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
N. Friedman et al., 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, Biochemistry, 33(8), 1994, pp. 1971-1976
Bacteriorhodopsin pigments lacking the retinal-lys-216 covalent bond w
ere prepared by reconstituting the K216G mutant protein with retinal a
lkylamine Schiff bases. The procedure follows the approach of Zhukovsk
y et al. [Zhukovsky, E., Robinson, P., and Oprian, D. (1991) Science 2
51, 558-560] in the case of visual (rhodopsin) pigments. Reconstitutio
n leads to a mixture of three pigments. One of them, bR(K216G)/566a, a
bsorbs (pH = 6.9) at 566 nm. Its absorption is pH-dependent, exhibitin
g a purple to blue transition. The pigment's laser-induced photocycle
patterns are similar to those of wild-type all-trans-6R. A second comp
onent, bR(K216G)/566b, exhibits an independent photocycle reminiscent
of that of wild-type 13-cis-bR. A third pigment component, bR(K216G)/6
30, absorbs around 630 nm. Experiments in the presence of a pH dye ind
icator show that illumination of bR(K216G)/566 produces a detectable p
roton gradient. It is cancluded that a covalent bond between the retin
al chromophore and the protein backbone is not a prerequisite for the
basic structure and photochemical features of bR or for its proton pum
p activity.