Effect of a light-induced pH gradient on purple-to-blue and purple-to-red transitions of bacteriorhodopsin
- Institute of Physical and Chemical Research, Saitama (Japan)
Bacteriorhodopsin-containing vesicles that were able to alkalize the extravesicular medium by greater than 1.5 pH units under illumination, i.e., inside-out vesicles, were reconstituted by reverse-phase evaporation with Halobacterium halobium polar lipids or exogenous phospholipids. Acid titration of a dark-adapted sample was accompanied by a color change from purple to blue (pKa = 2.5-4.5 in 0.15 M K2SO4), and alkali titration resulted in the formation of a red species absorbing maximally at 480 nm (pKa = 7 to greater than 9), the pKa values and the extents of these color changes being dependent on the nature of lipid. When a vesicle suspension at neutral or weakly acidic pH was irradiated by continuous light so that a large pH gradient was generated across the membrane, either a purple-to-blue or a purple-to-red transition took place. The light-induced purple-to-red transition was significant in an unbuffered vesicle suspension and correlated with the pH change in the extravesicular medium. The result suggests that the purple-to-red transition is driven from the extravesicular side, i.e., from the C-terminal membrane surface. In the presence of buffer molecules outside, the dominant color change induced in the light was the purple-to-blue transition, which seemed to be due to a large decrease in the intravesicular pH. But an apparently inconsistent result was obtained when the extravesicular medium was acidified by a HCl pulse, which was accompanied by a rapid color change to blue. We arrived at the following explanation: The two bR isomers, one containing all-trans-retinal and the other 13-cis-retinal, respond differently to pH changes in the extravesicular and the intravesicular medium. In this relation, full light adaptation was not achieved when the light-induced purple-to-blue transition was significant.
- OSTI ID:
- 6370684
- Journal Information:
- Biochemistry; (USA), Vol. 29:29; ISSN 0006-2960
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
RHODOPSIN
PHOTOCHEMISTRY
LIPIDS
LIPOSOMES
PH VALUE
PHOTOSYNTHETIC BACTERIA
PHOTOSYNTHETIC MEMBRANES
SPECTROPHOTOMETRY
VISIBLE RADIATION
CELL CONSTITUENTS
CHEMISTRY
ELECTROMAGNETIC RADIATION
MEMBRANES
ORGANIC COMPOUNDS
ORGANOIDS
PIGMENTS
PROTEINS
RADIATIONS
550200* - Biochemistry