuzi, M. Tanio, A. Naito, J.K. Lanyi, R. Needleman, H. Saito, Irreversible conformational change of bacterio-opsin induced by binding of retinal during... 

In the halobacterial system, the situation seems to be exactly opposite only the all-trans and 13-cis isomers react with bacterio-opsin. In both bacteriorhodopsin and rhodopsin, the ability to react is comparatively insensitive to modifications of the (3-ionone ring. 

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