Evidence for the involvement of more than one metal cation in the Schiff base deprotonation process during the photocycle of bacteriorhodopsin
The removal of metal cations inhibits the deprotonation process of the protonated Schiff base during the photocycle of bacteriorhodopsin. To understand the nature of the involvement of these cations, a spectroscopic and kinetic study was carried out on bacteriorhodopsin samples in which the native Ca/sup 2 +/ and Mg/sup 2 +/ were replaced by Eu/sup 3 +/, a luminescent cation. The decay of Eu/sup 3 +/ emission in bacteriorhodopsin can be fitted to a minimum of three decay components, which are assigned to Eu/sup 3 +/ emission from three different sites. This is supported by the response of the decay components to the presence of /sup 2/H/sub 2/O and to the changes in the Eu/sup 3 +//bR molar ratio. The number of water molecules coordinated to Eu/sup 3 +/ in each site is determined from the change in its emission lifetime when /sup 2/H/sub 2/O replaces H/sub 2/O. Most of the emission originates from two wet sites of low crystal-field symmetry-e.g., surface sites. Protonated Schiff base deprotonation has no discernible effect on the emission decay of protein-bound Eu/sup 3 +/, suggesting an indirect involvement of metal cations in the deprotonation process. Adding Eu/sup 3 +/ to deionized bacteriorhodopsin increases the emission intensity of each Eu/sup 3 +/ site linearly, but the extent of the deprotonation (and color) changes sigmoidally. This suggests that if only the emitting Eu/sup 3 +/ ions cause the deprotonation and bacteriorhodopsin color change, ions in more than one site must be involved-e.g., by inducing protein conformation changes. The latter could allow deprotonation by the interaction between the protonated Schiff base and a positive field of cations either on the surface or within the protein.
- Research Organization:
- Univ. of California, Los Angeles, CA (United States)
- OSTI ID:
- 5460830
- Journal Information:
- Proc. Natl. Acad. Sci. U.S.A.; (United States), Vol. 84:12
- Country of Publication:
- United States
- Language:
- English
Similar Records
Perturbation effects on the observed deprotonation processes in the photocycle of bacteriorhodopsin
On the molecular mechanisms of solar energy storage during the photocycle of the other photosynthetic system in nature, bacteriorhodopsin
Related Subjects
59 BASIC BIOLOGICAL SCIENCES
EUROPIUM COMPOUNDS
EMISSION SPECTRA
RHODOPSIN
PHOTOCHEMICAL REACTIONS
BIOCHEMICAL REACTION KINETICS
CALCIUM COMPOUNDS
CATIONS
HEAVY WATER
MAGNESIUM COMPOUNDS
PHOTOSYNTHETIC BACTERIA
SCHIFF BASES
ALKALINE EARTH METAL COMPOUNDS
CHARGED PARTICLES
CHEMICAL REACTIONS
HYDROGEN COMPOUNDS
IMINES
IONS
KINETICS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PIGMENTS
PROTEINS
RARE EARTH COMPOUNDS
REACTION KINETICS
SPECTRA
WATER
140505* - Solar Energy Conversion- Photochemical
Photobiological
& Thermochemical Conversion- (1980-)
550201 - Biochemistry- Tracer Techniques