A covalent link between the chromophore and the protein backbone of bacteriorhodopsin is not required for forming a photochemically active pigment analogous to the wild type.
Bacteriorhodopsin pigments lacking the retinal-Lys-216 covalent bond were prepared by reconstituting the K216G mutant protein with retinal alkylamine Schiff bases. The procedure follows the approach of Zhukovsky et al. [Zhukovsky, E., Robinson, P., & Oprian, D. (1991) Science 251, 558-560] in the case of visual (rhodopsin) pigments. Reconstitution leads to a mixture of three pigments. One of them, bR(K216G)/566a, absorbs (pH = 6.9) at 566 nm. Its absorption is pH-dependent, exhibiting a purple to blue transition. The pigment's laser-induced photocycle patterns are similar to those of wild-type all-trans-bR. A second component, bR(K216G)/566b, exhibits an independent photocycle reminiscent of that of wild-type 13-cis-bR. A third pigment component, bR(K216G)/630, absorbs around 630 nm. Experiments in the presence of a pH dye indicator show that illumination of bR(K216G)/566 produces a detectable proton gradient. It is concluded that a covalent bond between the retinal chromophore and the protein backbone is not a prerequisite for the basic structure and photochemical features of bR or for its proton pump activity.
Friedman
N
N
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel.
Druckmann
S
S
Lanyi
J
J
Needleman
R
R
Lewis
A
A
Ottolenghi
M
M
Sheves
M
M
D001436:Bacteriorhodopsins / Q000737:chemistry*; D019617:Halobacterium salinarum / Q000737:chemistry; D010777:Photochemistry; D010860:Pigments, Biological / Q000737:chemistry*; D017494:Proton Pumps; D012172:Retinaldehyde / Q000737:chemistry; D012545:Schiff Bases / Q000737:chemistry
D010860:Pigments, Biological; D017494:Proton Pumps; D012545:Schiff Bases; D001436:Bacteriorhodopsins; D012172:Retinaldehyde